On Fri, Nov 22, 2019 at 04:11:53PM -0500, Jason Merrill wrote:
> On 11/8/19 4:24 PM, Marek Polacek wrote:
> > After much weeping and gnashing of teeth, here's a patch to handle
> > dynamic_cast
> > in constexpr evaluation. While the change in the standard is trivial (see
> > <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1327r1.html>), the
> > change in the compiler is less so.
> >
> > When build_dynamic_cast realizes that a dynamic_cast needs a run-time
> > check, it
> > generates a call to __dynamic_cast -- see dyncast.cc in libsupc++ for its
> > definition. The gist of my approach is to evaluate such a call at compile
> > time.
> >
> > This should be easy in theory: let the constexpr machinery find out the
> > dynamic
> > type and then handle a sidecast and upcast. That's ultimately what the
> > patch
> > is trying to do but there was a number of hindrances.
> >
> > 1) We can't use __dynamic_cast's type_info parameters, this type is not a
> > literal class. But that means we have no idea what we're converting to!
>
> get_tinfo_decl sets the TREE_TYPE of the DECL_NAME of the tinfo decl to the
> relevant type, can't you use that?
Yes, lovely. I hadn't noticed that :(.
It doesn't say if this is a reference dynamic_cast or a pointer dynamic_cast,
so I checked OBJ to wheedle that information out of it.
> > 2) [class.cdtor] says that when a dynamic_cast is used in a constructor or
> > destructor and the operand of the dynamic_cast refers to the object under
> > construction or destruction, this object is considered to be a most derived
> > object.
>
> This means that during the 'tor the vtable pointer refers to the type_info
> for that class and the offset-to-top is 0. Can you use that?
I can't seem to: For e.g.
struct C : A, C2 { A *c = dynamic_cast<A*>(static_cast<C2*>(this)); };
the object under construction is C, the call to __dynamic_cast will be
__dynamic_cast (SAVE_EXPR <&((struct C *) this)->D.2119>, &_ZTI2C2, &_ZTI1A, -2)
here, OBJ is f.D.2156.D.2119 and ctx->global->ctor_object is f.D.2156. So OBJ
refers to the object under construction.
But I don't see C anywhere; CLASSTYPE_TYPEINFO_VAR of OBJTYPE is _ZTI2C2.
Am I looking into the wrong place?
> > + /* Given dynamic_cast<T>(v),
> > +
> > + [expr.dynamic.cast] If C is the class type to which T points or
> > refers,
> > + the runtime check logically executes as follows:
> > +
> > + If, in the most derived object pointed (referred) to by v, v points
> > + (refers) to a public base class subobject of a C object, and if only
> > + one object of type C is derived from the subobject pointed (referred)
> > + to by v the result points (refers) to that C object.
> > +
> > + In this case, HINT >= 0. This is a downcast. */
>
> Please avoid using up/down to refer to inheritance relationships, people
> disagree about what they mean. :)
OK, dropped that terminology.
> > + if (hint >= 0)
> > + {
> > + /* We now have something like
> > +
> > + g.D.2181.D.2154.D.2102.D.2093
> > + ^~~~~~
> > + OBJ
> > +
> > + and we're looking for a component with type TYPE. */
> > + tree objtype = TREE_TYPE (obj);
> > + tree ctor_object = ctx->global->ctor_object;
> > +
> > + for (;;)
> > + {
> > + /* Unfortunately, we can't rely on HINT, we need to do some
> > + verification here:
> > +
> > + 1) Consider
> > + dynamic_cast<E*>((A*)(B*)(D*)&e);
> > + and imagine that there's an accessible base A from E (so HINT
> > + is >= 0), but it's a different A than where OBJ points to.
> > + We need to check that the one we're accessing via E->D->B->A is
> > + in fact accessible. If e.g. B on this path is private, we gotta
> > + fail. So check that every base on the way can be reached from
> > + the preceding class.
> > +
> > + 2) Further, consider
> > +
> > + struct A { virtual void a(); };
> > + struct AA : A {};
> > + struct B : A {};
> > + struct Y : AA, private B {};
> > +
> > + dynamic_cast<Y*>((A*)(B*)&y);
> > +
> > + Here HINT is >=0, because A is a public unique base of Y,
> > + but that's not the A accessed via Y->B->A. */
> > + if (!accessible_base_p (TREE_TYPE (obj), objtype, false)
> > + || !accessible_base_p (type, TREE_TYPE (obj), false))
> > + {
> > + if (reference_p)
> > + {
> > + if (!ctx->quiet)
> > + {
> > + error_at (loc, "reference %<dynamic_cast%> failed");
> > + inform (loc, "static type %qT of its operand is a "
> > + "non-public base class of dynamic type %qT",
> > + objtype, type);
> > + }
> > + *non_constant_p = true;
> > + }
> > + return integer_zero_node;
> > + }
> > +
> > + if (same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (obj), type))
> > + /* The result points to the TYPE object. */
> > + return cp_build_addr_expr (obj, complain);
> > + else if (TREE_CODE (obj) == COMPONENT_REF
> > + && DECL_FIELD_IS_BASE (TREE_OPERAND (obj, 1)))
> > + obj = TREE_OPERAND (obj, 0);
>
> In the structural_type_p patch I tried to set TREE_PRIVATE/TREE_PROTECTED in
> build_base_field_1, would that be useful to check instead of
> accessible_base_p? Though I'm not sure I was successful, I got a bug report
> about structural type vs. private bases that I haven't looked at yet.
This works now, awesome. I remember being disappointed that that didn't work,
because accessible_base_p seems pretty expensive in comparison to just
checking a flag... I've used TREE_PRIVATE/TREE_PROTECTED accessible_base_p
instead of accessible_base_p in the following. (This patch is based on trunk
with this https://gcc.gnu.org/ml/gcc-patches/2019-12/msg00720.html in.)
Thanks!
Bootstrapped/regtested on x86_64-linux, built cmcstl2, ok for trunk?
2019-12-11 Marek Polacek <pola...@redhat.com>
PR c++/88337 - Implement P1327R1: Allow dynamic_cast in constexpr.
* call.c (is_base_field_ref): No longer static.
* constexpr.c (struct constexpr_global_ctx): Add ctor_object member
and initialize it.
(cxx_dynamic_cast_fn_p): New function.
(cxx_eval_dynamic_cast_fn): Likewise.
(cxx_eval_call_expression): Call cxx_eval_dynamic_cast_fn for a call
to __dynamic_cast. Save the object a constexpr constructor is
constructing.
(potential_constant_expression_1): Don't give up on
cxx_dynamic_cast_fn_p.
* cp-tree.h (is_base_field_ref): Declare.
* rtti.c (build_dynamic_cast_1): When creating a call to
__dynamic_cast, use the location of the original expression.
* g++.dg/cpp2a/constexpr-dynamic1.C: New test.
* g++.dg/cpp2a/constexpr-dynamic10.C: New test.
* g++.dg/cpp2a/constexpr-dynamic11.C: New test.
* g++.dg/cpp2a/constexpr-dynamic12.C: New test.
* g++.dg/cpp2a/constexpr-dynamic13.C: New test.
* g++.dg/cpp2a/constexpr-dynamic14.C: New test.
* g++.dg/cpp2a/constexpr-dynamic2.C: New test.
* g++.dg/cpp2a/constexpr-dynamic3.C: New test.
* g++.dg/cpp2a/constexpr-dynamic4.C: New test.
* g++.dg/cpp2a/constexpr-dynamic5.C: New test.
* g++.dg/cpp2a/constexpr-dynamic6.C: New test.
* g++.dg/cpp2a/constexpr-dynamic7.C: New test.
* g++.dg/cpp2a/constexpr-dynamic8.C: New test.
* g++.dg/cpp2a/constexpr-dynamic9.C: New test.
diff --git gcc/cp/call.c gcc/cp/call.c
index cbd5747d6ca..17d7e44575b 100644
--- gcc/cp/call.c
+++ gcc/cp/call.c
@@ -8225,7 +8225,7 @@ call_copy_ctor (tree a, tsubst_flags_t complain)
/* Return true iff T refers to a base field. */
-static bool
+bool
is_base_field_ref (tree t)
{
STRIP_NOPS (t);
diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
index 19e09c74760..daaf83193ea 100644
--- gcc/cp/constexpr.c
+++ gcc/cp/constexpr.c
@@ -1031,8 +1031,11 @@ struct constexpr_global_ctx {
auto_vec<tree, 16> heap_vars;
/* Cleanups that need to be evaluated at the end of CLEANUP_POINT_EXPR. */
vec<tree> *cleanups;
+ /* For a constructor, this is the object we're constructing. */
+ tree ctor_object;
/* Constructor. */
- constexpr_global_ctx () : constexpr_ops_count (0), cleanups (NULL) {}
+ constexpr_global_ctx () : constexpr_ops_count (0), cleanups (NULL),
+ ctor_object (NULL_TREE) {}
};
/* The constexpr expansion context. CALL is the current function
@@ -1692,6 +1695,247 @@ is_std_allocator_allocate (tree fndecl)
return decl_in_std_namespace_p (decl);
}
+/* Return true if FNDECL is __dynamic_cast. */
+
+static inline bool
+cxx_dynamic_cast_fn_p (tree fndecl)
+{
+ return (cxx_dialect >= cxx2a
+ && id_equal (DECL_NAME (fndecl), "__dynamic_cast")
+ && CP_DECL_CONTEXT (fndecl) == global_namespace);
+}
+
+/* Evaluate a call to __dynamic_cast (permitted by P1327R1).
+
+ The declaration of __dynamic_cast is:
+
+ void* __dynamic_cast (const void* __src_ptr,
+ const __class_type_info* __src_type,
+ const __class_type_info* __dst_type,
+ ptrdiff_t __src2dst);
+
+ where src2dst has the following possible values
+
+ >-1: src_type is a unique public non-virtual base of dst_type
+ dst_ptr + src2dst == src_ptr
+ -1: unspecified relationship
+ -2: src_type is not a public base of dst_type
+ -3: src_type is a multiple public non-virtual base of dst_type
+
+ Since literal types can't have virtual bases, we only expect hint >=0
+ or -2. */
+
+static tree
+cxx_eval_dynamic_cast_fn (const constexpr_ctx *ctx, tree call,
+ bool *non_constant_p, bool *overflow_p)
+{
+ /* T will be something like
+ __dynamic_cast ((B*) b, &_ZTI1B, &_ZTI1D, 8)
+ dismantle it. */
+ gcc_assert (call_expr_nargs (call) == 4);
+ tsubst_flags_t complain = ctx->quiet ? tf_none : tf_warning_or_error;
+ tree obj = CALL_EXPR_ARG (call, 0);
+ tree type = CALL_EXPR_ARG (call, 2);
+ HOST_WIDE_INT hint = int_cst_value (CALL_EXPR_ARG (call, 3));
+ location_t loc = cp_expr_loc_or_input_loc (call);
+
+ /* Get the target type of the dynamic_cast. */
+ gcc_assert (TREE_CODE (type) == ADDR_EXPR);
+ type = TREE_OPERAND (type, 0);
+ type = TREE_TYPE (DECL_NAME (type));
+
+ /* TYPE can only be either T* or T&. We can't know which of these it
+ is by looking at TYPE, but OBJ will be "(T*) x" in the first case,
+ and something like "(T*)(T&)(T*) x" in the second case. */
+ bool reference_p = false;
+ while (CONVERT_EXPR_P (obj))
+ {
+ reference_p |= TYPE_REF_P (TREE_TYPE (obj));
+ obj = TREE_OPERAND (obj, 0);
+ }
+
+ /* Evaluate the object so that we know its dynamic type. */
+ obj = cxx_eval_constant_expression (ctx, obj, /*lval*/false, non_constant_p,
+ overflow_p);
+ if (*non_constant_p)
+ return call;
+
+ /* We expect OBJ to be in form of &d.D.2102 when HINT == 0,
+ but when HINT is > 0, it can also be something like
+ &d.D.2102 + 18446744073709551608, which includes the BINFO_OFFSET. */
+ if (TREE_CODE (obj) == POINTER_PLUS_EXPR)
+ obj = TREE_OPERAND (obj, 0);
+ /* If OBJ doesn't refer to a base field, we're done. */
+ if (!is_base_field_ref (obj))
+ return integer_zero_node;
+ STRIP_NOPS (obj);
+ /* Strip the &. */
+ if (TREE_CODE (obj) == ADDR_EXPR)
+ obj = TREE_OPERAND (obj, 0);
+
+ /* Given dynamic_cast<T>(v),
+
+ [expr.dynamic.cast] If C is the class type to which T points or refers,
+ the runtime check logically executes as follows:
+
+ If, in the most derived object pointed (referred) to by v, v points
+ (refers) to a public base class subobject of a C object, and if only
+ one object of type C is derived from the subobject pointed (referred)
+ to by v the result points (refers) to that C object.
+
+ In this case, HINT >= 0. */
+ if (hint >= 0)
+ {
+ /* We now have something like
+
+ g.D.2181.D.2154.D.2102.D.2093
+ ^~~~~~
+ OBJ
+
+ and we're looking for a component with type TYPE. */
+ tree objtype = TREE_TYPE (obj);
+ tree ctor_object = ctx->global->ctor_object;
+
+ for (;;)
+ {
+ if (same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (obj), type))
+ /* The result points to the TYPE object. */
+ return cp_build_addr_expr (obj, complain);
+ else if (TREE_CODE (obj) == COMPONENT_REF
+ && DECL_FIELD_IS_BASE (TREE_OPERAND (obj, 1)))
+ {
+ /* Unfortunately, we can't rely on HINT, we need to do some
+ verification here:
+
+ 1) Consider
+ dynamic_cast<E*>((A*)(B*)(D*)&e);
+ and imagine that there's an accessible base A from E (so
+ HINT is >= 0), but it's a different A than where OBJ points
+ to. We need to check that the one we're accessing via
+ E->D->B->A is in fact accessible. If e.g. B on this path
+ is private, we gotta fail. So check that every base on the
+ way can be reached from the preceding class.
+
+ 2) Further, consider
+
+ struct A { virtual void a(); };
+ struct AA : A {};
+ struct B : A {};
+ struct Y : AA, private B {};
+
+ dynamic_cast<Y*>((A*)(B*)&y);
+
+ Here HINT is >=0, because A is a public unique base of Y,
+ but that's not the A accessed via Y->B->A. */
+ if (TREE_PRIVATE (TREE_OPERAND (obj, 1))
+ || TREE_PROTECTED (TREE_OPERAND (obj, 1)))
+ {
+ if (reference_p)
+ {
+ if (!ctx->quiet)
+ {
+ error_at (loc, "reference %<dynamic_cast%> failed");
+ inform (loc, "static type %qT of its operand is a "
+ "non-public base class of dynamic type %qT",
+ objtype, type);
+ }
+ *non_constant_p = true;
+ }
+ return integer_zero_node;
+ }
+ obj = TREE_OPERAND (obj, 0);
+ }
+ else if (ctor_object != NULL_TREE)
+ {
+ /* If we get here, it means we didn't find TYPE in OBJ. But
+ HINT told us that the source type is a unique public base
+ of TYPE. So it must be the case that we're looking at an
+ object under construction. Try again. */
+ obj = ctor_object;
+ ctor_object = NULL_TREE;
+ }
+ else
+ /* Make sure we've either found a problem or the desired type. */
+ gcc_unreachable ();
+ }
+ }
+ /* Otherwise, if v points (refers) to a public base class subobject of the
+ most derived object, and the type of the most derived object has a base
+ class, of type C, that is unambiguous and public, the result points
+ (refers) to the C subobject of the most derived object.
+
+ But it can also be an invalid case. */
+ else if (hint == -2)
+ {
+ tree objtype = TREE_TYPE (obj);
+ bool obj_accessible_p = true;
+ /* Get the most derived object. */
+ while (TREE_CODE (obj) == COMPONENT_REF
+ && DECL_FIELD_IS_BASE (TREE_OPERAND (obj, 1)))
+ {
+ if (TREE_PRIVATE (TREE_OPERAND (obj, 1))
+ || TREE_PROTECTED (TREE_OPERAND (obj, 1)))
+ obj_accessible_p = false;
+ /* [class.cdtor] When a dynamic_cast is used in a constructor ...
+ or in a destructor ... if the operand of the dynamic_cast refers
+ to the object under construction or destruction, this object is
+ considered to be a most derived object that has the type of the
+ constructor or destructor's class. */
+ if (ctx->global->ctor_object
+ && cp_tree_equal (obj, ctx->global->ctor_object))
+ break;
+ obj = TREE_OPERAND (obj, 0);
+ }
+
+ tree mdtype = TREE_TYPE (obj);
+ /* Check that OBJ refers to a public base class subobject of most derived
+ object. */
+ if (!obj_accessible_p)
+ {
+ if (reference_p)
+ {
+ if (!ctx->quiet)
+ {
+ error_at (loc, "reference %<dynamic_cast%> failed");
+ inform (loc, "static type %qT of its operand is a non-public"
+ " base class of dynamic type %qT", objtype, mdtype);
+ }
+ *non_constant_p = true;
+ }
+ return integer_zero_node;
+ }
+
+ /* Check that the type of the most derived object has a base class
+ of type TYPE that is unambiguous and public. */
+ base_kind b_kind;
+ tree binfo = lookup_base (mdtype, type, ba_check, &b_kind, tf_none);
+ if (!binfo || binfo == error_mark_node)
+ {
+ if (reference_p)
+ {
+ if (!ctx->quiet)
+ {
+ error_at (loc, "reference %<dynamic_cast%> failed");
+ if (b_kind == bk_ambig)
+ inform (loc, "%qT is an ambiguous base class of dynamic "
+ "type %qT of its operand", type, mdtype);
+ else
+ inform (loc, "dynamic type %qT of its operand does not "
+ "have an unambiguous public base class %qT",
+ mdtype, type);
+ }
+ *non_constant_p = true;
+ }
+ return integer_zero_node;
+ }
+ /* If so, return the TYPE subobject of the most derived object. */
+ obj = convert_to_base_statically (obj, binfo);
+ return cp_build_addr_expr (obj, complain);
+ }
+ else
+ gcc_unreachable ();
+}
+
/* Subroutine of cxx_eval_constant_expression.
Evaluate the call expression tree T in the context of OLD_CALL expression
evaluation. */
@@ -1857,6 +2101,9 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree
t,
gcc_assert (arg1);
return arg1;
}
+ else if (cxx_dynamic_cast_fn_p (fun))
+ return cxx_eval_dynamic_cast_fn (ctx, t, non_constant_p, overflow_p);
+
if (!ctx->quiet)
{
if (!lambda_static_thunk_p (fun))
@@ -2094,6 +2341,11 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree
t,
else
ctx->global->values.put (res, NULL_TREE);
+ /* Stash the object we're constructing. */
+ tree save_ctor_object = ctx->global->ctor_object;
+ if (new_obj)
+ ctx->global->ctor_object = new_obj;
+
/* Track the callee's evaluated SAVE_EXPRs and TARGET_EXPRs so that
we can forget their values after the call. */
constexpr_ctx ctx_with_save_exprs = *ctx;
@@ -2127,6 +2379,9 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree
t,
}
}
+ if (new_obj)
+ ctx->global->ctor_object = save_ctor_object;
+
/* At this point, the object's constructor will have run, so
the object is no longer under construction, and its possible
'const' semantics now apply. Make a note of this fact by
@@ -6729,7 +6984,8 @@ potential_constant_expression_1 (tree t, bool want_rval,
bool strict, bool now,
&& (!cxx_placement_new_fn (fun)
|| TREE_CODE (t) != CALL_EXPR
|| current_function_decl == NULL_TREE
- || !is_std_construct_at (current_function_decl)))
+ || !is_std_construct_at (current_function_decl))
+ && !cxx_dynamic_cast_fn_p (fun))
{
if (flags & tf_error)
{
diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
index b47698e1d0c..fc00ac1bcb6 100644
--- gcc/cp/cp-tree.h
+++ gcc/cp/cp-tree.h
@@ -6275,6 +6275,7 @@ extern void complain_about_bad_argument (location_t
arg_loc,
tree from_type, tree to_type,
tree fndecl, int parmnum);
extern void maybe_inform_about_fndecl_for_bogus_argument_init (tree, int);
+extern bool is_base_field_ref (tree);
/* A class for recording information about access failures (e.g. private
diff --git gcc/cp/rtti.c gcc/cp/rtti.c
index 1b6b87ba8d6..9a242dc64e4 100644
--- gcc/cp/rtti.c
+++ gcc/cp/rtti.c
@@ -777,6 +777,7 @@ build_dynamic_cast_1 (location_t loc, tree type, tree expr,
dynamic_cast_node = dcast_fn;
}
result = build_cxx_call (dcast_fn, 4, elems, complain);
+ SET_EXPR_LOCATION (result, loc);
if (tc == REFERENCE_TYPE)
{
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic1.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic1.C
new file mode 100644
index 00000000000..e8ba63d9609
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic1.C
@@ -0,0 +1,40 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Downcast.
+
+struct B {
+ virtual void baz () {}
+};
+
+struct D : B { };
+
+constexpr bool
+fn ()
+{
+ bool ok = true;
+ B b;
+ B *b1 = &b;
+ if (D *pd = dynamic_cast<D*>(b1))
+ ok = false;
+
+ D d;
+ B *b2 = &d;
+ if (D *pd = dynamic_cast<D*>(b2))
+ /*OK*/;
+ else
+ ok = false;
+
+ return ok;
+}
+
+static_assert(fn ());
+
+constexpr D d;
+constexpr B b;
+constexpr B *b1 = const_cast<B*>(&b);
+constexpr B *b2 = const_cast<D*>(&d);
+static_assert(dynamic_cast<D*>(b2) == &d);
+static_assert(&dynamic_cast<D&>(*b2) == &d);
+static_assert(dynamic_cast<const B*>(&d) == &d);
+static_assert(&dynamic_cast<const B&>(d) == &d);
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic10.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic10.C
new file mode 100644
index 00000000000..b05249d9795
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic10.C
@@ -0,0 +1,12 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Virtual base.
+
+struct C { virtual void a(); };
+struct B { virtual void b(); };
+struct A : virtual B, C { virtual void c(); }; // { dg-error ".struct A. has
virtual base classes" }
+
+constexpr A a; // { dg-error "call" }
+
+constexpr bool b1 = (dynamic_cast<C&>((B&)a), false); // { dg-error "reference
.dynamic_cast. failed" }
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic11.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic11.C
new file mode 100644
index 00000000000..391ca99e0b5
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic11.C
@@ -0,0 +1,29 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// dynamic_cast in a constructor.
+
+struct V {
+ virtual void f();
+};
+
+struct A : V { };
+
+struct B : V {
+ constexpr B(V*, A*);
+};
+
+struct D : A, B {
+ constexpr D() : B((A*)this, this) { }
+};
+
+constexpr B::B(V* v, A* a)
+{
+ // well-defined: v of type V*, V base of B results in B*
+ dynamic_cast<B*>(v);
+
+ // FIXME: UB in constexpr should be detected.
+ dynamic_cast<B*>(a); // undefined behavior, a has type A*, A not a
base of B
+}
+
+constexpr D d;
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic12.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic12.C
new file mode 100644
index 00000000000..f6d081a9de9
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic12.C
@@ -0,0 +1,23 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// dynamic_cast in a destructor.
+
+struct A2 { virtual void a2(); };
+
+struct A : A2 { virtual void a(); };
+
+struct C2 { virtual void c2(); };
+
+struct B : A, C2 {
+ constexpr ~B();
+};
+
+constexpr B::~B()
+{
+ A *a = dynamic_cast<A*>((C2*)this);
+}
+
+struct D : B { virtual void d(); };
+
+constexpr D d;
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic13.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic13.C
new file mode 100644
index 00000000000..203067a2581
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic13.C
@@ -0,0 +1,86 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Adopted from g++.old-deja/g++.other/dyncast1.C.
+
+// 1. downcast
+// 1.1. single inheritance case
+
+struct A { virtual void a(); };
+struct AA : A {};
+struct B : A {};
+struct BB : B {};
+class C : B {};
+struct D : C {};
+
+struct CC : B {};
+class DD : CC {};
+
+class CCC : protected B {};
+class DDD : protected CCC {};
+
+constexpr D d;
+static_assert (dynamic_cast<D*> ((A*)&d) == nullptr);
+static_assert (dynamic_cast<D*> ((B*)&d) == nullptr);
+static_assert (&d == dynamic_cast<D*> ((C*)&d));
+static_assert (dynamic_cast<C*> ((B*)&d) == nullptr);
+
+constexpr DD dd;
+static_assert (dynamic_cast<DD*> ((A*)&dd) == nullptr);
+static_assert (dynamic_cast<DD*> ((B*)&dd) == nullptr);
+
+constexpr DDD ddd;
+static_assert (dynamic_cast<DDD*> ((A*)&ddd) == nullptr);
+static_assert (dynamic_cast<DDD*> ((B*)&ddd) == nullptr);
+static_assert (dynamic_cast<CCC*> ((B*)&ddd) == nullptr);
+
+// 1.2. multiple inheritance case
+// 1.2.1. all bases are public
+
+struct E : D, CC {};
+struct EE : CC, D {}; //Will search in reverse order.
+
+constexpr E e;
+static_assert (dynamic_cast<E*> ((A*)(D*)&e) == nullptr);
+static_assert (dynamic_cast<E*> ((B*)(D*)&e) == nullptr);
+static_assert (&e == dynamic_cast<E*> ((C*)(D*)&e));
+static_assert (&e == dynamic_cast<E*> ((B*)(CC*)&e));
+static_assert ((CC*)&e == dynamic_cast<CC*> ((B*)(CC*)&e));
+
+constexpr EE ee;
+static_assert (dynamic_cast<EE*> ((A*)(D*)&ee) == nullptr);
+static_assert (dynamic_cast<EE*> ((B*)(D*)&ee) == nullptr);
+static_assert (&ee == dynamic_cast<EE*> ((C*)(D*)&ee));
+static_assert (&ee == dynamic_cast<EE*> ((B*)(CC*)&ee));
+static_assert ((CC*)&ee == dynamic_cast<CC*> ((B*)(CC*)&ee));
+
+// 1.2.2 one or more branches are nonpublic
+
+struct X : private BB, E {};
+struct Y : AA, private B {};
+
+class XX : BB, E {};
+
+constexpr X x;
+static_assert (&x == dynamic_cast<X*>((B*)(CC*)(E*)&x));
+
+constexpr XX xx;
+static_assert (dynamic_cast<XX*>((B*)(CC*)(E*)&xx) == nullptr);
+
+constexpr Y y;
+static_assert (dynamic_cast<Y*>((B*)&y) == nullptr);
+static_assert (dynamic_cast<Y*>((A*)(B*)&y) == nullptr);
+
+// 2. crosscast
+
+struct J { virtual void j(); };
+struct K : CC, private J {};
+class KK : J, CC{};
+
+static_assert (dynamic_cast<CC*> ((B*)(D*)&e) == nullptr);
+static_assert ((CC*)&e == dynamic_cast<CC*> ((C*)(D*)&e));
+
+constexpr K k;
+static_assert (dynamic_cast<J*> ((B*)&k) == nullptr);
+constexpr KK kk;
+static_assert (dynamic_cast<J*> ((CC*)&kk) == nullptr);
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic14.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic14.C
new file mode 100644
index 00000000000..77e59ee44b1
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic14.C
@@ -0,0 +1,105 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Adopted from g++.old-deja/g++.other/dyncast1.C.
+// But use reference dynamic_cast.
+
+// 1. downcast
+// 1.1. single inheritance case
+
+struct A { virtual void a(); };
+struct AA : A {};
+struct B : A {};
+struct BB : B {};
+class C : B {};
+struct D : C {};
+
+struct CC : B {};
+class DD : CC {};
+
+class CCC : protected B {};
+class DDD : protected CCC {};
+
+constexpr D d;
+constexpr bool b01 = (dynamic_cast<D&> ((A&)d), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const A. of its operand is a non-public base
class of dynamic type .const D." "" { target *-*-* } .-1 }
+constexpr bool b02 = (dynamic_cast<D&> ((B&)d), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const B. of its operand is a non-public base
class of dynamic type .const D." "" { target *-*-* } .-1 }
+static_assert (&d == &dynamic_cast<const D&> ((C&)d));
+constexpr bool b03 = (dynamic_cast<C&> ((B&)d), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const B. of its operand is a non-public base
class of dynamic type .const D." "" { target *-*-* } .-1 }
+
+constexpr DD dd;
+constexpr bool b04 = (dynamic_cast<DD&> ((A&)dd), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const A. of its operand is a non-public base
class of dynamic type .const DD." "" { target *-*-* } .-1 }
+constexpr bool b05 = (dynamic_cast<DD&> ((B&)dd), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const B. of its operand is a non-public base
class of dynamic type .const DD." "" { target *-*-* } .-1 }
+
+constexpr DDD ddd;
+constexpr bool b06 = (dynamic_cast<DDD&> ((A&)ddd), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const A. of its operand is a non-public base
class of dynamic type .const DDD." "" { target *-*-* } .-1 }
+constexpr bool b07 = (dynamic_cast<DDD&> ((B&)ddd), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const B. of its operand is a non-public base
class of dynamic type .const DDD." "" { target *-*-* } .-1 }
+constexpr bool b08 = (dynamic_cast<CCC&> ((B&)ddd), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const B. of its operand is a non-public base
class of dynamic type .const DDD." "" { target *-*-* } .-1 }
+
+// 1.2. multiple inheritance case
+// 1.2.1. all bases are public
+
+struct E : D, CC {};
+struct EE : CC, D {}; //Will search in reverse order.
+
+constexpr E e;
+constexpr bool b09 = (dynamic_cast<E&> ((A&)(D&)e), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .A. of its operand is a non-public base class of
dynamic type .E." "" { target *-*-* } .-1 }
+constexpr bool b10 = (dynamic_cast<E&> ((B&)(D&)e), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .B. of its operand is a non-public base class of
dynamic type .E." "" { target *-*-* } .-1 }
+static_assert (&e == &dynamic_cast<E&> ((C&)(D&)e));
+static_assert (&e == &dynamic_cast<E&> ((B&)(CC&)e));
+static_assert (&(CC&)e == &dynamic_cast<CC&> ((B&)(CC&)e));
+
+constexpr EE ee;
+constexpr bool b11 = (dynamic_cast<EE&> ((A&)(D&)ee), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .A. of its operand is a non-public base class of
dynamic type .EE." "" { target *-*-* } .-1 }
+constexpr bool b12 = (dynamic_cast<EE&> ((B&)(D&)ee), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .B. of its operand is a non-public base class of
dynamic type .EE." "" { target *-*-* } .-1 }
+static_assert (&ee == &dynamic_cast<EE&> ((C&)(D&)ee));
+static_assert (&ee == &dynamic_cast<EE&> ((B&)(CC&)ee));
+static_assert (&(CC&)ee == &dynamic_cast<CC&> ((B&)(CC&)ee));
+
+// 1.2.2 one or more branches are nonpublic
+
+struct X : private BB, E {};
+struct Y : AA, private B {};
+
+class XX : BB, E {};
+
+constexpr X x;
+static_assert (&x == &dynamic_cast<X&>((B&)(CC&)(E&)x));
+
+constexpr XX xx;
+constexpr bool b13 = (dynamic_cast<XX&>((B&)(CC&)(E&)xx), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .B. of its operand is a non-public base class of
dynamic type .const XX." "" { target *-*-* } .-1 }
+
+constexpr Y y;
+constexpr bool b14 = (dynamic_cast<Y&>((B&)y), true); // { dg-error "reference
.dynamic_cast. failed" }
+// { dg-message "static type .const B. of its operand is a non-public base
class of dynamic type .const Y." "" { target *-*-* } .-1 }
+constexpr bool b15 = (dynamic_cast<Y&>((A&)(B&)y), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .A. of its operand is a non-public base class of
dynamic type .Y." "" { target *-*-* } .-1 }
+
+// 2. crosscast
+
+struct J { virtual void j(); };
+struct K : CC, private J {};
+class KK : J, CC{};
+
+constexpr bool b16 = (dynamic_cast<CC&> ((B&)(D&)e), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .B. of its operand is a non-public base class of
dynamic type .CC." "" { target *-*-* } .-1 }
+static_assert (&(CC&)e == &dynamic_cast<CC&> ((C&)(D&)e));
+
+constexpr K k;
+constexpr bool b17 = (dynamic_cast<J&> ((B&)k), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "dynamic type .const K. of its operand does not have an
unambiguous public base class .J." "" { target *-*-* } .-1 }
+constexpr KK kk;
+constexpr bool b18 = (dynamic_cast<J&> ((CC&)kk), true); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const CC. of its operand is a non-public base
class of dynamic type .const KK." "" { target *-*-* } .-1 }
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic2.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic2.C
new file mode 100644
index 00000000000..aae03f691ca
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic2.C
@@ -0,0 +1,41 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Downcast, with hint > 0.
+
+struct B {
+ virtual void baz () {}
+};
+
+struct B2 {
+ virtual void baz2 () {}
+};
+
+struct D : B, B2 { };
+
+constexpr bool
+fn ()
+{
+ // try &/&&, add address test
+ bool ok = true;
+ B2 b;
+ B2 *b1 = &b;
+ if (D *pd = dynamic_cast<D*>(b1))
+ ok = false;
+
+ D d;
+ B2 *b2 = &d;
+ if (D *pd = dynamic_cast<D*>(b2))
+ /*OK*/;
+ else
+ ok = false;
+
+ return ok;
+}
+
+static_assert(fn ());
+
+constexpr D d;
+constexpr B2 *b = const_cast<D*>(&d);
+static_assert(dynamic_cast<D*>(b) == &d);
+static_assert(&dynamic_cast<D&>(*b) == &d);
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic3.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic3.C
new file mode 100644
index 00000000000..c3e09808e32
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic3.C
@@ -0,0 +1,33 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Sidecast.
+
+struct A {
+ virtual void afn () {}
+};
+
+struct B {
+ virtual void bfn () {}
+};
+
+struct D : A, B { };
+
+constexpr bool
+fn ()
+{
+ bool ok = true;
+ D d;
+ A *a = &d;
+ if (B *bp = dynamic_cast<B*>(a))
+ /*OK*/;
+ else
+ ok = false;
+
+ A &ar = d;
+ B &br = dynamic_cast<B&>(ar);
+
+ return ok;
+}
+
+static_assert(fn ());
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic4.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic4.C
new file mode 100644
index 00000000000..a9eca8b6d96
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic4.C
@@ -0,0 +1,55 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// From clang's constant-expression-cxx2a.cpp.
+
+struct A2 { virtual void a2(); };
+struct A : A2 { virtual void a(); };
+struct B : A {};
+struct C2 { virtual void c2(); };
+struct C : A, C2 { A *c = dynamic_cast<A*>(static_cast<C2*>(this)); };
+struct D { virtual void d(); };
+struct E { virtual void e(); };
+struct F : B, C, D, private E { void *f =
dynamic_cast<void*>(static_cast<D*>(this)); };
+struct Padding { virtual void padding(); };
+struct G : Padding, F {};
+
+constexpr G g;
+
+// During construction of C, A is unambiguous subobject of dynamic type C.
+static_assert(g.c == (C*)&g);
+// ... but in the complete object, the same is not true, so the runtime fails.
+static_assert(dynamic_cast<const A*>(static_cast<const C2*>(&g)) == nullptr);
+
+// dynamic_cast<void*> produces a pointer to the object of the dynamic type.
+static_assert(g.f == (void*)(F*)&g);
+static_assert(dynamic_cast<const void*>(static_cast<const D*>(&g)) == &g);
+
+constexpr int d_a = (dynamic_cast<const A&>(static_cast<const D&>(g)), 0); //
{ dg-error "reference .dynamic_cast. failed" }
+// { dg-message ".A. is an ambiguous base class of dynamic type .const G." ""
{ target *-*-* } .-1 }
+
+// Can navigate from A2 to its A...
+static_assert(&dynamic_cast<A&>((A2&)(B&)g) == &(A&)(B&)g);
+// ... and from B to its A ...
+static_assert(&dynamic_cast<A&>((B&)g) == &(A&)(B&)g);
+// ... but not from D.
+static_assert(&dynamic_cast<A&>((D&)g) == &(A&)(B&)g); // { dg-error
"non-constant condition for static assertion|reference .dynamic_cast. failed" }
+// { dg-message ".A. is an ambiguous base class of dynamic type .const G." ""
{ target *-*-* } .-1 }
+
+// Can cast from A2 to sibling class D.
+static_assert(&dynamic_cast<D&>((A2&)(B&)g) == &(D&)g);
+
+// Cannot cast from private base E to derived class F.
+constexpr int e_f = (dynamic_cast<F&>((E&)g), 0); // { dg-error "reference
.dynamic_cast. failed" }
+// { dg-message "static type .const E. of its operand is a non-public base
class of dynamic type .const G." "" { target *-*-* } .-1 }
+
+// Cannot cast from B to private sibling E.
+constexpr int b_e = (dynamic_cast<E&>((B&)g), 0); // { dg-error "reference
.dynamic_cast. failed" }
+// { dg-message "dynamic type .const G. of its operand does not have an
unambiguous public base class .E." "" { target *-*-* } .-1 }
+
+struct Unrelated { virtual void unrelated(); };
+
+constexpr int b_unrelated = (dynamic_cast<Unrelated&>((B&)g), 0); // {
dg-error "reference .dynamic_cast. failed" }
+// { dg-message "dynamic type .const G. of its operand does not have an
unambiguous public base class .Unrelated." "" { target *-*-* } .-1 }
+constexpr int e_unrelated = (dynamic_cast<Unrelated&>((E&)g), 0); // {
dg-error "reference .dynamic_cast. failed" }
+// { dg-message "static type .const E. of its operand is a non-public base
class of dynamic type .const G." "" { target *-*-* } .-1 }
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic5.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic5.C
new file mode 100644
index 00000000000..743b3018d2f
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic5.C
@@ -0,0 +1,22 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Multiple levels.
+
+struct A { virtual void a(); };
+struct B : A { virtual void b(); };
+struct C : B { virtual void c(); };
+struct D : C { virtual void d(); };
+struct E : D { virtual void e(); };
+struct F : E { virtual void f(); };
+
+constexpr F f;
+
+// F->C->A->B == F->C->B
+static_assert (&dynamic_cast<B&>((A&)(C&)f) == &(B&)(C&)f);
+// F->A->E == F->E
+static_assert (&dynamic_cast<E&>((A&)f) == &(E&)f);
+// F->E->D->C->B->A->C == F->C
+static_assert (&dynamic_cast<C&>((A&)(B&)(C&)(D&)(E&)f) == &(C&)f);
+// F->B->F == F
+static_assert (&dynamic_cast<F&>((B&)f) == &f);
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic6.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic6.C
new file mode 100644
index 00000000000..c010d811885
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic6.C
@@ -0,0 +1,25 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Private base.
+
+struct P1 { virtual void p1(); };
+struct P2 { virtual void p2(); };
+struct B : private P1 { virtual void b(); };
+struct C { virtual void c(); };
+struct A : B, C, private P2 { virtual void a(); };
+
+constexpr A a;
+
+// P1 is a non-public base of A.
+constexpr bool b1 = (dynamic_cast<B&>((P1&)a), false); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const P1. of its operand is a non-public base
class of dynamic type .const A." "" { target *-*-* } .-1 }
+
+// Don't error here.
+static_assert (dynamic_cast<B*>((P1*)&a) == nullptr);
+
+constexpr bool b2 = (dynamic_cast<C&>((P2&)a), false); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const P2. of its operand is a non-public base
class of dynamic type .const A." "" { target *-*-* } .-1 }
+
+static_assert (dynamic_cast<C*>((P1*)&a) == nullptr);
+static_assert (dynamic_cast<C*>((P2*)&a) == nullptr);
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic7.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic7.C
new file mode 100644
index 00000000000..8d3e7d39208
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic7.C
@@ -0,0 +1,25 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Protected base.
+
+struct P1 { virtual void p1(); };
+struct P2 { virtual void p2(); };
+struct B : protected P1 { virtual void b(); };
+struct C { virtual void c(); };
+struct A : B, C, protected P2 { virtual void a(); };
+
+constexpr A a;
+
+// P1 is a non-public base of A.
+constexpr bool b1 = (dynamic_cast<B&>((P1&)a), false); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const P1. of its operand is a non-public base
class of dynamic type .const A." "" { target *-*-* } .-1 }
+
+// Don't error here.
+static_assert (dynamic_cast<B*>((P1*)&a) == nullptr);
+
+constexpr bool b2 = (dynamic_cast<C&>((P2&)a), false); // { dg-error
"reference .dynamic_cast. failed" }
+// { dg-message "static type .const P2. of its operand is a non-public base
class of dynamic type .const A." "" { target *-*-* } .-1 }
+
+static_assert (dynamic_cast<C*>((P1*)&a) == nullptr);
+static_assert (dynamic_cast<C*>((P2*)&a) == nullptr);
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic8.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic8.C
new file mode 100644
index 00000000000..e2320a0cebb
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic8.C
@@ -0,0 +1,24 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Unrelated type.
+
+struct B { virtual void b(); };
+struct P1 { virtual void p1(); };
+struct P2 { virtual void p2(); };
+struct A : public B, private P1, protected P2 { virtual void a(); };
+
+constexpr A a;
+
+struct U { virtual void u(); };
+
+constexpr bool b1 = (dynamic_cast<U&>((B&)a), 0); // { dg-error "reference
.dynamic_cast. failed" }
+// { dg-message "dynamic type .const A. of its operand does not have an
unambiguous public base class .U." "" { target *-*-* } .-1 }
+constexpr bool b2 = (dynamic_cast<U&>((P1&)a), 0); // { dg-error "reference
.dynamic_cast. failed" }
+// { dg-message "static type .const P1. of its operand is a non-public base
class of dynamic type .const A." "" { target *-*-* } .-1 }
+constexpr bool b3 = (dynamic_cast<U&>((P2&)a), 0); // { dg-error "reference
.dynamic_cast. failed" }
+// { dg-message "static type .const P2. of its operand is a non-public base
class of dynamic type .const A." "" { target *-*-* } .-1 }
+
+static_assert (dynamic_cast<U*>((B*)&a) == nullptr);
+static_assert (dynamic_cast<U*>((P1*)&a) == nullptr);
+static_assert (dynamic_cast<U*>((P2*)&a) == nullptr);
diff --git gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic9.C
gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic9.C
new file mode 100644
index 00000000000..dc836747a2f
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/constexpr-dynamic9.C
@@ -0,0 +1,17 @@
+// PR c++/88337 - Implement P1327R1: Allow dynamic_cast/typeid in constexpr.
+// { dg-do compile { target c++2a } }
+
+// Ambiguous base.
+
+struct A { virtual void a(); };
+struct B : A { virtual void b(); };
+struct C : A { virtual void c(); };
+struct D { virtual void a(); };
+struct E : B, C, D { virtual void d(); };
+
+constexpr E e;
+
+constexpr bool b1 = (dynamic_cast<A&>((D&)e), false); // { dg-error "reference
.dynamic_cast. failed" }
+// { dg-message ".A. is an ambiguous base class of dynamic type .const E. of
its operand" "" { target *-*-* } .-1 }
+
+static_assert (dynamic_cast<A*>((D*)&e) == nullptr);
