On 3/6/23 17:01, Marek Polacek wrote:
On Mon, Mar 06, 2023 at 11:12:56AM -0500, Jason Merrill wrote:
On 3/3/23 12:51, Marek Polacek wrote:
Similarly to PR107938, this also started with r11-557, whereby cp_finish_decl
can call check_initializer even in a template for a constexpr initializer.
Here we are rejecting
extern const Q q;
template<int>
constexpr auto p = q(0);
even though q has a constexpr operator(). It's deemed non-const by
decl_maybe_constant_var_p because even though 'q' is const it is not
of integral/enum type. I think the fix is for p_c_e to treat q(0) as
potentially-constant, as below.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/12?
PR c++/107939
gcc/cp/ChangeLog:
* constexpr.cc (is_constexpr_function_object): New.
(potential_constant_expression_1): Treat an object with constexpr
operator() as potentially-constant.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/var-templ74.C: Remove dg-error.
* g++.dg/cpp1y/var-templ77.C: New test.
---
gcc/cp/constexpr.cc | 23 ++++++++++++++++++++++-
gcc/testsuite/g++.dg/cpp1y/var-templ74.C | 2 +-
gcc/testsuite/g++.dg/cpp1y/var-templ77.C | 14 ++++++++++++++
3 files changed, 37 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ77.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index acf9847a4d1..7d786f332b4 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8929,6 +8929,24 @@ check_for_return_continue (tree *tp, int *walk_subtrees,
void *data)
return NULL_TREE;
}
+/* Return true iff TYPE is a class with constexpr operator(). */
+
+static bool
+is_constexpr_function_object (tree type)
+{
+ if (!CLASS_TYPE_P (type))
+ return false;
+
+ for (tree f = TYPE_FIELDS (type); f; f = DECL_CHAIN (f))
+ if (TREE_CODE (f) == FUNCTION_DECL
+ && DECL_OVERLOADED_OPERATOR_P (f)
+ && DECL_OVERLOADED_OPERATOR_IS (f, CALL_EXPR)
+ && DECL_DECLARED_CONSTEXPR_P (f))
+ return true;
+
+ return false;
+}
+
/* Return true if T denotes a potentially constant expression. Issue
diagnostic as appropriate under control of FLAGS. If WANT_RVAL is true,
an lvalue-rvalue conversion is implied. If NOW is true, we want to
@@ -9160,7 +9178,10 @@ potential_constant_expression_1 (tree t, bool want_rval,
bool strict, bool now,
}
else if (fun)
{
- if (RECUR (fun, rval))
+ if (VAR_P (fun)
+ && is_constexpr_function_object (TREE_TYPE (fun)))
+ /* Could be an object with constexpr operator(). */;
I guess if fun is not a function pointer, we don't know if we're using it as
an lvalue or rvalue
Presumably the operator function could return this, making it an lvalue?
I'm not sure I'm really clear on this.
I mean just calling the operator uses the variable as an lvalue, by
passing its address as 'this'.
, so we want to pass 'any' for want_rval, which should
make this work;
Yes, want_rval==false means that p_c_e/VAR_DECL will not issue the
hard error.
I don't think we need to be specific about constexpr op(),
as a constexpr conversion operator to fn* could also do the trick.
Ah, those surrogate classes. I couldn't reproduce the problem with
them, though I'm adding a test for it anyway.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
OK, thanks.
-- >8 --
Similarly to PR107938, this also started with r11-557, whereby cp_finish_decl
can call check_initializer even in a template for a constexpr initializer.
Here we are rejecting
extern const Q q;
template<int>
constexpr auto p = q(0);
even though q has a constexpr operator(). It's deemed non-const by
decl_maybe_constant_var_p because even though 'q' is const it is not
of integral/enum type.
If fun is not a function pointer, we don't know if we're using it as an
lvalue or rvalue, so with this patch we pass 'any' for want_rval. With
that, p_c_e/VAR_DECL doesn't flat out reject the underlying VAR_DECL.
PR c++/107939
gcc/cp/ChangeLog:
* constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>: Pass
'any' when recursing on a VAR_DECL and not a pointer to function.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/var-templ74.C: Remove dg-error.
* g++.dg/cpp1y/var-templ77.C: New test.
---
gcc/cp/constexpr.cc | 8 ++++--
gcc/testsuite/g++.dg/cpp1y/var-templ74.C | 2 +-
gcc/testsuite/g++.dg/cpp1y/var-templ77.C | 32 ++++++++++++++++++++++++
3 files changed, 39 insertions(+), 3 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ77.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index 364695b762c..3079561f2e8 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9179,8 +9179,12 @@ potential_constant_expression_1 (tree t, bool want_rval,
bool strict, bool now,
}
else if (fun)
{
- if (RECUR (fun, rval))
- /* Might end up being a constant function pointer. */;
+ if (RECUR (fun, FUNCTION_POINTER_TYPE_P (fun) ? rval : any))
+ /* Might end up being a constant function pointer. But it
+ could also be a function object with constexpr op(), so
+ we pass 'any' so that the underlying VAR_DECL is deemed
+ as potentially-constant even though it wasn't declared
+ constexpr. */;
else
return false;
}
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
index 4e2e800a6eb..c76a7d949ac 100644
--- a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
@@ -9,7 +9,7 @@ struct Q {
extern const Q q;
template<int>
-constexpr const Q* p = q(0); // { dg-bogus "not usable" "PR107939" { xfail
*-*-* } }
+constexpr const Q* p = q(0);
void
g ()
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
new file mode 100644
index 00000000000..0c56d70a034
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
@@ -0,0 +1,32 @@
+// PR c++/107939
+// { dg-do compile { target c++14 } }
+
+struct Q {
+ struct P {
+ const Q* p;
+ };
+ int n;
+ constexpr P operator()(int) const { return {this}; }
+};
+
+extern const Q q;
+template<int>
+constexpr auto p = q(0);
+static_assert(p<0>.p == &q, "");
+
+constexpr int
+fn (int)
+{
+ return 42;
+}
+
+struct Sur {
+ using FN = int(int);
+ constexpr operator FN*() const { return &fn; }
+};
+
+extern const Sur sur;
+template<int>
+constexpr int aja = sur (0);
+static_assert(aja<0> == 42, "");
+static_assert(sur(1) == 42, "");
base-commit: 553ff2524f412be4e02e2ffb1a0a3dc3e2280742
