Hi!
https://eel.is/c++draft/bit.cast#3 says that std::bit_cast isn't constexpr
if To, From and the types of all subobjects have certain properties which the
check_bit_cast_type checks (such as it isn't a pointer, reference, union,
member pointer, volatile). The function doesn't cp_walk_tree though, so
I've missed one important case, for ARRAY_TYPEs we need to recurse on the
element type. I think we don't need to handle VECTOR_TYPEs/COMPLEX_TYPEs,
because those will not have a pointer/reference/union/member pointer in
the element type and if the element type is volatile, I think the whole
derived type is volatile as well.
Bootstrapped/regtested on x86_64-linux and i686-linux, committed to trunk
as obvious.
2024-04-16 Jakub Jelinek <ja...@redhat.com>
PR c++/114706
* constexpr.cc (check_bit_cast_type): Handle ARRAY_TYPE.
* g++.dg/cpp2a/bit-cast17.C: New test.
--- gcc/cp/constexpr.cc.jj 2024-04-12 19:57:58.792749776 +0200
+++ gcc/cp/constexpr.cc 2024-04-15 17:24:00.181449078 +0200
@@ -4843,6 +4843,8 @@ check_bit_cast_type (const constexpr_ctx
if (TREE_CODE (field) == FIELD_DECL
&& check_bit_cast_type (ctx, loc, TREE_TYPE (field), orig_type))
return true;
+ if (TREE_CODE (type) == ARRAY_TYPE)
+ return check_bit_cast_type (ctx, loc, TREE_TYPE (type), orig_type);
return false;
}
--- gcc/testsuite/g++.dg/cpp2a/bit-cast17.C.jj 2024-04-15 17:13:20.192052326
+0200
+++ gcc/testsuite/g++.dg/cpp2a/bit-cast17.C 2024-04-15 17:38:04.758093926
+0200
@@ -0,0 +1,31 @@
+// PR c++/114706
+// { dg-do compile { target c++20 } }
+
+namespace std {
+template<typename T, typename F>
+constexpr T
+bit_cast (const F& f) noexcept
+{
+ return __builtin_bit_cast (T, f);
+}
+}
+// { dg-error "'__builtin_bit_cast' is not a constant expression because
'const A' contains a union type" "" { target *-*-* } .-3 }
+// { dg-error "'__builtin_bit_cast' is not a constant expression because 'A'
contains a union type" "" { target *-*-* } .-4 }
+// { dg-error "'__builtin_bit_cast' is not a constant expression because
'const C' contains a union type" "" { target *-*-* } .-5 }
+// { dg-error "'__builtin_bit_cast' is not a constant expression because 'C'
contains a union type" "" { target *-*-* } .-6 }
+// { dg-error "'__builtin_bit_cast' is not a constant expression because
'const E' contains a pointer type" "" { target *-*-* } .-7 }
+// { dg-error "'__builtin_bit_cast' is not a constant expression because 'E'
contains a pointer type" "" { target *-*-* } .-8 }
+
+union U { int a; int b; };
+struct A { U c[2]; };
+struct B { int d[2]; };
+struct C { U e[2][2]; };
+struct D { int f[2][2]; };
+struct E { int *g[3]; };
+struct F { char h[sizeof (int *) * 3]; };
+constexpr B i = std::bit_cast<B> (A{}); // { dg-message "in 'constexpr'
expansion of 'std::bit_cast<" }
+constexpr A j = std::bit_cast<A> (B{}); // { dg-message "in 'constexpr'
expansion of 'std::bit_cast<" }
+constexpr D k = std::bit_cast<D> (C{}); // { dg-message "in 'constexpr'
expansion of 'std::bit_cast<" }
+constexpr C l = std::bit_cast<C> (D{}); // { dg-message "in 'constexpr'
expansion of 'std::bit_cast<" }
+constexpr F m = std::bit_cast<F> (E{}); // { dg-message "in 'constexpr'
expansion of 'std::bit_cast<" }
+constexpr E n = std::bit_cast<E> (F{}); // { dg-message "in 'constexpr'
expansion of 'std::bit_cast<" }
Jakub
