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c++: Handle ARRAY_TYPE in check_bit_cast_type [PR114706]

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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.

2024-04-16  Jakub Jelinek  <jakub@redhat.com>

	PR c++/114706
	* constexpr.cc (check_bit_cast_type): Handle ARRAY_TYPE.

	* g++.dg/cpp2a/bit-cast17.C: New test.
This commit is contained in:
Jakub Jelinek 2024-04-16 09:39:19 +02:00
parent 6e925ba0a8
commit 79ff53453e
2 changed files with 33 additions and 0 deletions
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2
gcc/cp/constexpr.cc
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@ -4843,6 +4843,8 @@ check_bit_cast_type (const constexpr_ctx *ctx, location_t loc, tree type,
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;
}

31
gcc/testsuite/g++.dg/cpp2a/bit-cast17.C Normal file
View file

@ -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<" }