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c++: alias of decltype(lambda) is opaque [PR116714, PR107390]

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Here for

  using type = decltype([]{});
  static_assert(is_same_v<type, type>);

we strip the alias ahead of time during template argument coercion
which effectively transforms the template-id into

  is_same_v<decltype([]{}), decltype([]{})>

which is wrong because later substitution into the template-id will
produce two new lambdas with distinct types and cause is_same_v to
return false.

This demonstrates that such aliases should be considered opaque (a
notion that we recently introduced in r15-2331-g523836716137d0).
(An alternative solution might be to consider memoizing lambda-expr
substitution rather than always producing a new lambda, but this is
much simpler.)

	PR c++/116714
	PR c++/107390

gcc/cp/ChangeLog:

	* pt.cc (dependent_opaque_alias_p): Also return true for a
	decltype(lambda) alias.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-uneval18.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
This commit is contained in:
Patrick Palka 2024-09-18 13:50:43 -04:00
parent fe1ed68000
commit 82c2acd0bc
2 changed files with 48 additions and 2 deletions
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11
gcc/cp/pt.cc
View file

@ -6759,8 +6759,15 @@ dependent_opaque_alias_p (const_tree t)
{
return (TYPE_P (t)
&& typedef_variant_p (t)
&& any_dependent_type_attributes_p (DECL_ATTRIBUTES
(TYPE_NAME (t))));
&& (any_dependent_type_attributes_p (DECL_ATTRIBUTES
(TYPE_NAME (t)))
/* Treat a dependent decltype(lambda) alias as opaque so that we
don't prematurely strip it when used as a template argument.
Otherwise substitution into each occurrence of the (stripped)
alias would incorrectly yield a distinct lambda type. */
|| (TREE_CODE (t) == DECLTYPE_TYPE
&& TREE_CODE (DECLTYPE_TYPE_EXPR (t)) == LAMBDA_EXPR
&& !typedef_variant_p (DECL_ORIGINAL_TYPE (TYPE_NAME (t))))));
}
/* Return the number of innermost template parameters in TMPL. */

39
gcc/testsuite/g++.dg/cpp2a/lambda-uneval18.C Normal file
View file

@ -0,0 +1,39 @@
// PR c++/116714
// PR c++/107390
// { dg-do compile { target c++20 } }
template<class T, class U>
inline constexpr bool is_same_v = __is_same(T, U);
template<class T, class U>
struct is_same { static constexpr bool value = false; };
template<class T>
struct is_same<T, T> { static constexpr bool value = true; };
template<class>
void f() {
using type = decltype([]{});
static_assert(is_same_v<type, type>);
static_assert(is_same<type, type>::value);
};
template<class>
void g() {
using ty1 = decltype([]{});
using ty2 = ty1;
static_assert(is_same_v<ty1, ty2>);
static_assert(is_same<ty1, ty2>::value);
};
template<class>
void h() {
using ty1 = decltype([]{});
using ty2 = decltype([]{});
static_assert(!is_same_v<ty1, ty2>);
static_assert(!is_same<ty1, ty2>::value);
};
template void f<int>();
template void g<int>();
template void h<int>();