On Mon, Sep 16, 2024 at 8:12 AM Patrick Palka <ppa...@redhat.com> wrote:
>
> Bootstrapped and regtested on x86_64-pc-linuxgnu, does this look
> OK for trunk? Sadly the prerequisity patch r15-2331-g523836716137d0
> probably isn't suitable for backporting, so I reckon this should be
> trunk-only.
>
> -- >8 --
>
> Here we're prematurely stripping the decltype(lambda) alias used inside
> the template-id during ahead of time template argument coercion, which
> means we treat it as if it were
>
> is_same_v<decltype([]{}), decltype([]{})>
>
> which instead yields false since now we're substituting into the lambda
> twice, and every such substitution yields a unique lambda. This
> demonstrates that such aliases should be considered opaque, a notion which
> coincidentally we recently introduced in r15-2331-g523836716137d0.
I wonder if this fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106221 too.
While https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107390 looks it
would be fixed here too.
Thanks,
Andrew
>
> PR c++/116714
>
> 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.
> ---
> gcc/cp/pt.cc | 6 ++++--
> gcc/testsuite/g++.dg/cpp2a/lambda-uneval18.C | 20 ++++++++++++++++++++
> 2 files changed, 24 insertions(+), 2 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-uneval18.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 04987f66746..a72a1eadbc7 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -6764,8 +6764,10 @@ 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)))
> + || (TREE_CODE (t) == DECLTYPE_TYPE
> + && TREE_CODE (DECLTYPE_TYPE_EXPR (t)) == LAMBDA_EXPR)));
> }
>
> /* Return the number of innermost template parameters in TMPL. */
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-uneval18.C
> b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval18.C
> new file mode 100644
> index 00000000000..2942f8305c7
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval18.C
> @@ -0,0 +1,20 @@
> +// PR c++/116714
> +// { 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 void f<int>();
> --
> 2.46.1.506.ged155187b4
>
