[Bug c++/107390] New: template-nested lambda type uniqueness

["development at jordi dot vilar.cat" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107390

            Bug ID: 107390
           Summary: template-nested lambda type uniqueness
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: development at jordi dot vilar.cat
  Target Milestone: ---

type obtained from a lambda (with c++20 decltype()) nested into a template
(either class o function) is not resolved as a unique type, making this simple
code to fail (compiled with g++-12 -std=c++20):

#include <type_traits>

template<typename> struct test1
{
    using type0 = decltype([]{});
    using type1 = std::type_identity_t<type0>;

    static_assert(std::is_same_v<type0, type1>);
};

template<typename> void test2()
{
    using type0 = decltype([]{});
    using type1 = std::type_identity_t<type0>;

    static_assert(std::is_same_v<type0, type1>);
}

int main(int, char**)
{
    test1<void>{};
    test2<void>();
}

both static_assert fail identically:

 error: static assertion failed
    8 |     static_assert(std::is_same_v<type0, type1>);
      |                   ~~~~~^~~~~~~~~~~~~~~~~~~~~~~
<source>:8:24: note: 'std::is_same_v<test1<void>::<lambda()>,
test1<void>::<lambda()> >' evaluates to false
<source>:16:24: error: static assertion failed
   16 |     static_assert(std::is_same_v<type0, type1>);
      |                   ~~~~~^~~~~~~~~~~~~~~~~~~~~~~
<source>:16:24: note: 'std::is_same_v<test2<void>()::<lambda()>,
test2<void>()::<lambda()> >' evaluates to false

This compiles correctly if lambdas are not nested inside a template.