Aliases and uniqueness of lambda in default argument of a class template parameter

[cor3ntin]

Consider

template<typename T, typename F = decltype([](){} )>
struct Type {};

template<typename A>
using C = Type<A>;

static_assert(std::same_as<Type<int>,Type<int>>); // #1
static_assert(std::same_as<C<int>,C<int>>); // #2

https://godbolt.org/z/6ofdP1bPz

All compilers reject #1. Only clang rejects #2.

This seems vaguely related, but distinct from CWG2794

Given https://eel.is/c++draft/temp.alias#2

When a template-id refers to the specialization of an alias template, it is equivalent to the associated type obtained by substitution of its template-arguments for the template-parameters in the defining-type-id of the alias template.

And https://eel.is/c++draft/temp#arg.general-9.sentence-1

When a simple-template-id does not name a function, a default template-argument is implicitly instantiated when the value of that default argument is needed.

It's clear as mud (to me) what the expected behavior should be.
CWG discussion https://lists.isocpp.org/core/2025/01/16895.php

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: cor3ntin (cor3ntin)

Consider

template<typename T, typename F = decltype([](){} )>
struct Type {};

template<typename A>
using C = Type<A>;

static_assert(std::same_as&lt;Type&lt;int&gt;,Type&lt;int&gt;&gt;); // #<!-- -->1
static_assert(std::same_as&lt;C&lt;int&gt;,C&lt;int&gt;&gt;); // #<!-- -->2

https://godbolt.org/z/6ofdP1bPz

All compilers reject #1. Only clang reject #2.
This seems vaguely related, but distinct from
https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2794

Given https://eel.is/c++draft/temp.alias#2
> When a template-id refers to the specialization of an alias template, it is equivalent to the associated type obtained by substitution of its template-arguments for the template-parameters in the defining-type-id of the alias template.

And https://eel.is/c++draft/temp#arg.general-9.sentence-1
> When a simple-template-id does not name a function, a default template-argument is implicitly instantiated when the value of that default argument is needed.

It's clear as mud (to me) what the expected behavior should be. I'll contact CWG

[cor3ntin]

@t3nsor @zygoloid