bogus diagnostic `deduction_guide_bad_trailing_return_type`

[hokein]

The following code is passed in gcc and msvc, but clang gives a bogus diagnostic deduced type 'B<T>' (aka 'A<type-parameter-0-0>') of deduction guide is not written as a specialization of template 'A'.

The reason is that when clang compares the equality of two Template-kind TemplateName, it doesn't seem to consider the TypeAliasTemplateDecl case.

template<typename T> struct A {};
A(void) -> A<int>;

template<typename T> using B = A<T>; 

template<typename T> A(T) -> B<T>; 

[Bekenn]

The prior behavior was correct; this change should be reverted.

Relevant excerpt from the standard:

    deduction-guide:
        explicit-specifier_opt template-name ( parameter-declaration-clause ) requires-clause_opt -> simple-template-id ;

[temp.deduct.guide]/3:

The simple-template-id shall name a class template specialization.

This is saying that the template-name in the simple-template-id in the return type shall be the actual name of a class template rather than some equivalent alias template. To make it even more clear:

The template-name shall be the same identifier as the template-name of the simple-template-id.

This is saying that the name of the deduction guide must match the name of the template in the return type.

[a-tarasyuk]

@Bekenn thanks for highlighting that detail in the spec. @hokein @erichkeane I've created PR to revert these changes #118165.

[Bekenn]

Thanks for addressing this!

[hokein]

Thanks for pointing this out.

I should have checked the standard before filing the bug. Based on the standard's specifications, I think the previous behavior was correct, and we should revert the change.