Out-of-line member function definition with a constrained placeholder return type is rejected

[gwllx]

Issue Summary

An out-of-line member function definition of a template class with a constrained placeholder return type that includes template arguments is rejected with the error: "template argument list must match the parameter list".

Reproducing Source Code

template <class T, class U> concept C = true;
template <class T> struct S {
  constexpr C<int> auto f() noexcept;
};
template <class T>
constexpr C<int> auto S<T>::f() noexcept { return 0; }

https://godbolt.org/z/YnTEzvY77

Note: If the type-constraint C has only 1 template arg such that the spelling is C auto, or if S itself is not a template class, this is accepted.

Command-line Options

cpfe --c++20

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

Suggested patch: concept_template_in_return_type.patch

This patch recognises the template args following the concept-id within decl_specifier and bypasses them to ensure the declarator prescan starts following them.

Reproducing (Standard) Configuration(s)

linux-gcc-debug

Reproducing (Non-standard) Configuration

No response

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