Spurious "redeclaration of deduction guide" for guides that differ only in a type-constraint

[stevensudit-corvid]

Issue Summary

The front end reports two deduction guides as a redeclaration when they have the same parameters and differ only in a type-constraint on a template parameter, as in template<A T>.

gcc and clang accept the code. I ran into it with a wrapper class that has one guide per concept.

Reproducing Source Code

template<class T> concept A = true;
template<class T> struct w {};
template<class T> w(T) -> w<T>;
template<A T> w(T) -> w<T>;

Command-line Options

cpfe --c++23 repro.cpp

Type of Issue

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

Additional Details

The diagnostic, at 158320e (version 7.0):

"repro.cpp", line 4: error: redeclaration of deduction guide "(T)->w<T>"
          (previous declaration at line 3)
  template<A T> w(T) -> w<T>;
                ^

--c++20 and --c++26 give the same error, and so do the GNU, Clang and Microsoft modes.

The same pair compiles when the constraint is written as a requires-clause after the template head:

template<class T> w(T) -> w<T>;
template<class T> requires A<T> w(T) -> w<T>;

It also compiles with w(A auto) -> w<int>; as the second guide. Two guides constrained by different concepts, template<A T> and template<B T>, fail like the example above. Ordinary function templates that differ only in a type-constraint compile.

The check is deduction_guides_are_redeclarations (symbol_tbl.c:7306). It compares the function type, the trailing requires-clause and the template-head requires-clause. It does not compare type-constraints on the template parameters, which is the one place these two guides differ.

The diagnostic was added by EDGcpfe/25301 (Changes, 6/10/26, "Missing diagnostic for redeclaration of deduction guide"), so this is new in 6.9.

Reproducing (Standard) Configuration(s)

linux-gcc-release

Reproducing (Non-standard) Configuration

No response

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