[Bug c++/97099] New: [temp.deduct.guide]p3 not implemented properly

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97099

            Bug ID: 97099
           Summary: [temp.deduct.guide]p3 not implemented properly
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

We fail to give the "redeclared" errors in the following test:

template<typename> struct S { };

// [temp.deduct.guide]p3: Two deduction guide declarations in the same
// translation unit for the same class template shall not have equivalent
// parameter-declaration-clauses.
S() -> S<int>;
S() -> S<int>; // { dg-error "redeclared" }
S() -> S<float>; // { dg-error "ambiguating new declaration of" }

S(bool) -> S<int>;
explicit S(bool) -> S<int>; // { dg-error "redeclared" }


explicit S(char) -> S<int>;
S(char) -> S<int>; // { dg-error "redeclared" }

// OK: Use SFINAE.
template<typename T> S(T) -> S<typename T::foo>;
template<typename T> S(T) -> S<typename T::bar>;

// OK: Non-template wins.
S(int) -> S<int>;
template<typename T = int> S(int) -> S<char>;