[Bug c++/99644] New: Add fix-it hint for

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99644

            Bug ID: 99644
           Summary: Add fix-it hint for
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Given:

template<typename T> struct S { };
template<typename T> requires (sizeof(T) == 1) struct S { };

This fails to compile with -std=gnu++20 as expected:

con.C:2:55: error: redeclaration 'template<class T> struct S' with different
constraints
    2 | template<typename T> requires (sizeof(T) == 1) struct S { };
      |                                                       ^
con.C:1:29: note: original declaration appeared here
    1 | template<typename T> struct S { };
      |                             ^



The user probably meant to declare the constrained template as a partial
specialization, not a redeclaration of the primary template (I do this all the
time). So a fix-it hint suggesting to add a template argument list would be
helpful:

note: did you mean to declare a partial specialization?
    2 | template<typename T> requires (sizeof(T) == 1) struct S { };
      |                                                        ^
      |                                                        <T>

This could be suggested whenever a redeclaration of a template has exactly the
same template-parameter-list as the original declaration but different
constraints. The fix-it hint should suggest adding a template argument list
containing each of the template parameters.