[Bug c++/114947] New: [modules] ICE when processing class-scope constrained partial specialisations

["nshead at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114947

            Bug ID: 114947
           Summary: [modules] ICE when processing class-scope constrained
                    partial specialisations
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nshead at gcc dot gnu.org
  Target Milestone: ---

The following code ICEs on trunk:


  // part.cpp
  module M:part;

  struct A {
    template <typename> struct R;
    template <typename T> requires false struct R<T>;
    template <typename T> requires true struct R<T>;
  };

  template <typename T> int foo(T, int);
  template <typename T> int foo(T, A);

  void go() { foo(0, 0); }


  // m.cpp
  export module M;
  export import :part;


This is ultimately because 'set_defining_module' is not called on the partial
specialisation declarations of R.  This is a regression since r15-101, found
while reducing the following testcase that also ICEs even before this change:


  // a.cpp
  module;
  #include <format>
  export module M:a;

  // b.cpp
  module;
  #include <format>
  export module M:b;

  // m.cpp
  export module M;
  export import :a;
  export import :b;