P2615 (Meaningful exports) is not fully implemented: Diagnostic required

[ckwastra]

Test code (CE):

export module M;
export template <class> struct S {};

// ❌ - Clang/MSVC/EDG: OK
// ✅ - GCC:
//      - error: declaration of partial specialization in unbraced
//        export-declaration
//      - note: a specialization is always exported alongside its
//        primary template
export template <class T> struct S<T *> {};

// ❌ - Clang/MSVC/EDG: OK
// ✅ - GCC: error: explicit instantiations are not permitted here
export template struct S<int>;

// ❌ - Clang/MSVC/EDG: OK
// ✅ - GCC:
//      - error: explicit specializations are not permitted here
//      - note: a specialization is always exported alongside its
//              primary template
export template <> struct S<void> {};

P2615R1 forbids applying export directly to partial specializations and explicit instantiations/specializations, as doing so is meaningless. (These declarations are allowed to appear within an export block for convenience.) The code above should be diagnosed accordingly.

Related: #147135.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Vincent X (ckwastra)

Test code ([CE](https://godbolt.org/z/Yov6xcKq7)):

export module M;
export template <class> struct S {};

// ❌ - Clang/MSVC/EDG: OK
// ✅ - GCC:
//      - error: declaration of partial specialization in unbraced
//        export-declaration
//      - note: a specialization is always exported alongside its
//        primary template
export template <class T> struct S<T *> {};

// ❌ - Clang/MSVC/EDG: OK
// ✅ - GCC: error: explicit instantiations are not permitted here
export template struct S<int>;

// ❌ - Clang/MSVC/EDG: OK
// ✅ - GCC:
//      - error: explicit specializations are not permitted here
//      - note: a specialization is always exported alongside its
//              primary template
export template <> struct S<void> {};

P2615R1 forbids applying export directly to partial specializations and explicit instantiations/specializations, as doing so is meaningless. (These declarations are allowed to appear within an export block for convenience.) The code above should be diagnosed accordingly.

Related: #147135.

[shafik]

CC @ChuanqiXu9 @mpark