[all-commits] [llvm/llvm-project] 3a4863: [Clang][Sema] Diagnose friend declarations with en...

[Krystian Stasiowski via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 3a48630a4b25d50abefd945742c247f17bd61156
      https://github.com/llvm/llvm-project/commit/3a48630a4b25d50abefd945742c247f17bd61156
  Author: Krystian Stasiowski <sdkrystian at gmail.com>
  Date:   2024-02-13 (Tue, 13 Feb 2024)

  Changed paths:
    M clang/docs/ReleaseNotes.rst
    M clang/include/clang/Basic/DiagnosticSemaKinds.td
    M clang/include/clang/Sema/DeclSpec.h
    M clang/include/clang/Sema/Sema.h
    M clang/lib/Parse/ParseTentative.cpp
    M clang/lib/Sema/DeclSpec.cpp
    M clang/lib/Sema/SemaDecl.cpp
    M clang/lib/Sema/SemaDeclCXX.cpp
    M clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
    M clang/test/CXX/dcl.dcl/dcl.spec/dcl.type/dcl.type.elab/p3.cpp
    A clang/test/CXX/dcl.dcl/dcl.spec/dcl.type/dcl.type.elab/p4.cpp
    M clang/test/CXX/drs/dr16xx.cpp
    M clang/test/CXX/drs/dr23xx.cpp
    M clang/test/CXX/temp/temp.decls/temp.class/temp.mem.enum/p1.cpp
    M clang/test/FixIt/fixit-c++11.cpp
    M clang/test/Parser/cxx-decl.cpp
    M clang/test/Parser/cxx0x-decl.cpp
    M clang/test/SemaCXX/cxx98-compat.cpp
    M clang/test/SemaCXX/enum-scoped.cpp

  Log Message:
  -----------
  [Clang][Sema] Diagnose friend declarations with enum elaborated-type-specifier in all language modes (#80171)

According to [dcl.type.elab] p4:
> If an _elaborated-type-specifier_ appears with the `friend` specifier
as an entire _member-declaration_, the _member-declaration_ shall have
one of the following forms:
>     `friend` _class-key_ _nested-name-specifier_(opt) _identifier_ `;`
>     `friend` _class-key_ _simple-template-id_ `;`
> `friend` _class-key_ _nested-name-specifier_ `template`(opt)
_simple-template-id_ `;`

Notably absent from this list is the `enum` form of an
_elaborated-type-specifier_ "`enum` _nested-name-specifier_(opt)
_identifier_", which appears to be intentional per the resolution of
CWG2363.

Most major implementations accept these declarations, so the diagnostic
is a pedantic warning across all C++ versions.

In addition to the trivial cases previously diagnosed in C++98, we now
diagnose cases where the _elaborated-type-specifier_ has a dependent
_nested-name-specifier_:
```
template<typename T>
struct A
{
    enum class E;
};

struct B
{
    template<typename T>
    friend enum A<T>::E; // pedantic warning: elaborated enumeration type cannot be a friend
};

template<typename T>
struct C
{
    friend enum T::E;  // pedantic warning: elaborated enumeration type cannot be a friend
};
```