[Clang] Allow omitting `typename` in befriended constructors parameters

[cor3ntin]

Summary

Fixes #63119

[aaron.ballman]

In the patch summary: consstructors -> constructors

Can you also add a test case to cover "unless that parameter-declaration appears in a default argument" from http://eel.is/c++draft/temp.res#general-4.3.3 ?

[cor3ntin]

Add test for default parameters

[aaron.ballman]

clang/test/CXX/temp/temp.res/p4.cpp

182–183

Hmmm, I was expecting a missing 'typename' test case, so perhaps add a friend declaration that's a definition instead?

[cor3ntin]

clang/test/CXX/temp/temp.res/p4.cpp

182–183

that is not possible for a constructor afaik

[aaron.ballman]

LGTM!

clang/test/CXX/temp/temp.res/p4.cpp

182–183

Oh shoot, that's a good point. Nevermiiiiiiind. :-)

clang/docs/ReleaseNotes.rst

- Fix handling of constexpr dynamic memory allocations in template
   arguments. (`#62462 <https://github.com/llvm/llvm-project/issues/62462>`_)
 - Some predefined expressions are now treated as string literals in MSVC
   compatibility mode.
   (`#114 <https://github.com/llvm/llvm-project/issues/114>`_)
 - Fix parsing of `auto(x)`, when it is surrounded by parentheses.
   (`#62494 <https://github.com/llvm/llvm-project/issues/62494>`_)
 - Fix handling of generic lambda used as template arguments.
   (`#62611 <https://github.com/llvm/llvm-project/issues/62611>`_)
+- Allow omitting ``typename`` in the parameter declaration of a friend
+  constructor declaration.
+  (`#63119 <https://github.com/llvm/llvm-project/issues/63119>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 - Preserve ``namespace`` definitions that follow malformed declarations.
 
 Miscellaneous Bug Fixes
 ^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Parse/ParseDecl.cpp

bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
   ParsedAttributes Attrs(AttrFactory);
   MaybeParseMicrosoftAttributes(Attrs);
 
   // Check whether the next token(s) are part of a declaration
   // specifier, in which case we have the start of a parameter and,
   // therefore, we know that this is a constructor.
   // Due to an ambiguity with implicit typename, the above is not enough.
   // Additionally, check to see if we are a friend.
+  // If we parsed a scope specifier as well as friend,
+  // we might be parsing a friend constructor.
   bool IsConstructor = false;
-  if (isDeclarationSpecifier(IsFriend ? ImplicitTypenameContext::No
-                                      : ImplicitTypenameContext::Yes))
+  if (isDeclarationSpecifier(IsFriend && !SS.isSet()
+                                 ? ImplicitTypenameContext::No
     IsConstructor = true;
   else if (Tok.is(tok::identifier) ||
            (Tok.is(tok::annot_cxxscope) && NextToken().is(tok::identifier))) {
     // We've seen "C ( X" or "C ( X::Y", but "X" / "X::Y" is not a type.
     // This might be a parenthesized member name, but is more likely to
     // be a constructor declaration with an invalid argument type. Keep
     // looking.
     if (Tok.is(tok::annot_cxxscope))

clang/test/CXX/temp/temp.res/p4.cpp

 struct C {
   C(T::type); // implicit typename context
   friend C (T::fn)(); // not implicit typename context, declarator-id of friend declaration
   C(T::type::*x)[3]; // not implicit typename context, pointer-to-member type
 };
 
 template <typename T>
 C<T>::C(T::type) {}
+
+namespace GH63119 {
+struct X {
+    X(int);
+    X(auto);
+    void f(int);
+};
+template<typename T> struct S {
+  friend X::X(T::type);
+  friend X::X(T::type = (int)(void(*)(typename T::type))(nullptr)); // expected-error {{friend declaration specifying a default argument must be a definition}}
+  friend X::X(T::type = (int)(void(*)(T::type))(nullptr)); // expected-error {{friend declaration specifying a default argument must be a definition}} \
+                                                           // expected-error {{expected expression}}
+  friend void X::f(T::type);
+};
+}