[Clang][Sema][Parse] Delay parsing of noexcept-specifiers in friend function declarations (#90517)

According to [class.mem.general] p8:
> A complete-class context of a class (template) is a
> - function body,
> - default argument,
> - default template argument,
> - _noexcept-specifier_, or
> - default member initializer
>
> within the member-specification of the class or class template.

When testing #90152, it came to my attention that we do _not_ consider
the _noexcept-specifier_ of a friend function declaration to be a
complete-class context (something which the Microsoft standard library
depends on). Although a comment states that this is "consistent with
what other implementations do", the only other implementation that
exhibits this behavior is GCC (MSVC and EDG both late-parse the
_noexcept-specifier_).

This patch changes _noexcept-specifiers_ of friend function declarations
to be late parsed, which is in agreement with the standard & majority of
implementations. Pre-#90152, our existing implementation falls "in
between" the implementation consensus: within non-template classes, we
would not find latter declared members (qualified and unqualified),
while within class templates we would not find latter declared member
when named with a unqualified name, we would find members named with a
qualified name (even when lookup context is the current instantiation).
Therefore, this _shouldn't_ be a breaking change -- any code that didn't
compile will continue to not compile (since a _noexcept-specifier_ is
not part of the deduction substitution
loci (see [temp.deduct.general] p7), and any code which
did compile should continue to do so.

1 files changed, 14 insertions, 6 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 7431c25..69fabf6 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -7416,12 +7416,20 @@ void Parser::ParseFunctionDeclarator(Declarator &D,
       std::optional<Sema::CXXThisScopeRAII> ThisScope;
       InitCXXThisScopeForDeclaratorIfRelevant(D, DS, ThisScope);
 
-      // Parse exception-specification[opt].
-      // FIXME: Per [class.mem]p6, all exception-specifications at class scope
-      // should be delayed, including those for non-members (eg, friend
-      // declarations). But only applying this to member declarations is
-      // consistent with what other implementations do.
-      bool Delayed = D.isFirstDeclarationOfMember() &&
+      // C++ [class.mem.general]p8:
+      //   A complete-class context of a class (template) is a
+      //     - function body,
+      //     - default argument,
+      //     - default template argument,
+      //     - noexcept-specifier, or
+      //     - default member initializer
+      //   within the member-specification of the class or class template.
+      //
+      // Parse exception-specification[opt]. If we are in the
+      // member-specification of a class or class template, this is a
+      // complete-class context and parsing of the noexcept-specifier should be
+      // delayed (even if this is a friend declaration).
+      bool Delayed = D.getContext() == DeclaratorContext::Member &&
                      D.isFunctionDeclaratorAFunctionDeclaration();
       if (Delayed && Actions.isLibstdcxxEagerExceptionSpecHack(D) &&
           GetLookAheadToken(0).is(tok::kw_noexcept) &&