[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, 7 insertions, 6 deletions

diff --git a/clang/lib/Sema/SemaExceptionSpec.cpp b/clang/lib/Sema/SemaExceptionSpec.cpp
index c9dd6bb..41bf273 100644
--- a/clang/lib/Sema/SemaExceptionSpec.cpp
+++ b/clang/lib/Sema/SemaExceptionSpec.cpp
@@ -258,13 +258,14 @@ Sema::UpdateExceptionSpec(FunctionDecl *FD,
 }
 
 static bool exceptionSpecNotKnownYet(const FunctionDecl *FD) {
-  auto *MD = dyn_cast<CXXMethodDecl>(FD);
-  if (!MD)
+  ExceptionSpecificationType EST =
+      FD->getType()->castAs<FunctionProtoType>()->getExceptionSpecType();
+  if (EST == EST_Unparsed)
+    return true;
+  else if (EST != EST_Unevaluated)
     return false;
-
-  auto EST = MD->getType()->castAs<FunctionProtoType>()->getExceptionSpecType();
-  return EST == EST_Unparsed ||
-         (EST == EST_Unevaluated && MD->getParent()->isBeingDefined());
+  const DeclContext *DC = FD->getLexicalDeclContext();
+  return DC->isRecord() && cast<RecordDecl>(DC)->isBeingDefined();
 }
 
 static bool CheckEquivalentExceptionSpecImpl(