[clang] [Clang] resolve record declaration of defaulted comparison method by using the first argument (PR #96228)

[Younan Zhang via cfe-commits]

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@@ -9192,7 +9194,7 @@ ComputeDefaultedComparisonExceptionSpec(Sema &S, SourceLocation Loc,
     EnterExpressionEvaluationContext Context(
         S, Sema::ExpressionEvaluationContext::Unevaluated);
 
-    CXXRecordDecl *RD = cast<CXXRecordDecl>(FD->getLexicalParent());
+    auto RD = getRecordDeclFromFirstParameter(FD);
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zyn0217 wrote:

Hmmm... Does `cast<CXXRecordDecl>(FD->getDeclContext())` work? That way, we can avoid the `getRecordDeclFromFirstParameter` whole thing.
I suspect the current approach isn't right here because 1. we have to ensure FD is a CXXMethodDecl; 2. the parameter list could be empty and thus we probably run into an out-of-bound error.

https://github.com/llvm/llvm-project/pull/96228