[clang] Fix crash on defaulted comparison recovery in Sema and AST (PR #210668)

[Oliver Hunt via cfe-commits]

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@@ -9042,7 +9042,7 @@ bool Sema::CheckExplicitlyDefaultedComparison(Scope *S, FunctionDecl *FD,
 
   // Perform any unqualified lookups we're going to need to default this
   // function.
-  if (S) {
+  if (S && !FD->isInvalidDecl()) {
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ojhunt wrote:

I don't believe we should be getting here if the function is invalid

https://github.com/llvm/llvm-project/pull/210668