[clang] [Clang] prevent constexpr crash on invalid overrides (PR #184048)

[Oleksandr Tarasiuk via cfe-commits]

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@@ -9247,9 +9247,12 @@ bool Sema::AddOverriddenMethods(CXXRecordDecl *DC, CXXMethodDecl *MD) {
         continue;
       if (Overridden.insert(BaseMD).second) {
         MD->addOverriddenMethod(BaseMD);
-        CheckOverridingFunctionReturnType(MD, BaseMD);
-        CheckOverridingFunctionAttributes(MD, BaseMD);
-        CheckOverridingFunctionExceptionSpec(MD, BaseMD);
+        bool Invalid = false;
+        Invalid |= CheckOverridingFunctionReturnType(MD, BaseMD);
+        Invalid |= CheckOverridingFunctionAttributes(MD, BaseMD);
+        Invalid |= CheckOverridingFunctionExceptionSpec(MD, BaseMD);
+        if (Invalid)
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a-tarasyuk wrote:

That’s the case I was thinking about — whether it should be treated as invalid at this stage or not. For now, I decided to keep it as-is.

Some checks depend on `New` decl not being marked as invalid when diagnosing inconsistent override control. Marking it as invalid at this stage may cause certain diagnostics to be skipped

https://github.com/llvm/llvm-project/blob/e8275dfe2a030e6f06a47ca23746c7dab00ea01f/clang/lib/Sema/SemaDeclCXX.cpp#L7323-L7327

https://github.com/llvm/llvm-project/blob/e8275dfe2a030e6f06a47ca23746c7dab00ea01f/clang/lib/Sema/SemaDeclCXX.cpp#L3311-L3313

Removing the `isInvalidDecl` checks would likely introduce noisy diagnostics for unrelated invalid methods, since this runs as a broad late pass over class methods. WDYT?

https://github.com/llvm/llvm-project/pull/184048