[clang] [clang] Disable missing definition warning on pure virtual functions (PR #74510)

[Shafik Yaghmour via cfe-commits]

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@@ -18931,7 +18931,7 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
   //   constant evaluated
   bool NeededForConstantEvaluation =
       isPotentiallyConstantEvaluatedContext(*this) &&
-      isImplicitlyDefinableConstexprFunction(Func);
+      isImplicitlyDefinableConstexprFunction(Func) && !Func->isPure();
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shafik wrote:

I am not sure why moving it down silences the diagnostic, I would think doing `(OdrUse == OdrUseContext::Used || NeededForConstantEvaluation || !Func->isPure())` should work. 

Can you explain in more detail?

https://github.com/llvm/llvm-project/pull/74510