[clang] [Clang] Treat constexpr-unknown value as invalid in `evaluateValue` (PR #128409)

[Eli Friedman via cfe-commits]

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@@ -2587,6 +2587,9 @@ APValue *VarDecl::evaluateValueImpl(SmallVectorImpl<PartialDiagnosticAt> &Notes,
       !Notes.empty())
     Result = false;
 
+  if (Eval->Evaluated.allowConstexprUnknown())
+    Result = false;
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efriedma-quic wrote:

I think this check should be inside `EvaluateAsInitializer()`... and all the other `Evaluate` entry points also include this check.

https://github.com/llvm/llvm-project/pull/128409