[clang] [clang] fix constexpr-unknown handling of self-references. (PR #132990)

[via cfe-commits]

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@@ -3485,11 +3485,27 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
 
   APValue::LValueBase Base(VD, Frame ? Frame->Index : 0, Version);
 
+  auto CheckUninitReference = [&] {
+    if (!Result->hasValue() && VD->getType()->isReferenceType()) {
+      // We can't use an uninitialized reference directly.
+      if (!AllowConstexprUnknown) {
+        if (!Info.checkingPotentialConstantExpression())
+          Info.FFDiag(E, diag::note_constexpr_use_uninit_reference);
+        return false;
+      }
+      Result = &Info.CurrentCall->createConstexprUnknownAPValues(VD, Base);
+    }
+    return true;
+  };
+
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cor3ntin wrote:

I think this could use a comment

https://github.com/llvm/llvm-project/pull/132990