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

[Yingwei Zheng via cfe-commits]

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@@ -17000,6 +16998,18 @@ bool Expr::EvaluateAsInitializer(APValue &Value, const ASTContext &Ctx,
 
     if (!Info.discardCleanups())
       llvm_unreachable("Unhandled cleanup; missing full expression marker?");
+
+    if (Value.allowConstexprUnknown()) {
+      assert(Value.isLValue() && "Expected an lvalue");
+      auto Base = Value.getLValueBase();
+      const auto *NewVD = Base.dyn_cast<const ValueDecl *>();
+      if (!NewVD)
+        NewVD = VD;
+      Info.FFDiag(getExprLoc(), diag::note_constexpr_var_init_non_constant, 1)
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dtcxzyw wrote:

> If you need more information to generate a good diagnostic, you could pass that information to createConstexprUnknownAPValues, and store it in the APValue itself, I guess. Computing the whole diagnostic in advance seems like it would be too expensive, though.

That is what the current implementation does.


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