[clang] [clang] Fix pointer comparisons between pointers to constexpr-unknown (PR #147663)

[Eli Friedman via cfe-commits]

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@@ -14544,6 +14538,23 @@ EvaluateComparisonBinaryOperator(EvalInfo &Info, const BinaryOperator *E,
           (LHSValue.Base && isZeroSized(RHSValue)))
         return DiagComparison(
             diag::note_constexpr_pointer_comparison_zero_sized);
+      // A constexpr-unknown reference can be equal to any other lvalue, except
+      // for variables allocated during constant evaluation. (The "lifetime
+      // [...] includes the entire constant evaluation", so it has to be
+      // distinct from anything allocated during constant evaluation.)
+      //
+      // Theoretically we could handle other cases, but the standard doesn't say
+      // what other cases we need to handle; it just says an "equality
+      // operator where the result is unspecified" isn't a constant expression.
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efriedma-quic wrote:

Okay, I removed the checks for objects from the current evaluation, and my comment about the standard.

I'd like to get this in because it's fixing a clear miscompile of pointer comparisons; we can worry about the finer details in a followup once the CWG decides what to do.

https://github.com/llvm/llvm-project/pull/147663