[clang] [Clang] Add __builtin_is_within_lifetime to implement P2641R4's std::is_within_lifetime (PR #91895)

[Hana Dusíková via cfe-commits]

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@@ -3927,6 +3935,9 @@ findSubobject(EvalInfo &Info, const Expr *E, const CompleteObject &Obj,
             // Placement new onto an inactive union member makes it active.
             O->setUnion(Field, APValue());
           } else {
+            // Pointer to/into inactive union member: Not within lifetime
+            if (handler.AccessKind == AK_IsWithinLifetime)
+              return false;
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hanickadot wrote:

same here

https://github.com/llvm/llvm-project/pull/91895