[clang] Fix references to complete types in attribute references (PR #209537)

[Erich Keane via cfe-commits]

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@@ -3212,8 +3212,17 @@ static bool HandleLValueMember(EvalInfo &Info, const Expr *E, LValue &LVal,
                                const FieldDecl *FD,
                                const ASTRecordLayout *RL = nullptr) {
   if (!RL) {
-    if (FD->getParent()->isInvalidDecl()) return false;
-    RL = &Info.Ctx.getASTRecordLayout(FD->getParent());
+    const RecordDecl *RD = FD->getParent();
+    if (RD->isInvalidDecl())
+      return false;
+    // There are some cases where the base is not yet complete but we haven't
+    // disagnosed (such as in a template instantation of an attribute that
+    // references the expression, ala enable_if).  These aren't necessarily
+    // constant expressions so we return 'false', but they might be, so we don't
+    // diagnose.
+    if (!RD->isCompleteDefinition())
+      return false;
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erichkeane wrote:

@tbaederr for visibility.

I tried my test with `-fexperimental-new-constant-interpreter` and it doesn't reproduce, so I think that means no? 

https://github.com/llvm/llvm-project/pull/209537