[clang] [Clang] Implement P0963R3 "Structured binding declaration as a condition" (PR #130228)

[Younan Zhang via cfe-commits]

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@@ -5218,16 +5218,28 @@ static bool EvaluateVarDecl(EvalInfo &Info, const VarDecl *VD) {
   return true;
 }
 
+static bool EvaluateDecompositionDeclInit(EvalInfo &Info,
+                                          const DecompositionDecl *DD);
+
 static bool EvaluateDecl(EvalInfo &Info, const Decl *D) {
   bool OK = true;
 
   if (const VarDecl *VD = dyn_cast<VarDecl>(D))
     OK &= EvaluateVarDecl(Info, VD);
 
-  if (const DecompositionDecl *DD = dyn_cast<DecompositionDecl>(D))
-    for (auto *BD : DD->flat_bindings())
-      if (auto *VD = BD->getHoldingVar())
-        OK &= EvaluateDecl(Info, VD);
+  if (const DecompositionDecl *DD = dyn_cast<DecompositionDecl>(D);
+      DD && !DD->isDecisionVariable())
+    OK &= EvaluateDecompositionDeclInit(Info, DD);
+
+  return OK;
+}
+
+static bool EvaluateDecompositionDeclInit(EvalInfo &Info,
+                                          const DecompositionDecl *DD) {
+  bool OK = true;
+  for (auto *BD : DD->flat_bindings())
+    if (auto *VD = BD->getHoldingVar())
+      OK &= EvaluateDecl(Info, VD);
----------------
zyn0217 wrote:

And we crash during `isPotentialConstantExpr()`, wherein we set `CheckingPotentialConstantExpression = true` that results in `EvaluateVarDecl()` returning true because `EvaluateInPlace()` it called bailed out of evaluation in this case. So in short, we can't short-circuit the initialization of the binding decls because they're necessary in `CheckingPotentialConstantExpression`.

https://github.com/llvm/llvm-project/pull/130228