Handle returning from an expansion properly in constant evaluation

Author: Sirraide

clang/lib/AST/ExprConstant.cpp

@@ -5956,19 +5956,24 @@ static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
 
     // No need to push an extra scope for these since they're already
     // CompoundStmts.
+    EvalStmtResult ESR = ESR_Succeeded;
     for (const Stmt *Instantiation : Expansion->getInstantiations()) {
-      EvalStmtResult ESR = EvaluateStmt(Result, Info, Instantiation);
+      ESR = EvaluateStmt(Result, Info, Instantiation);
       if (ESR == ESR_Failed ||
           ShouldPropagateBreakContinue(Info, Expansion, &Scope, ESR))
         return ESR;
       if (ESR != ESR_Continue) {
         // Succeeded here actually means we encountered a 'break'.
-        assert(ESR == ESR_Succeeded);
+        assert(ESR == ESR_Succeeded || ESR == ESR_Returned);
         break;
       }
     }
 
-    return Scope.destroy() ? ESR_Succeeded : ESR_Failed;
+    // Map Continue back to Succeeded if we fell off the end of the loop.
+    if (ESR == ESR_Continue)
+      ESR = ESR_Succeeded;
+
+    return Scope.destroy() ? ESR : ESR_Failed;
   }
 
   case Stmt::SwitchStmtClass:

clang/test/SemaCXX/cxx2c-expansion-statements.cpp

@@ -918,3 +918,24 @@ static_assert(lifetime_extension_instantiate_expansions<int>() == 47);
 static_assert(lifetime_extension_dependent_expansion_stmt<int>() == 47);
 static_assert(foo<int>().lifetime_extension_multiple_instantiations<int>() == 47);
 }
+
+template <typename... Ts>
+constexpr int return_from_expansion(Ts... ts) {
+  template for (int i : {1, 2, 3}) {
+    return (ts + ...);
+  }
+  __builtin_unreachable();
+}
+
+static_assert(return_from_expansion(4, 5, 6) == 15);
+
+void not_constexpr();
+
+constexpr int empty_expansion_consteval() {
+  template for (auto _ : {}) {
+    not_constexpr();
+  }
+  return 3;
+}
+
+static_assert(empty_expansion_consteval() == 3);