Improved handing of evolving types in iteration statements

src/compiler/checker.ts

@@ -180,6 +180,7 @@ namespace ts {
         let jsxElementClassType: Type;
 
         let deferredNodes: Node[];
+        let inFlowCheck = false;
 
         const tupleTypes: Map<TupleType> = {};
         const unionTypes: Map<UnionType> = {};
@@ -7528,7 +7529,11 @@ namespace ts {
 
         function getFlowTypeOfReference(reference: Node, declaredType: Type, initialType: Type) {
             let key: string;
-            return reference.flowNode ? getTypeAtFlowNode(reference.flowNode) : declaredType;
+            const saveInFlowCheck = inFlowCheck;
+            inFlowCheck = true;
+            const result = reference.flowNode ? getTypeAtFlowNode(reference.flowNode) : declaredType;
+            inFlowCheck = saveInFlowCheck;
+            return result;
 
             function getTypeAtFlowNode(flow: FlowNode): Type {
                 while (true) {
@@ -8537,7 +8542,7 @@ namespace ts {
             const args = getEffectiveCallArguments(callTarget);
             const argIndex = indexOf(args, arg);
             if (argIndex >= 0) {
-                const signature = getResolvedSignature(callTarget);
+                const signature = getResolvedOrAnySignature(callTarget);
                 return getTypeAtPosition(signature, argIndex);
             }
             return undefined;
@@ -11071,6 +11076,20 @@ namespace ts {
             return resolveCall(node, callSignatures, candidatesOutArray, headMessage);
         }
 
+        function resolveSignature(node: CallLikeExpression, candidatesOutArray?: Signature[]): Signature {
+            switch (node.kind) {
+                case SyntaxKind.CallExpression:
+                    return resolveCallExpression(<CallExpression>node, candidatesOutArray);
+                case SyntaxKind.NewExpression:
+                    return resolveNewExpression(<NewExpression>node, candidatesOutArray);
+                case SyntaxKind.TaggedTemplateExpression:
+                    return resolveTaggedTemplateExpression(<TaggedTemplateExpression>node, candidatesOutArray);
+                case SyntaxKind.Decorator:
+                    return resolveDecorator(<Decorator>node, candidatesOutArray);
+            }
+            Debug.fail("Branch in 'resolveSignature' should be unreachable.");
+        }
+
         // candidatesOutArray is passed by signature help in the language service, and collectCandidates
         // must fill it up with the appropriate candidate signatures
         function getResolvedSignature(node: CallLikeExpression, candidatesOutArray?: Signature[]): Signature {
@@ -11079,26 +11098,23 @@ namespace ts {
             // However, it is possible that either candidatesOutArray was not passed in the first time,
             // or that a different candidatesOutArray was passed in. Therefore, we need to redo the work
             // to correctly fill the candidatesOutArray.
-            if (!links.resolvedSignature || candidatesOutArray) {
-                links.resolvedSignature = anySignature;
-
-                if (node.kind === SyntaxKind.CallExpression) {
-                    links.resolvedSignature = resolveCallExpression(<CallExpression>node, candidatesOutArray);
-                }
-                else if (node.kind === SyntaxKind.NewExpression) {
-                    links.resolvedSignature = resolveNewExpression(<NewExpression>node, candidatesOutArray);
-                }
-                else if (node.kind === SyntaxKind.TaggedTemplateExpression) {
-                    links.resolvedSignature = resolveTaggedTemplateExpression(<TaggedTemplateExpression>node, candidatesOutArray);
-                }
-                else if (node.kind === SyntaxKind.Decorator) {
-                    links.resolvedSignature = resolveDecorator(<Decorator>node, candidatesOutArray);
-                }
-                else {
-                    Debug.fail("Branch in 'getResolvedSignature' should be unreachable.");
-                }
+            const cached = links.resolvedSignature;
+            if (cached && cached !== anySignature && !candidatesOutArray) {
+                return cached;
             }
-            return links.resolvedSignature;
+            links.resolvedSignature = anySignature;
+            const result = resolveSignature(node, candidatesOutArray);
+            // If signature resolution originated in control flow type analysis (for example to compute the
+            // assigned type in a flow assignment) we don't cache the result as it may be based on temporary
+            // types from the control flow analysis.
+            links.resolvedSignature = inFlowCheck ? cached : result;
+            return result;
+        }
+
+        function getResolvedOrAnySignature(node: CallLikeExpression) {
+            // If we're already in the process of resolving the given signature, don't resolve again as
+            // that could cause infinite recursion. Instead, return anySignature.
+            return getNodeLinks(node).resolvedSignature === anySignature ? anySignature : getResolvedSignature(node);
         }
 
         function getInferredClassType(symbol: Symbol) {