Fix arity checking for partial overload resolution

src/compiler/checker.ts

@@ -4331,25 +4331,32 @@ module ts {
         }
 
         function signatureHasCorrectArity(node: CallExpression, signature: Signature): boolean {
-            var args = node.arguments || emptyArray;
-            var isCorrect = args.length >= signature.minArgumentCount &&
-                (signature.hasRestParameter || args.length <= signature.parameters.length) &&
-                (!node.typeArguments || signature.typeParameters && node.typeArguments.length === signature.typeParameters.length);
-
-            // For error recovery, since we have parsed OmittedExpressions for any extra commas
-            // in the argument list, if we see any OmittedExpressions, just return true.
-            // The reason this is ok is because omitted expressions here are syntactically
-            // illegal, and will cause a parse error.
-            // Note: It may be worth keeping the upper bound check on arity, but removing
-            // the lower bound check if there are omitted expressions.
-            if (!isCorrect) {
-                // Technically this type assertion is not safe because args could be initialized to emptyArray
-                // above.
-                if ((<NodeArray<Node>>args).hasTrailingComma || forEach(args, arg => arg.kind === SyntaxKind.OmittedExpression)) {
-                    return true;
-                }
+            if (!node.arguments) {
+                // This only happens when we have something of the form:
+                //     new C
+                //
+                return signature.minArgumentCount === 0;
             }
-            return isCorrect;
+
+            // For IDE scenarios, since we may have an incomplete call, we make two modifications
+            // to arity checking.
+            //    1. A trailing comma is tantamount to adding another argument
+            //    2. If the call is incomplete (no closing paren) allow fewer arguments than expected
+            var args = node.arguments;
+            var numberOfArgs = args.hasTrailingComma ? args.length + 1 : args.length;
+            var hasTooManyArguments = !signature.hasRestParameter && numberOfArgs > signature.parameters.length;
+            var hasRightNumberOfTypeArguments = !node.typeArguments ||
+                (signature.typeParameters && node.typeArguments.length === signature.typeParameters.length);
+
+            if (hasTooManyArguments || !hasRightNumberOfTypeArguments) {
+                return false;
+            }
+
+            // If we are missing the close paren, the call is incomplete, and we should skip
+            // the lower bound check.
+            var callIsIncomplete = args.end === node.end;
+            var hasEnoughArguments = numberOfArgs >= signature.minArgumentCount;
+            return callIsIncomplete || hasEnoughArguments;
         }
 
         // If type has a single call signature and no other members, return that signature. Otherwise, return undefined.