From 3344a42686b4a3b4470497ef20a3f3e3aea11cd5 Mon Sep 17 00:00:00 2001
From: Anders Hejlsberg <andersh@microsoft.com>
Date: Thu, 7 Jan 2016 10:34:22 -0800
Subject: [PATCH] Check for numeric index signature instead of array-like type
 Better error message when object with numeric index signature is indexed with
 a string

---
 src/compiler/checker.ts              | 21 +++++++++++++++------
 src/compiler/diagnosticMessages.json |  4 ++++
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 62ab2080d28..ca562a5c353 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -8581,10 +8581,17 @@ namespace ts {
         }
 
         /**
-         * Return true if given node is an expression consisting of an identifier (possibly parenthesized)
-         * that references a variable declared in a containing for-in statement for an array-like object.
+         * Return true if the given type is considered to have numeric property names.
          */
-        function isForInVariableForArrayLikeObject(expr: Expression) {
+        function hasNumericPropertyNames(type: Type) {
+            return getIndexTypeOfType(type, IndexKind.Number) && !getIndexTypeOfType(type, IndexKind.String);
+        }
+
+        /**
+         * Return true if given node is an expression consisting of an identifier (possibly parenthesized)
+         * that references a for-in variable for an object with numeric property names.
+         */
+        function isForInVariableForNumericPropertyNames(expr: Expression) {
             const e = skipParenthesizedNodes(expr);
             if (e.kind === SyntaxKind.Identifier) {
                 const symbol = getResolvedSymbol(<Identifier>e);
@@ -8595,7 +8602,7 @@ namespace ts {
                         if (node.kind === SyntaxKind.ForInStatement &&
                             child === (<ForInStatement>node).statement &&
                             getForInVariableSymbol(<ForInStatement>node) === symbol &&
-                            isArrayLikeType(checkExpression((<ForInStatement>node).expression))) {
+                            hasNumericPropertyNames(checkExpression((<ForInStatement>node).expression))) {
                             return true;
                         }
                         child = node;
@@ -8666,7 +8673,7 @@ namespace ts {
             if (isTypeAnyOrAllConstituentTypesHaveKind(indexType, TypeFlags.StringLike | TypeFlags.NumberLike | TypeFlags.ESSymbol)) {
 
                 // Try to use a number indexer.
-                if (isTypeAnyOrAllConstituentTypesHaveKind(indexType, TypeFlags.NumberLike) || isForInVariableForArrayLikeObject(node.argumentExpression)) {
+                if (isTypeAnyOrAllConstituentTypesHaveKind(indexType, TypeFlags.NumberLike) || isForInVariableForNumericPropertyNames(node.argumentExpression)) {
                     const numberIndexType = getIndexTypeOfType(objectType, IndexKind.Number);
                     if (numberIndexType) {
                         return numberIndexType;
@@ -8681,7 +8688,9 @@ namespace ts {
 
                 // Fall back to any.
                 if (compilerOptions.noImplicitAny && !compilerOptions.suppressImplicitAnyIndexErrors && !isTypeAny(objectType)) {
-                    error(node, Diagnostics.Index_signature_of_object_type_implicitly_has_an_any_type);
+                    error(node, getIndexTypeOfType(objectType, IndexKind.Number) ?
+                        Diagnostics.Element_implicitly_has_an_any_type_because_index_expression_is_not_of_type_number :
+                        Diagnostics.Index_signature_of_object_type_implicitly_has_an_any_type);
                 }
 
                 return anyType;
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index f7268d6d990..4a0a1dba5ac 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -2422,6 +2422,10 @@
         "category": "Error",
         "code": 7013
     },
+    "Element implicitly has an 'any' type because index expression is not of type 'number'.": {
+        "category": "Error",
+        "code": 7015
+    },
     "Property '{0}' implicitly has type 'any', because its 'set' accessor lacks a type annotation.": {
         "category": "Error",
         "code": 7016