(RFC) utils/rttc.ts: Add integer check for array indices (#282)

* utils/rttc.ts: Add integer check for array indices

* Fix allowed range of array indices

* Use term "array index" in error message and add detail for other numbers

Author: thomastanck

src/utils/__tests__/__snapshots__/rttc.ts.snap

@@ -8317,8 +8317,8 @@ Object {
 
 exports[`Member expression type combinations: Invalid type combinations return TypeError 54`] = `
 Object {
-  "elaborate": "Expected number as prop, got boolean.",
-  "explain": "Expected number as prop, got boolean.",
+  "elaborate": "Expected array index as prop, got boolean.",
+  "explain": "Expected array index as prop, got boolean.",
   "left": Array [
     2,
   ],
@@ -8328,8 +8328,8 @@ Object {
 
 exports[`Member expression type combinations: Invalid type combinations return TypeError 55`] = `
 Object {
-  "elaborate": "Expected number as prop, got string.",
-  "explain": "Expected number as prop, got string.",
+  "elaborate": "Expected array index as prop, got string.",
+  "explain": "Expected array index as prop, got string.",
   "left": Array [
     2,
   ],
@@ -8339,8 +8339,8 @@ Object {
 
 exports[`Member expression type combinations: Invalid type combinations return TypeError 56`] = `
 Object {
-  "elaborate": "Expected number as prop, got function.",
-  "explain": "Expected number as prop, got function.",
+  "elaborate": "Expected array index as prop, got function.",
+  "explain": "Expected array index as prop, got function.",
   "left": Array [
     2,
   ],
@@ -8350,8 +8350,8 @@ Object {
 
 exports[`Member expression type combinations: Invalid type combinations return TypeError 57`] = `
 Object {
-  "elaborate": "Expected number as prop, got function.",
-  "explain": "Expected number as prop, got function.",
+  "elaborate": "Expected array index as prop, got function.",
+  "explain": "Expected array index as prop, got function.",
   "left": Array [
     2,
   ],
@@ -8361,8 +8361,8 @@ Object {
 
 exports[`Member expression type combinations: Invalid type combinations return TypeError 58`] = `
 Object {
-  "elaborate": "Expected number as prop, got object.",
-  "explain": "Expected number as prop, got object.",
+  "elaborate": "Expected array index as prop, got object.",
+  "explain": "Expected array index as prop, got object.",
   "left": Array [
     2,
   ],
@@ -8374,8 +8374,8 @@ Object {
 
 exports[`Member expression type combinations: Invalid type combinations return TypeError 59`] = `
 Object {
-  "elaborate": "Expected number as prop, got array.",
-  "explain": "Expected number as prop, got array.",
+  "elaborate": "Expected array index as prop, got array.",
+  "explain": "Expected array index as prop, got array.",
   "left": Array [
     2,
   ],
@@ -8387,8 +8387,8 @@ Object {
 
 exports[`Member expression type combinations: Invalid type combinations return TypeError 60`] = `
 Object {
-  "elaborate": "Expected number as prop, got undefined.",
-  "explain": "Expected number as prop, got undefined.",
+  "elaborate": "Expected array index as prop, got undefined.",
+  "explain": "Expected array index as prop, got undefined.",
   "left": Array [
     2,
   ],
@@ -8398,8 +8398,8 @@ Object {
 
 exports[`Member expression type combinations: Invalid type combinations return TypeError 61`] = `
 Object {
-  "elaborate": "Expected number as prop, got null.",
-  "explain": "Expected number as prop, got null.",
+  "elaborate": "Expected array index as prop, got null.",
+  "explain": "Expected array index as prop, got null.",
   "left": Array [
     2,
   ],
@@ -8577,6 +8577,39 @@ Object {
 }
 `;
 
+exports[`Member expression type combinations: Invalid type combinations return TypeError 80`] = `
+Object {
+  "elaborate": "Expected array index as prop, got other number.",
+  "explain": "Expected array index as prop, got other number.",
+  "left": Array [
+    2,
+  ],
+  "right": -1,
+}
+`;
+
+exports[`Member expression type combinations: Invalid type combinations return TypeError 81`] = `
+Object {
+  "elaborate": "Expected array index as prop, got other number.",
+  "explain": "Expected array index as prop, got other number.",
+  "left": Array [
+    2,
+  ],
+  "right": 0.5,
+}
+`;
+
+exports[`Member expression type combinations: Invalid type combinations return TypeError 82`] = `
+Object {
+  "elaborate": "Expected array index as prop, got other number.",
+  "explain": "Expected array index as prop, got other number.",
+  "left": Array [
+    2,
+  ],
+  "right": 4294967295,
+}
+`;
+
 exports[`Ternary/if test expression type combinations: Invalid type combinations return TypeError 1`] = `
 Object {
   "elaborate": "Expected boolean as condition, got number.",

src/utils/__tests__/rttc.ts

@@ -289,6 +289,14 @@ describe('Member expression type combinations:', () => {
     })
   })
 
+  // Extra tests for array indices integral check.
+  valid.push([arr, 0])
+  valid.push([arr, 10])
+  valid.push([arr, 2 ** 32 - 2])
+  invalid.push([arr, -1])
+  invalid.push([arr, 0.5])
+  invalid.push([arr, 2 ** 32 - 1])
+
   test('Valid type combinations are OK', () => {
     valid.forEach(([left, right]: [Value, Value]) => {
       const context = mockRuntimeContext()

src/utils/rttc.ts

@@ -35,6 +35,10 @@ const typeOf = (v: Value) => {
 }
 
 const isNumber = (v: Value) => typeOf(v) === 'number'
+// See section 4 of https://2ality.com/2012/12/arrays.html
+// v >>> 0 === v checks that v is a valid unsigned 32-bit int
+// tslint:disable-next-line:no-bitwise
+const isArrayIndex = (v: Value) => isNumber(v) && v >>> 0 === v && v < 2 ** 32 - 1
 const isString = (v: Value) => typeOf(v) === 'string'
 const isBool = (v: Value) => typeOf(v) === 'boolean'
 const isObject = (v: Value) => typeOf(v) === 'object'
@@ -95,7 +99,11 @@ export const checkMemberAccess = (node: es.Node, obj: Value, prop: Value) => {
   if (isObject(obj)) {
     return isString(prop) ? undefined : new TypeError(node, ' as prop', 'string', typeOf(prop))
   } else if (isArray(obj)) {
-    return isNumber(prop) ? undefined : new TypeError(node, ' as prop', 'number', typeOf(prop))
+    return isArrayIndex(prop)
+      ? undefined
+      : isNumber(prop)
+      ? new TypeError(node, ' as prop', 'array index', 'other number')
+      : new TypeError(node, ' as prop', 'array index', typeOf(prop))
   } else {
     return new TypeError(node, '', 'object or array', typeOf(obj))
   }