Improve unification by moving more logic to getIndexedAccessType

Author: ahejlsberg

src/compiler/checker.ts

@@ -5738,14 +5738,24 @@ namespace ts {
             return indexedAccessTypes[objectType.id] || (indexedAccessTypes[objectType.id] = createIndexedAccessType(objectType, indexType));
         }
 
-        function getPropertyTypeForIndexType(objectType: Type, indexType: Type, accessNode?: ElementAccessExpression | IndexedAccessTypeNode) {
+        function getPropertyTypeForIndexType(objectType: Type, indexType: Type, accessNode: ElementAccessExpression | IndexedAccessTypeNode, cacheSymbol: boolean) {
             const accessExpression = accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression ? <ElementAccessExpression>accessNode : undefined;
-            if (indexType.flags & (TypeFlags.StringLiteral | TypeFlags.NumberLiteral | TypeFlags.EnumLiteral)) {
-                const prop = getPropertyOfType(objectType, escapeIdentifier((<LiteralType>indexType).text));
+            const propName = indexType.flags & (TypeFlags.StringLiteral | TypeFlags.NumberLiteral | TypeFlags.EnumLiteral) ?
+                (<LiteralType>indexType).text :
+                accessExpression && checkThatExpressionIsProperSymbolReference(accessExpression.argumentExpression, indexType, /*reportError*/ false) ?
+                    getPropertyNameForKnownSymbolName((<Identifier>(<PropertyAccessExpression>accessExpression.argumentExpression).name).text) :
+                    undefined;
+            if (propName) {
+                const prop = getPropertyOfType(objectType, propName);
                 if (prop) {
-                    if (accessExpression && isAssignmentTarget(accessExpression) && isReadonlySymbol(prop)) {
-                        error(accessExpression.argumentExpression, Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property, symbolToString(prop));
-                        return unknownType;
+                    if (accessExpression) {
+                        if (isAssignmentTarget(accessExpression) && (isReferenceToReadonlyEntity(accessExpression, prop) || isReferenceThroughNamespaceImport(accessExpression))) {
+                            error(accessExpression.argumentExpression, Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property, symbolToString(prop));
+                            return unknownType;
+                        }
+                        if (cacheSymbol) {
+                            getNodeLinks(accessNode).resolvedSymbol = prop;
+                        }
                     }
                     return getTypeOfSymbol(prop);
                 }
@@ -5798,18 +5808,19 @@ namespace ts {
                 }
                 return getIndexedAccessTypeForTypeParameter(objectType, <TypeParameter>indexType);
             }
+            const apparentType = getApparentType(objectType);
             if (indexType.flags & TypeFlags.Union && !(indexType.flags & TypeFlags.Primitive)) {
                 const propTypes: Type[] = [];
                 for (const t of (<UnionType>indexType).types) {
-                    const propType = getPropertyTypeForIndexType(objectType, t, accessNode);
+                    const propType = getPropertyTypeForIndexType(apparentType, t, accessNode, /*cacheSymbol*/ false);
                     if (propType === unknownType) {
                         return unknownType;
                     }
                     propTypes.push(propType);
                 }
                 return getUnionType(propTypes);
             }
-            return getPropertyTypeForIndexType(objectType, indexType, accessNode);
+            return getPropertyTypeForIndexType(apparentType, indexType, accessNode, /*cacheSymbol*/ true);
         }
 
         function getTypeFromIndexedAccessTypeNode(node: IndexedAccessTypeNode) {
@@ -11644,55 +11655,9 @@ namespace ts {
                 return unknownType;
             }
 
-            const propName = getPropertyNameForIndexedAccess(indexExpression, indexType);
-            if (propName !== undefined) {
-                const prop = getPropertyOfType(getApparentType(objectType), propName);
-                if (prop) {
-                    getNodeLinks(node).resolvedSymbol = prop;
-                    if (isAssignmentTarget(node)) {
-                        if (isReferenceToReadonlyEntity(<Expression>node, prop) || isReferenceThroughNamespaceImport(<Expression>node)) {
-                            error(indexExpression, Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property, propName);
-                            return unknownType;
-                        }
-                    }
-                    return getTypeOfSymbol(prop);
-                }
-            }
-
             return getIndexedAccessType(objectType, indexType, node);
         }
 
-        /**
-         * If indexArgumentExpression is a string literal or number literal, returns its text.
-         * If indexArgumentExpression is a constant value, returns its string value.
-         * If indexArgumentExpression is a well known symbol, returns the property name corresponding
-         *    to this symbol, as long as it is a proper symbol reference.
-         * Otherwise, returns undefined.
-         */
-        function getPropertyNameForIndexedAccess(indexExpression: Expression, indexType: Type): string {
-            if (indexExpression.kind === SyntaxKind.StringLiteral || indexExpression.kind === SyntaxKind.NumericLiteral) {
-                return (<LiteralExpression>indexExpression).text;
-            }
-            if (indexExpression.kind === SyntaxKind.ElementAccessExpression || indexExpression.kind === SyntaxKind.PropertyAccessExpression) {
-                const value = getConstantValue(<ElementAccessExpression | PropertyAccessExpression>indexExpression);
-                if (value !== undefined) {
-                    return value.toString();
-                }
-            }
-            if (checkThatExpressionIsProperSymbolReference(indexExpression, indexType, /*reportError*/ false)) {
-                const rightHandSideName = (<Identifier>(<PropertyAccessExpression>indexExpression).name).text;
-                return getPropertyNameForKnownSymbolName(rightHandSideName);
-            }
-            return undefined;
-        }
-
-        /**
-         * A proper symbol reference requires the following:
-         *   1. The property access denotes a property that exists
-         *   2. The expression is of the form Symbol.<identifier>
-         *   3. The property access is of the primitive type symbol.
-         *   4. Symbol in this context resolves to the global Symbol object
-         */
         function checkThatExpressionIsProperSymbolReference(expression: Expression, expressionType: Type, reportError: boolean): boolean {
             if (expressionType === unknownType) {
                 // There is already an error, so no need to report one.