Narrow on element access of literal (#26424)

* Narrow literal element accesses

This means that, for example, the tuple `[number, string?]` allows its
second element to be narrowed with element access:

```ts
export function f(pair: [number, string?]): string {
  return pair[1] ? pair[1] : 'nope';
}
```

* Update baselines

* Cleanup

* More cleanup

* Test dashes in property names

* More cleanup

* Delete undead code

src/compiler/binder.ts

@@ -723,6 +723,7 @@ namespace ts {
                 case SyntaxKind.Identifier:
                 case SyntaxKind.ThisKeyword:
                 case SyntaxKind.PropertyAccessExpression:
+                case SyntaxKind.ElementAccessExpression:
                     return isNarrowableReference(expr);
                 case SyntaxKind.CallExpression:
                     return hasNarrowableArgument(<CallExpression>expr);
@@ -737,10 +738,11 @@ namespace ts {
         }
 
         function isNarrowableReference(expr: Expression): boolean {
-            return expr.kind === SyntaxKind.Identifier ||
-                expr.kind === SyntaxKind.ThisKeyword ||
-                expr.kind === SyntaxKind.SuperKeyword ||
-                expr.kind === SyntaxKind.PropertyAccessExpression && isNarrowableReference((<PropertyAccessExpression>expr).expression);
+            return expr.kind === SyntaxKind.Identifier || expr.kind === SyntaxKind.ThisKeyword || expr.kind === SyntaxKind.SuperKeyword ||
+                isPropertyAccessExpression(expr) && isNarrowableReference(expr.expression) ||
+                isElementAccessExpression(expr) && expr.argumentExpression &&
+                (isStringLiteral(expr.argumentExpression) || isNumericLiteral(expr.argumentExpression)) &&
+                isNarrowableReference(expr.expression);
         }
 
         function hasNarrowableArgument(expr: CallExpression) {
@@ -2066,6 +2068,7 @@ namespace ts {
                     }
                     return checkStrictModeIdentifier(<Identifier>node);
                 case SyntaxKind.PropertyAccessExpression:
+                case SyntaxKind.ElementAccessExpression:
                     if (currentFlow && isNarrowableReference(<Expression>node)) {
                         node.flowNode = currentFlow;
                     }

src/compiler/checker.ts

@@ -9158,7 +9158,8 @@ namespace ts {
                             getNodeLinks(accessNode!).resolvedSymbol = prop;
                         }
                     }
-                    return getTypeOfSymbol(prop);
+                    const propType = getTypeOfSymbol(prop);
+                    return accessExpression ? getFlowTypeOfReference(accessExpression, propType) : propType;
                 }
                 if (isTupleType(objectType)) {
                     const restType = getRestTypeOfTupleType(objectType);
@@ -13778,9 +13779,10 @@ namespace ts {
                 case SyntaxKind.SuperKeyword:
                     return target.kind === SyntaxKind.SuperKeyword;
                 case SyntaxKind.PropertyAccessExpression:
-                    return target.kind === SyntaxKind.PropertyAccessExpression &&
-                        (<PropertyAccessExpression>source).name.escapedText === (<PropertyAccessExpression>target).name.escapedText &&
-                        isMatchingReference((<PropertyAccessExpression>source).expression, (<PropertyAccessExpression>target).expression);
+                case SyntaxKind.ElementAccessExpression:
+                    return (isPropertyAccessExpression(target) || isElementAccessExpression(target)) &&
+                        getAccessedPropertyName(source as PropertyAccessExpression | ElementAccessExpression) === getAccessedPropertyName(target) &&
+                        isMatchingReference((source as PropertyAccessExpression | ElementAccessExpression).expression, target.expression);
                 case SyntaxKind.BindingElement:
                     if (target.kind !== SyntaxKind.PropertyAccessExpression) return false;
                     const t = target as PropertyAccessExpression;
@@ -13796,6 +13798,12 @@ namespace ts {
             return false;
         }
 
+        function getAccessedPropertyName(access: PropertyAccessExpression | ElementAccessExpression): __String | undefined {
+            return isPropertyAccessExpression(access) ? access.name.escapedText :
+                isStringLiteral(access.argumentExpression) || isNumericLiteral(access.argumentExpression) ? escapeLeadingUnderscores(access.argumentExpression.text) :
+                undefined;
+        }
+
         function containsMatchingReference(source: Node, target: Node) {
             while (source.kind === SyntaxKind.PropertyAccessExpression) {
                 source = (<PropertyAccessExpression>source).expression;
@@ -14438,7 +14446,10 @@ namespace ts {
                     else if (flags & FlowFlags.Start) {
                         // Check if we should continue with the control flow of the containing function.
                         const container = (<FlowStart>flow).container;
-                        if (container && container !== flowContainer && reference.kind !== SyntaxKind.PropertyAccessExpression && reference.kind !== SyntaxKind.ThisKeyword) {
+                        if (container && container !== flowContainer &&
+                            reference.kind !== SyntaxKind.PropertyAccessExpression &&
+                            reference.kind !== SyntaxKind.ElementAccessExpression &&
+                            reference.kind !== SyntaxKind.ThisKeyword) {
                             flow = container.flowNode!;
                             continue;
                         }
@@ -14555,7 +14566,10 @@ namespace ts {
                     type = narrowTypeBySwitchOnDiscriminant(type, flow.switchStatement, flow.clauseStart, flow.clauseEnd);
                 }
                 else if (isMatchingReferenceDiscriminant(expr, type)) {
-                    type = narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => narrowTypeBySwitchOnDiscriminant(t, flow.switchStatement, flow.clauseStart, flow.clauseEnd));
+                    type = narrowTypeByDiscriminant(
+                        type,
+                        expr as PropertyAccessExpression | ElementAccessExpression,
+                        t => narrowTypeBySwitchOnDiscriminant(t, flow.switchStatement, flow.clauseStart, flow.clauseEnd));
                 }
                 return createFlowType(type, isIncomplete(flowType));
             }
@@ -14671,14 +14685,23 @@ namespace ts {
             }
 
             function isMatchingReferenceDiscriminant(expr: Expression, computedType: Type) {
-                return expr.kind === SyntaxKind.PropertyAccessExpression &&
-                    computedType.flags & TypeFlags.Union &&
-                    isMatchingReference(reference, (<PropertyAccessExpression>expr).expression) &&
-                    isDiscriminantProperty(computedType, (<PropertyAccessExpression>expr).name.escapedText);
+                if (!(computedType.flags & TypeFlags.Union) ||
+                    expr.kind !== SyntaxKind.PropertyAccessExpression && expr.kind !== SyntaxKind.ElementAccessExpression) {
+                    return false;
+                }
+                const access = expr as PropertyAccessExpression | ElementAccessExpression;
+                const name = getAccessedPropertyName(access);
+                if (!name) {
+                    return false;
+                }
+                return isMatchingReference(reference, access.expression) && isDiscriminantProperty(computedType, name);
             }
 
-            function narrowTypeByDiscriminant(type: Type, propAccess: PropertyAccessExpression, narrowType: (t: Type) => Type): Type {
-                const propName = propAccess.name.escapedText;
+            function narrowTypeByDiscriminant(type: Type, access: PropertyAccessExpression | ElementAccessExpression, narrowType: (t: Type) => Type): Type {
+                const propName = getAccessedPropertyName(access);
+                if (!propName) {
+                    return type;
+                }
                 const propType = getTypeOfPropertyOfType(type, propName);
                 const narrowedPropType = propType && narrowType(propType);
                 return propType === narrowedPropType ? type : filterType(type, t => isTypeComparableTo(getTypeOfPropertyOfType(t, propName)!, narrowedPropType!));
@@ -14689,7 +14712,7 @@ namespace ts {
                     return getTypeWithFacts(type, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy);
                 }
                 if (isMatchingReferenceDiscriminant(expr, declaredType)) {
-                    return narrowTypeByDiscriminant(type, <PropertyAccessExpression>expr, t => getTypeWithFacts(t, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy));
+                    return narrowTypeByDiscriminant(type, <PropertyAccessExpression | ElementAccessExpression>expr, t => getTypeWithFacts(t, assumeTrue ? TypeFacts.Truthy : TypeFacts.Falsy));
                 }
                 if (containsMatchingReferenceDiscriminant(reference, expr)) {
                     return declaredType;
@@ -14740,10 +14763,10 @@ namespace ts {
                             return narrowTypeByEquality(type, operator, left, assumeTrue);
                         }
                         if (isMatchingReferenceDiscriminant(left, declaredType)) {
-                            return narrowTypeByDiscriminant(type, <PropertyAccessExpression>left, t => narrowTypeByEquality(t, operator, right, assumeTrue));
+                            return narrowTypeByDiscriminant(type, <PropertyAccessExpression | ElementAccessExpression>left, t => narrowTypeByEquality(t, operator, right, assumeTrue));
                         }
                         if (isMatchingReferenceDiscriminant(right, declaredType)) {
-                            return narrowTypeByDiscriminant(type, <PropertyAccessExpression>right, t => narrowTypeByEquality(t, operator, left, assumeTrue));
+                            return narrowTypeByDiscriminant(type, <PropertyAccessExpression | ElementAccessExpression>right, t => narrowTypeByEquality(t, operator, left, assumeTrue));
                         }
                         if (containsMatchingReferenceDiscriminant(reference, left) || containsMatchingReferenceDiscriminant(reference, right)) {
                             return declaredType;
@@ -14982,6 +15005,7 @@ namespace ts {
                     case SyntaxKind.ThisKeyword:
                     case SyntaxKind.SuperKeyword:
                     case SyntaxKind.PropertyAccessExpression:
+                    case SyntaxKind.ElementAccessExpression:
                         return narrowTypeByTruthiness(type, expr, assumeTrue);
                     case SyntaxKind.CallExpression:
                         return narrowTypeByTypePredicate(type, <CallExpression>expr, assumeTrue);