Consider index signatures as optional properties in contextual type discrimination

src/compiler/checker.ts

@@ -10161,8 +10161,11 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
  return prop ? getTypeOfSymbol(prop) : undefined;
  }
 
- function getTypeOfPropertyOrIndexSignature(type: Type, name: __String): Type {
- return getTypeOfPropertyOfType(type, name) || getApplicableIndexInfoForName(type, name)?.type || unknownType;
+ function getTypeOfPropertyOrIndexSignature(type: Type, name: __String, addOptionalityToIndex: boolean): Type {
+ let propType;
+ return getTypeOfPropertyOfType(type, name) ||
+ (propType = getApplicableIndexInfoForName(type, name)?.type) && addOptionality(propType, /*isProperty*/ true, addOptionalityToIndex) ||
+ unknownType;
  }
 
  function isTypeAny(type: Type | undefined) {
@@ -22700,7 +22703,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
  let matched = false;
  for (let i = 0; i < types.length; i++) {
  if (include[i]) {
- const targetType = getTypeOfPropertyOfType(types[i], propertyName);
+ const targetType = getTypeOfPropertyOrIndexSignature(types[i], propertyName, /*addOptionalityToIndex*/ true);
  if (targetType && related(getDiscriminatingType(), targetType)) {
  matched = true;
  }
@@ -27013,7 +27016,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
  propType = removeNullable && optionalChain ? getOptionalType(propType) : propType;
  const narrowedPropType = narrowType(propType);
  return filterType(type, t => {
- const discriminantType = getTypeOfPropertyOrIndexSignature(t, propName);
+ const discriminantType = getTypeOfPropertyOrIndexSignature(t, propName, /*addOptionalityToIndex*/ false);
  return !(discriminantType.flags & TypeFlags.Never) && !(narrowedPropType.flags & TypeFlags.Never) && areTypesComparable(narrowedPropType, discriminantType);
  });
  }

tests/baselines/reference/discriminatedUnionWithIndexSignature.js

@@ -0,0 +1,44 @@
+//// [tests/cases/compiler/discriminatedUnionWithIndexSignature.ts] ////
+
+//// [discriminatedUnionWithIndexSignature.ts]
+export interface UnionAltA {
+ type?: 'text';
+}
+
+export interface UnionAltB {
+ type?: 'image' | 'video' | 'document';
+}
+
+export type ValueUnion = UnionAltA | UnionAltB;
+
+export type MapOrSingleton =
+ | {
+ [key: string]: ValueUnion;
+ }
+ | ValueUnion;
+
+const withoutAsConst: MapOrSingleton = {
+ 1: {
+ type: 'text' /*as const*/,
+ },
+};
+
+const withAsConst: MapOrSingleton = {
+ 1: {
+ type: 'text' as const,
+ },
+};
+
+//// [discriminatedUnionWithIndexSignature.js]
+"use strict";
+Object.defineProperty(exports, "__esModule", { value: true });
+var withoutAsConst = {
+ 1: {
+ type: 'text' /*as const*/,
+ },
+};
+var withAsConst = {
+ 1: {
+ type: 'text',
+ },
+};

tests/baselines/reference/discriminatedUnionWithIndexSignature.symbols

@@ -0,0 +1,59 @@
+//// [tests/cases/compiler/discriminatedUnionWithIndexSignature.ts] ////
+
+=== discriminatedUnionWithIndexSignature.ts ===
+export interface UnionAltA {
+>UnionAltA : Symbol(UnionAltA, Decl(discriminatedUnionWithIndexSignature.ts, 0, 0))
+
+ type?: 'text';
+>type : Symbol(UnionAltA.type, Decl(discriminatedUnionWithIndexSignature.ts, 0, 28))
+}
+
+export interface UnionAltB {
+>UnionAltB : Symbol(UnionAltB, Decl(discriminatedUnionWithIndexSignature.ts, 2, 1))
+
+ type?: 'image' | 'video' | 'document';
+>type : Symbol(UnionAltB.type, Decl(discriminatedUnionWithIndexSignature.ts, 4, 28))
+}
+
+export type ValueUnion = UnionAltA | UnionAltB;
+>ValueUnion : Symbol(ValueUnion, Decl(discriminatedUnionWithIndexSignature.ts, 6, 1))
+>UnionAltA : Symbol(UnionAltA, Decl(discriminatedUnionWithIndexSignature.ts, 0, 0))
+>UnionAltB : Symbol(UnionAltB, Decl(discriminatedUnionWithIndexSignature.ts, 2, 1))
+
+export type MapOrSingleton =
+>MapOrSingleton : Symbol(MapOrSingleton, Decl(discriminatedUnionWithIndexSignature.ts, 8, 47))
+
+ | {
+ [key: string]: ValueUnion;
+>key : Symbol(key, Decl(discriminatedUnionWithIndexSignature.ts, 12, 9))
+>ValueUnion : Symbol(ValueUnion, Decl(discriminatedUnionWithIndexSignature.ts, 6, 1))
+ }
+ | ValueUnion;
+>ValueUnion : Symbol(ValueUnion, Decl(discriminatedUnionWithIndexSignature.ts, 6, 1))
+
+const withoutAsConst: MapOrSingleton = {
+>withoutAsConst : Symbol(withoutAsConst, Decl(discriminatedUnionWithIndexSignature.ts, 16, 5))
+>MapOrSingleton : Symbol(MapOrSingleton, Decl(discriminatedUnionWithIndexSignature.ts, 8, 47))
+
+ 1: {
+>1 : Symbol(1, Decl(discriminatedUnionWithIndexSignature.ts, 16, 40))
+
+ type: 'text' /*as const*/,
+>type : Symbol(type, Decl(discriminatedUnionWithIndexSignature.ts, 17, 8))
+
+ },
+};
+
+const withAsConst: MapOrSingleton = {
+>withAsConst : Symbol(withAsConst, Decl(discriminatedUnionWithIndexSignature.ts, 22, 5))
+>MapOrSingleton : Symbol(MapOrSingleton, Decl(discriminatedUnionWithIndexSignature.ts, 8, 47))
+
+ 1: {
+>1 : Symbol(1, Decl(discriminatedUnionWithIndexSignature.ts, 22, 37))
+
+ type: 'text' as const,
+>type : Symbol(type, Decl(discriminatedUnionWithIndexSignature.ts, 23, 8))
+>const : Symbol(const)
+
+ },
+};

tests/baselines/reference/discriminatedUnionWithIndexSignature.types

@@ -0,0 +1,55 @@
+//// [tests/cases/compiler/discriminatedUnionWithIndexSignature.ts] ////
+
+=== discriminatedUnionWithIndexSignature.ts ===
+export interface UnionAltA {
+ type?: 'text';
+>type : "text" | undefined
+}
+
+export interface UnionAltB {
+ type?: 'image' | 'video' | 'document';
+>type : "image" | "video" | "document" | undefined
+}
+
+export type ValueUnion = UnionAltA | UnionAltB;
+>ValueUnion : UnionAltA | UnionAltB
+
+export type MapOrSingleton =
+>MapOrSingleton : ValueUnion | { [key: string]: ValueUnion; }
+
+ | {
+ [key: string]: ValueUnion;
+>key : string
+ }
+ | ValueUnion;
+
+const withoutAsConst: MapOrSingleton = {
+>withoutAsConst : MapOrSingleton
+>{ 1: { type: 'text' /*as const*/, },} : { 1: { type: "text"; }; }
+
+ 1: {
+>1 : { type: "text"; }
+>{ type: 'text' /*as const*/, } : { type: "text"; }
+
+ type: 'text' /*as const*/,
+>type : "text"
+>'text' : "text"
+
+ },
+};
+
+const withAsConst: MapOrSingleton = {
+>withAsConst : MapOrSingleton
+>{ 1: { type: 'text' as const, },} : { 1: { type: "text"; }; }
+
+ 1: {
+>1 : { type: "text"; }
+>{ type: 'text' as const, } : { type: "text"; }
+
+ type: 'text' as const,
+>type : "text"
+>'text' as const : "text"
+>'text' : "text"
+
+ },
+};

tests/cases/compiler/discriminatedUnionWithIndexSignature.ts

@@ -0,0 +1,29 @@
+// @strict: true
+
+export interface UnionAltA {
+ type?: 'text';
+}
+
+export interface UnionAltB {
+ type?: 'image' | 'video' | 'document';
+}
+
+export type ValueUnion = UnionAltA | UnionAltB;
+
+export type MapOrSingleton =
+ | {
+ [key: string]: ValueUnion;
+ }
+ | ValueUnion;
+
+const withoutAsConst: MapOrSingleton = {
+ 1: {
+ type: 'text' /*as const*/,
+ },
+};
+
+const withAsConst: MapOrSingleton = {
+ 1: {
+ type: 'text' as const,
+ },
+};