Let properties having primitive types discriminate object unions

src/compiler/binder.ts

@@ -342,7 +342,7 @@ interface ActiveLabel {
 export function getModuleInstanceState(node: ModuleDeclaration, visited?: Map<number, ModuleInstanceState | undefined>): ModuleInstanceState {
     if (node.body && !node.body.parent) {
         // getModuleInstanceStateForAliasTarget needs to walk up the parent chain, so parent pointers must be set on this tree already
-        setParent(node.body, node);
+        setParent(node.body as ModuleDeclaration["parent"], node);
         setParentRecursive(node.body, /*incremental*/ false);
     }
     return node.body ? getModuleInstanceStateCached(node.body, visited) : ModuleInstanceState.Instantiated;

src/compiler/checker.ts

@@ -15050,6 +15050,9 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (isLiteralType(type) || isPatternLiteralType(type)) {
                 checkFlags |= CheckFlags.HasLiteralType;
             }
+            if (isUnion && (type.flags & TypeFlags.Primitive)) {
+                checkFlags |= CheckFlags.HasPrimitiveType;
+            }
             if (type.flags & TypeFlags.Never && type !== uniqueLiteralType) {
                 checkFlags |= CheckFlags.HasNeverType;
             }
@@ -27281,16 +27284,29 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return false;
     }
 
-    function isDiscriminantProperty(type: Type | undefined, name: __String) {
+    // The flag `considerNonUniformPrimitivePropDiscriminant`, when enabled, introduces a new criterion for a property to be considered discriminant:
+    // 1 The property must be non-uniform
+    // 2 The property must include at least one primitive type as a possible type
+    function isDiscriminantProperty(type: Type | undefined, name: __String, considerNonUniformPrimitivePropDiscriminant: boolean = false) {
         if (type && type.flags & TypeFlags.Union) {
             const prop = getUnionOrIntersectionProperty(type as UnionType, name);
             if (prop && getCheckFlags(prop) & CheckFlags.SyntheticProperty) {
+                const propType = getTypeOfSymbol(prop);
                 // NOTE: cast to TransientSymbol should be safe because only TransientSymbols can have CheckFlags.SyntheticProperty
-                if ((prop as TransientSymbol).links.isDiscriminantProperty === undefined) {
-                    (prop as TransientSymbol).links.isDiscriminantProperty = ((prop as TransientSymbol).links.checkFlags & CheckFlags.Discriminant) === CheckFlags.Discriminant &&
-                        !isGenericType(getTypeOfSymbol(prop));
+                const transientSymbol = prop as TransientSymbol;
+                transientSymbol.links.isDiscriminantProperty ??= new Map();
+
+                let isDiscriminant = transientSymbol.links.isDiscriminantProperty.get(considerNonUniformPrimitivePropDiscriminant);
+
+                if (isDiscriminant === undefined) {
+                    isDiscriminant = (((transientSymbol.links.checkFlags & CheckFlags.Discriminant) === CheckFlags.Discriminant)
+                        || !!(considerNonUniformPrimitivePropDiscriminant && (transientSymbol.links.checkFlags & (CheckFlags.HasNonUniformType | CheckFlags.HasPrimitiveType))))
+                        && !isGenericType(propType);
+
+                    transientSymbol.links.isDiscriminantProperty.set(considerNonUniformPrimitivePropDiscriminant, isDiscriminant);
                 }
-                return !!(prop as TransientSymbol).links.isDiscriminantProperty;
+
+                return isDiscriminant;
             }
         }
         return false;
@@ -28858,7 +28874,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                     const name = getAccessedPropertyName(access);
                     if (name) {
                         const type = declaredType.flags & TypeFlags.Union && isTypeSubsetOf(computedType, declaredType) ? declaredType : computedType;
-                        if (isDiscriminantProperty(type, name)) {
+                        if (isDiscriminantProperty(type, name, /*considerNonUniformPrimitivePropDiscriminant*/ true)) {
                             return access;
                         }
                     }

src/compiler/types.ts

@@ -2756,7 +2756,7 @@ export interface ArrowFunction extends Expression, FunctionLikeDeclarationBase,
     readonly modifiers?: NodeArray<Modifier>;
     readonly equalsGreaterThanToken: EqualsGreaterThanToken;
     readonly body: ConciseBody;
-    readonly name: never;
+    readonly name?: never;
 }
 
 // The text property of a LiteralExpression stores the interpreted value of the literal in text form. For a StringLiteral,
@@ -6020,7 +6020,7 @@ export interface SymbolLinks {
     leftSpread?: Symbol;                        // Left source for synthetic spread property
     rightSpread?: Symbol;                       // Right source for synthetic spread property
     syntheticOrigin?: Symbol;                   // For a property on a mapped or spread type, points back to the original property
-    isDiscriminantProperty?: boolean;           // True if discriminant synthetic property
+    isDiscriminantProperty?: Map<boolean, boolean | undefined> // Key is a flag, value is true if discriminant synthetic property
     resolvedExports?: SymbolTable;              // Resolved exports of module or combined early- and late-bound static members of a class.
     resolvedMembers?: SymbolTable;              // Combined early- and late-bound members of a symbol
     exportsChecked?: boolean;                   // True if exports of external module have been checked
@@ -6071,6 +6071,7 @@ export const enum CheckFlags {
     Mapped            = 1 << 18,        // Property of mapped type
     StripOptional     = 1 << 19,        // Strip optionality in mapped property
     Unresolved        = 1 << 20,        // Unresolved type alias symbol
+    HasPrimitiveType  = 1 << 21,        // Synthetic property with at least one primitive type in constituents
     Synthetic = SyntheticProperty | SyntheticMethod,
     Discriminant = HasNonUniformType | HasLiteralType,
     Partial = ReadPartial | WritePartial,

tests/baselines/reference/api/typescript.d.ts

@@ -4989,7 +4989,7 @@ declare namespace ts {
         readonly modifiers?: NodeArray<Modifier>;
         readonly equalsGreaterThanToken: EqualsGreaterThanToken;
         readonly body: ConciseBody;
-        readonly name: never;
+        readonly name?: never;
     }
     interface LiteralLikeNode extends Node {
         text: string;

tests/baselines/reference/destructuringControlFlow.types

@@ -104,10 +104,10 @@ function f2(obj: [number, string] | null[]) {
 >    : ^^^^^^^^^^^^^^^^^^^^^^^^^
 >0 : 0
 >  : ^
->obj[1] : string | null
->       : ^^^^^^^^^^^^^
->obj : [number, string] | null[]
->    : ^^^^^^^^^^^^^^^^^^^^^^^^^
+>obj[1] : string
+>       : ^^^^^^
+>obj : [number, string]
+>    : ^^^^^^^^^^^^^^^^
 >1 : 1
 >  : ^
 
@@ -116,8 +116,8 @@ function f2(obj: [number, string] | null[]) {
 >   : ^^^^^^
 >obj[0] : number
 >       : ^^^^^^
->obj : [number, string] | null[]
->    : ^^^^^^^^^^^^^^^^^^^^^^^^^
+>obj : [number, string]
+>    : ^^^^^^^^^^^^^^^^
 >0 : 0
 >  : ^
 
@@ -126,8 +126,8 @@ function f2(obj: [number, string] | null[]) {
 >   : ^^^^^^
 >obj[1] : string
 >       : ^^^^^^
->obj : [number, string] | null[]
->    : ^^^^^^^^^^^^^^^^^^^^^^^^^
+>obj : [number, string]
+>    : ^^^^^^^^^^^^^^^^
 >1 : 1
 >  : ^
 
@@ -136,22 +136,22 @@ function f2(obj: [number, string] | null[]) {
 >   : ^^^^^^
 >d1 : string
 >   : ^^^^^^
->obj : [number, string] | null[]
->    : ^^^^^^^^^^^^^^^^^^^^^^^^^
+>obj : [number, string]
+>    : ^^^^^^^^^^^^^^^^
 
         ([c0, c1] = obj);
->([c0, c1] = obj) : [number, string] | null[]
->                 : ^^^^^^^^^^^^^^^^^^^^^^^^^
->[c0, c1] = obj : [number, string] | null[]
->               : ^^^^^^^^^^^^^^^^^^^^^^^^^
+>([c0, c1] = obj) : [number, string]
+>                 : ^^^^^^^^^^^^^^^^
+>[c0, c1] = obj : [number, string]
+>               : ^^^^^^^^^^^^^^^^
 >[c0, c1] : [number, string]
 >         : ^^^^^^^^^^^^^^^^
 >c0 : number
 >   : ^^^^^^
 >c1 : string
 >   : ^^^^^^
->obj : [number, string] | null[]
->    : ^^^^^^^^^^^^^^^^^^^^^^^^^
+>obj : [number, string]
+>    : ^^^^^^^^^^^^^^^^
     }
 }
 

tests/baselines/reference/discriminatedUnionTypes2.errors.txt

@@ -1,9 +1,10 @@
 discriminatedUnionTypes2.ts(27,30): error TS2353: Object literal may only specify known properties, and 'c' does not exist in type '{ a: null; b: string; }'.
 discriminatedUnionTypes2.ts(32,11): error TS2339: Property 'b' does not exist on type '{ a: 0; b: string; } | { a: T; c: number; }'.
   Property 'b' does not exist on type '{ a: T; c: number; }'.
+discriminatedUnionTypes2.ts(132,11): error TS2339: Property 'value' does not exist on type 'never'.
 
 
-==== discriminatedUnionTypes2.ts (2 errors) ====
+==== discriminatedUnionTypes2.ts (3 errors) ====
     function f10(x : { kind: false, a: string } | { kind: true, b: string } | { kind: string, c: string }) {
         if (x.kind === false) {
             x.a;
@@ -141,6 +142,8 @@ discriminatedUnionTypes2.ts(32,11): error TS2339: Property 'b' does not exist on
         }
         else {
             x.value;  // number
+              ~~~~~
+!!! error TS2339: Property 'value' does not exist on type 'never'.
         }
     }
 

tests/baselines/reference/discriminatedUnionTypes2.symbols

@@ -369,9 +369,7 @@ function foo1(x: RuntimeValue & { type: 'number' }) {
     }
     else {
         x.value;  // number
->x.value : Symbol(value, Decl(discriminatedUnionTypes2.ts, 122, 23))
 >x : Symbol(x, Decl(discriminatedUnionTypes2.ts, 126, 14))
->value : Symbol(value, Decl(discriminatedUnionTypes2.ts, 122, 23))
     }
 }
 

tests/baselines/reference/discriminatedUnionTypes2.types

@@ -578,12 +578,12 @@ function foo1(x: RuntimeValue & { type: 'number' }) {
     }
     else {
         x.value;  // number
->x.value : number
->        : ^^^^^^
->x : { type: "number"; value: number; } & { type: "number"; }
->  : ^^^^^^^^        ^^^^^^^^^      ^^^^^^^^^^^^^^        ^^^
->value : number
->      : ^^^^^^
+>x.value : any
+>        : ^^^
+>x : never
+>  : ^^^^^
+>value : any
+>      : ^^^
     }
 }
 

tests/baselines/reference/narrowUnionOfObjectsByPrimitiveProperty.errors.txt

@@ -0,0 +1,128 @@
+narrowUnionOfObjectsByPrimitiveProperty.ts(62,1): error TS2322: Type '{ prop: string | number; other: never; }' is not assignable to type '{ prop: string; other: string; } | { prop: number; other: number; }'.
+  Type '{ prop: string | number; other: never; }' is not assignable to type '{ prop: number; other: number; }'.
+    Types of property 'prop' are incompatible.
+      Type 'string | number' is not assignable to type 'number'.
+        Type 'string' is not assignable to type 'number'.
+narrowUnionOfObjectsByPrimitiveProperty.ts(70,1): error TS2322: Type '{ prop: string | number; other: never; }' is not assignable to type '{ prop: string; other: string; } | { prop: number; other: number; }'.
+  Type '{ prop: string | number; other: never; }' is not assignable to type '{ prop: number; other: number; }'.
+    Types of property 'prop' are incompatible.
+      Type 'string | number' is not assignable to type 'number'.
+        Type 'string' is not assignable to type 'number'.
+narrowUnionOfObjectsByPrimitiveProperty.ts(96,32): error TS2339: Property 'label' does not exist on type 'never'.
+
+
+==== narrowUnionOfObjectsByPrimitiveProperty.ts (3 errors) ====
+    export {}
+
+    interface State<Type> {
+      state: Type;
+    }
+
+    interface UserName {
+      first: string;
+      last?: string;
+    }
+
+    const nameState = {} as {
+      value: string;
+      state: State<string>;
+    } | {
+      value: UserName;
+      state: State<UserName>;
+    }
+
+    if (typeof nameState.value === "string") {
+      nameState.state satisfies State<string>;
+    } else {
+      nameState.state satisfies State<UserName>;
+    }
+
+
+    declare const arr: [string, number] | [number, string];
+    if (typeof arr[0] === "string") {
+      arr[1] satisfies number;
+    } else {
+      arr[1] satisfies string;
+    }
+
+
+    function aStringOrANumber<T extends { a: string } | { a: number }>(param: T): T extends { a: string } ? string : T extends { a: number } ? number : never {
+      if (typeof param.a === "string") {
+        return param.a.repeat(3);
+      }
+      if (typeof param.a === "number") {
+        return Math.exp(param.a);
+      }
+      throw new Error()
+    }
+
+    aStringOrANumber({ a: "string" })
+    aStringOrANumber({ a: 42 })
+
+
+    // The following two tests ensure that the discriminativeness of property 'prop'
+    // is treated differently in assignability and narrowing, and that the discriminativeness is properly cached.
+    declare let obj: { prop: string, other: string } | { prop: number, other: number }
+
+    // Here, we first perform narrowing, but the subsequent assignability should not be affected.
+    // We expect an error there because of an incorrect value assigned to 'prop'.
+    // See contextualTypeWithUnionTypeObjectLiteral.ts
+    if(typeof obj.prop === "string") {
+      obj.other.repeat(3);
+    } else {
+      Math.exp(obj.other);
+    }
+
+    obj = { prop: Math.random() > 0.5 ? "whatever" : 42, other: "irrelevant" as never }
+    ~~~
+!!! error TS2322: Type '{ prop: string | number; other: never; }' is not assignable to type '{ prop: string; other: string; } | { prop: number; other: number; }'.
+!!! error TS2322:   Type '{ prop: string | number; other: never; }' is not assignable to type '{ prop: number; other: number; }'.
+!!! error TS2322:     Types of property 'prop' are incompatible.
+!!! error TS2322:       Type 'string | number' is not assignable to type 'number'.
+!!! error TS2322:         Type 'string' is not assignable to type 'number'.
+
+
+    declare let obj2: { prop: string, other: string } | { prop: number, other: number }
+
+    // Here, we first assign a value to 'obj2' and then perform narrowing.
+    // We expect an error here because of an incorrect value assigned to 'prop', like above,
+    // but the subsequent narrowing should not be affected by the assignability.
+    obj2 = { prop: Math.random() > 0.5 ? "whatever" : 42, other: "irrelevant" as never }
+    ~~~~
+!!! error TS2322: Type '{ prop: string | number; other: never; }' is not assignable to type '{ prop: string; other: string; } | { prop: number; other: number; }'.
+!!! error TS2322:   Type '{ prop: string | number; other: never; }' is not assignable to type '{ prop: number; other: number; }'.
+!!! error TS2322:     Types of property 'prop' are incompatible.
+!!! error TS2322:       Type 'string | number' is not assignable to type 'number'.
+!!! error TS2322:         Type 'string' is not assignable to type 'number'.
+
+    if(typeof obj2.prop === "string") {
+      obj2.other.repeat(3);
+    } else {
+      Math.exp(obj2.other);
+    }
+
+
+    interface ILocalizedString {
+        original: string;
+        value: string;
+    }
+
+    type Opt = ({
+        label: ILocalizedString;
+        alias?: string;
+    } | {
+        label: string;
+        alias: string;
+    })
+
+    declare const opt: Opt
+
+    if (typeof opt.label === 'string') {
+        const l = opt.label;
+        const a = opt.alias ?? opt.label;
+                                   ~~~~~
+!!! error TS2339: Property 'label' does not exist on type 'never'.
+    } else {
+        const l = opt.label;
+        const a = opt.alias ?? opt.label.original;
+    }