Allow non-unit types in union discriminants

src/compiler/checker.ts

@@ -14212,12 +14212,26 @@ namespace ts {
             return undefined;
         }
 
+        function isDiscriminantType(type: Type): boolean {
+            if (type.flags & TypeFlags.Union) {
+                if (type.flags & (TypeFlags.Boolean | TypeFlags.EnumLiteral)) {
+                    return true;
+                }
+                let combined = 0;
+                for (const t of (<UnionType>type).types) combined |= t.flags;
+                if (combined & TypeFlags.Unit && !(combined & TypeFlags.Instantiable)) {
+                    return true;
+                }
+            }
+            return false;
+        }
+
         function isDiscriminantProperty(type: Type | undefined, name: __String) {
             if (type && type.flags & TypeFlags.Union) {
                 const prop = getUnionOrIntersectionProperty(<UnionType>type, name);
                 if (prop && getCheckFlags(prop) & CheckFlags.SyntheticProperty) {
                     if ((<TransientSymbol>prop).isDiscriminantProperty === undefined) {
-                        (<TransientSymbol>prop).isDiscriminantProperty = !!((<TransientSymbol>prop).checkFlags & CheckFlags.HasNonUniformType) && isLiteralType(getTypeOfSymbol(prop));
+                        (<TransientSymbol>prop).isDiscriminantProperty = !!((<TransientSymbol>prop).checkFlags & CheckFlags.HasNonUniformType) && isDiscriminantType(getTypeOfSymbol(prop));
                     }
                     return !!(<TransientSymbol>prop).isDiscriminantProperty;
                 }

tests/baselines/reference/discriminatedUnionTypes2.errors.txt

@@ -0,0 +1,86 @@
+tests/cases/conformance/types/union/discriminatedUnionTypes2.ts(27,30): error TS2322: Type '{ a: null; b: string; c: number; }' is not assignable to type '{ a: null; b: string; } | { a: string; c: number; }'.
+  Object literal may only specify known properties, and 'c' does not exist in type '{ a: null; b: string; }'.
+tests/cases/conformance/types/union/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; }'.
+
+
+==== tests/cases/conformance/types/union/discriminatedUnionTypes2.ts (2 errors) ====
+    function f10(x : { kind: false, a: string } | { kind: true, b: string } | { kind: string, c: string }) {
+        if (x.kind === false) {
+            x.a;
+        }
+        else if (x.kind === true) {
+            x.b;
+        }
+        else {
+            x.c;
+        }
+    }
+
+    function f11(x : { kind: false, a: string } | { kind: true, b: string } | { kind: string, c: string }) {
+        switch (x.kind) {
+            case false:
+                x.a;
+                break;
+            case true:
+                x.b;
+                break;
+            default:
+                x.c;
+        }
+    }
+
+    function f13(x: { a: null; b: string } | { a: string, c: number }) {
+        x = { a: null, b: "foo", c: 4};  // Error
+                                 ~~~~
+!!! error TS2322: Type '{ a: null; b: string; c: number; }' is not assignable to type '{ a: null; b: string; } | { a: string; c: number; }'.
+!!! error TS2322:   Object literal may only specify known properties, and 'c' does not exist in type '{ a: null; b: string; }'.
+    }
+
+    function f14<T>(x: { a: 0; b: string } | { a: T, c: number }) {
+        if (x.a === 0) {
+            x.b;  // Error
+              ~
+!!! error TS2339: Property 'b' does not exist on type '{ a: 0; b: string; } | { a: T; c: number; }'.
+!!! error TS2339:   Property 'b' does not exist on type '{ a: T; c: number; }'.
+        }
+    }
+
+    type Result<T> = { error?: undefined, value: T } | { error: Error };
+
+    function f15(x: Result<number>) {
+        if (!x.error) {
+            x.value;
+        }
+        else {
+            x.error.message;
+        }
+    }
+
+    f15({ value: 10 });
+    f15({ error: new Error("boom") });
+
+    // Repro from #24193
+
+    interface WithError {
+        error: Error
+        data: null
+    }
+
+    interface WithoutError<Data> {
+        error: null
+        data: Data
+    }
+
+    type DataCarrier<Data> = WithError | WithoutError<Data>
+
+    function f20<Data>(carrier: DataCarrier<Data>) {
+        if (carrier.error === null) {
+            const error: null = carrier.error
+            const data: Data = carrier.data
+        } else {
+            const error: Error = carrier.error
+            const data: null = carrier.data
+        }
+    }
+

tests/baselines/reference/discriminatedUnionTypes2.js

@@ -0,0 +1,128 @@
+//// [discriminatedUnionTypes2.ts]
+function f10(x : { kind: false, a: string } | { kind: true, b: string } | { kind: string, c: string }) {
+    if (x.kind === false) {
+        x.a;
+    }
+    else if (x.kind === true) {
+        x.b;
+    }
+    else {
+        x.c;
+    }
+}
+
+function f11(x : { kind: false, a: string } | { kind: true, b: string } | { kind: string, c: string }) {
+    switch (x.kind) {
+        case false:
+            x.a;
+            break;
+        case true:
+            x.b;
+            break;
+        default:
+            x.c;
+    }
+}
+
+function f13(x: { a: null; b: string } | { a: string, c: number }) {
+    x = { a: null, b: "foo", c: 4};  // Error
+}
+
+function f14<T>(x: { a: 0; b: string } | { a: T, c: number }) {
+    if (x.a === 0) {
+        x.b;  // Error
+    }
+}
+
+type Result<T> = { error?: undefined, value: T } | { error: Error };
+
+function f15(x: Result<number>) {
+    if (!x.error) {
+        x.value;
+    }
+    else {
+        x.error.message;
+    }
+}
+
+f15({ value: 10 });
+f15({ error: new Error("boom") });
+
+// Repro from #24193
+
+interface WithError {
+    error: Error
+    data: null
+}
+
+interface WithoutError<Data> {
+    error: null
+    data: Data
+}
+
+type DataCarrier<Data> = WithError | WithoutError<Data>
+
+function f20<Data>(carrier: DataCarrier<Data>) {
+    if (carrier.error === null) {
+        const error: null = carrier.error
+        const data: Data = carrier.data
+    } else {
+        const error: Error = carrier.error
+        const data: null = carrier.data
+    }
+}
+
+
+//// [discriminatedUnionTypes2.js]
+"use strict";
+function f10(x) {
+    if (x.kind === false) {
+        x.a;
+    }
+    else if (x.kind === true) {
+        x.b;
+    }
+    else {
+        x.c;
+    }
+}
+function f11(x) {
+    switch (x.kind) {
+        case false:
+            x.a;
+            break;
+        case true:
+            x.b;
+            break;
+        default:
+            x.c;
+    }
+}
+function f13(x) {
+    x = { a: null, b: "foo", c: 4 }; // Error
+}
+function f14(x) {
+    if (x.a === 0) {
+        x.b; // Error
+    }
+}
+function f15(x) {
+    if (!x.error) {
+        x.value;
+    }
+    else {
+        x.error.message;
+    }
+}
+f15({ value: 10 });
+f15({ error: new Error("boom") });
+function f20(carrier) {
+    if (carrier.error === null) {
+        var error = carrier.error;
+        var data = carrier.data;
+    }
+    else {
+        var error = carrier.error;
+        var data = carrier.data;
+    }
+}