Preserve literal types in contextual unions

src/compiler/types.ts

@@ -2786,7 +2786,7 @@ namespace ts {
         /* @internal */ getNullType(): Type;
         /* @internal */ getESSymbolType(): Type;
         /* @internal */ getNeverType(): Type;
-        /* @internal */ getUnionType(types: Type[], subtypeReduction?: boolean): Type;
+        /* @internal */ getUnionType(types: Type[], subtypeReduction?: UnionReduction): Type;
         /* @internal */ createArrayType(elementType: Type): Type;
         /* @internal */ createPromiseType(type: Type): Type;
 
@@ -2841,6 +2841,13 @@ namespace ts {
         /* @internal */ getAccessibleSymbolChain(symbol: Symbol, enclosingDeclaration: Node | undefined, meaning: SymbolFlags, useOnlyExternalAliasing: boolean): Symbol[] | undefined;
     }
 
+    /* @internal */
+    export const enum UnionReduction {
+        None = 0,
+        Literal,
+        Subtype
+    }
+
     export enum NodeBuilderFlags {
         None                                    = 0,
         // Options

src/services/codefixes/inferFromUsage.ts

@@ -305,7 +305,7 @@ namespace ts.codefix {
                             }
                         }
                         if (types.length) {
-                            const type = checker.getWidenedType(checker.getUnionType(types, /*subtypeReduction*/ true));
+                            const type = checker.getWidenedType(checker.getUnionType(types, UnionReduction.Subtype));
                             paramTypes[parameterIndex] = isRestParameter ? checker.createArrayType(type) : type;
                         }
                     }
@@ -542,12 +542,12 @@ namespace ts.codefix {
                 return checker.getStringType();
             }
             else if (usageContext.candidateTypes) {
-                return checker.getWidenedType(checker.getUnionType(map(usageContext.candidateTypes, t => checker.getBaseTypeOfLiteralType(t)), /*subtypeReduction*/ true));
+                return checker.getWidenedType(checker.getUnionType(map(usageContext.candidateTypes, t => checker.getBaseTypeOfLiteralType(t)), UnionReduction.Subtype));
             }
             else if (usageContext.properties && hasCallContext(usageContext.properties.get("then" as __String))) {
                 const paramType = getParameterTypeFromCallContexts(0, usageContext.properties.get("then" as __String).callContexts, /*isRestParameter*/ false, checker);
                 const types = paramType.getCallSignatures().map(c => c.getReturnType());
-                return checker.createPromiseType(types.length ? checker.getUnionType(types, /*subtypeReduction*/ true) : checker.getAnyType());
+                return checker.createPromiseType(types.length ? checker.getUnionType(types, UnionReduction.Subtype) : checker.getAnyType());
             }
             else if (usageContext.properties && hasCallContext(usageContext.properties.get("push" as __String))) {
                 return checker.createArrayType(getParameterTypeFromCallContexts(0, usageContext.properties.get("push" as __String).callContexts, /*isRestParameter*/ false, checker));
@@ -610,7 +610,7 @@ namespace ts.codefix {
             }
 
             if (types.length) {
-                const type = checker.getWidenedType(checker.getUnionType(types, /*subtypeReduction*/ true));
+                const type = checker.getWidenedType(checker.getUnionType(types, UnionReduction.Subtype));
                 return isRestParameter ? checker.createArrayType(type) : type;
             }
             return undefined;

tests/baselines/reference/contextualTypeShouldBeLiteral.js

@@ -0,0 +1,136 @@
+//// [contextualTypeShouldBeLiteral.ts]
+interface X {
+    type: 'x';
+    value: string;
+    method(): void;
+}
+
+interface Y {
+    type: 'y';
+    value: 'none' | 'done';
+    method(): void;
+}
+
+function foo(bar: X | Y) { }
+
+foo({
+    type: 'y',
+    value: 'done',
+    method() {
+        this;
+        this.type;
+        this.value;
+    }
+});
+
+interface X2 {
+    type1: 'x';
+    value: string;
+    method(): void;
+}
+
+interface Y2 {
+    type2: 'y';
+    value: 'none' | 'done';
+    method(): void;
+}
+
+function foo2(bar: X2 | Y2) { }
+
+foo2({
+    type2: 'y',
+    value: 'done',
+    method() {
+        this;
+        this.value;
+    }
+});
+
+interface X3 {
+    type: 'x';
+    value: 1 | 2 | 3;
+    xtra: number;
+}
+
+interface Y3 {
+    type: 'y';
+    value: 11 | 12 | 13;
+    ytra: number;
+}
+
+let xy: X3 | Y3 = {
+    type: 'y',
+    value: 11,
+    ytra: 12
+};
+
+xy;
+
+
+interface LikeA {
+    x: 'x';
+    y: 'y';
+    value: string;
+    method(): void;
+}
+
+interface LikeB {
+    x: 'xx';
+    y: 'yy';
+    value: number;
+    method(): void;
+}
+
+let xyz: LikeA | LikeB = {
+    x: 'x',
+    y: 'y',
+    value: "foo",
+    method() {
+        this;
+        this.x;
+        this.y;
+        this.value;
+    }
+};
+
+xyz;
+
+//// [contextualTypeShouldBeLiteral.js]
+"use strict";
+function foo(bar) { }
+foo({
+    type: 'y',
+    value: 'done',
+    method: function () {
+        this;
+        this.type;
+        this.value;
+    }
+});
+function foo2(bar) { }
+foo2({
+    type2: 'y',
+    value: 'done',
+    method: function () {
+        this;
+        this.value;
+    }
+});
+var xy = {
+    type: 'y',
+    value: 11,
+    ytra: 12
+};
+xy;
+var xyz = {
+    x: 'x',
+    y: 'y',
+    value: "foo",
+    method: function () {
+        this;
+        this.x;
+        this.y;
+        this.value;
+    }
+};
+xyz;