Distributive conditional type applied to 'never' produces 'never'

ahejlsberg · ahejlsberg · commit 56e6deefc73b · 2018-02-28T06:39:17.000-08:00
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -8229,8 +8229,8 @@ namespace ts {
                 const inferredTypes = map(inferences, inference => getTypeFromInference(inference) || emptyObjectType);
                 combinedMapper = combineTypeMappers(mapper, createTypeMapper(root.inferTypeParameters, inferredTypes));
             }
-            // Return union of trueType and falseType for any and never since they match anything
-            if (checkType.flags & TypeFlags.Any || (checkType.flags & TypeFlags.Never && !(extendsType.flags & TypeFlags.Never))) {
+            // Return union of trueType and falseType for 'any' since it matches anything
+            if (checkType.flags & TypeFlags.Any) {
                 return getUnionType([instantiateType(root.trueType, combinedMapper || mapper), instantiateType(root.falseType, mapper)]);
             }
             // Instantiate the extends type including inferences for 'infer T' type parameters
@@ -8911,7 +8911,7 @@ namespace ts {
             if (root.isDistributive) {
                 const checkType = <TypeParameter>root.checkType;
                 const instantiatedType = mapper(checkType);
-                if (checkType !== instantiatedType && instantiatedType.flags & TypeFlags.Union) {
+                if (checkType !== instantiatedType && instantiatedType.flags & (TypeFlags.Union | TypeFlags.Never)) {
                     return mapType(instantiatedType, t => getConditionalType(root, createReplacementMapper(checkType, t, mapper)));
                 }
             }
@@ -12449,6 +12449,9 @@ namespace ts {
         // is a union type, the mapping function is applied to each constituent type and a union
         // of the resulting types is returned.
         function mapType(type: Type, mapper: (t: Type) => Type, noReductions?: boolean): Type {
+            if (type.flags & TypeFlags.Never) {
+                return type;
+            }
             if (!(type.flags & TypeFlags.Union)) {
                 return mapper(type);
             }

Original file line number	Diff line number	Diff line change
`@@ -8229,8 +8229,8 @@ namespace ts {`
`8229`	`8229`	`const inferredTypes = map(inferences, inference => getTypeFromInference(inference) \|\| emptyObjectType);`
`8230`	`8230`	`combinedMapper = combineTypeMappers(mapper, createTypeMapper(root.inferTypeParameters, inferredTypes));`
`8231`	`8231`	`}`
`8232`		`- // Return union of trueType and falseType for any and never since they match anything`
`8233`		`- if (checkType.flags & TypeFlags.Any \|\| (checkType.flags & TypeFlags.Never && !(extendsType.flags & TypeFlags.Never))) {`
	`8232`	`+ // Return union of trueType and falseType for 'any' since it matches anything`
	`8233`	`+ if (checkType.flags & TypeFlags.Any) {`
`8234`	`8234`	`return getUnionType([instantiateType(root.trueType, combinedMapper \|\| mapper), instantiateType(root.falseType, mapper)]);`
`8235`	`8235`	`}`
`8236`	`8236`	`// Instantiate the extends type including inferences for 'infer T' type parameters`
`@@ -8911,7 +8911,7 @@ namespace ts {`
`8911`	`8911`	`if (root.isDistributive) {`
`8912`	`8912`	`const checkType = <TypeParameter>root.checkType;`
`8913`	`8913`	`const instantiatedType = mapper(checkType);`
`8914`		`- if (checkType !== instantiatedType && instantiatedType.flags & TypeFlags.Union) {`
	`8914`	`+ if (checkType !== instantiatedType && instantiatedType.flags & (TypeFlags.Union \| TypeFlags.Never)) {`
`8915`	`8915`	`return mapType(instantiatedType, t => getConditionalType(root, createReplacementMapper(checkType, t, mapper)));`
`8916`	`8916`	`}`
`8917`	`8917`	`}`
`@@ -12449,6 +12449,9 @@ namespace ts {`
`12449`	`12449`	`// is a union type, the mapping function is applied to each constituent type and a union`
`12450`	`12450`	`// of the resulting types is returned.`
`12451`	`12451`	`function mapType(type: Type, mapper: (t: Type) => Type, noReductions?: boolean): Type {`
	`12452`	`+ if (type.flags & TypeFlags.Never) {`
	`12453`	`+ return type;`
	`12454`	`+ }`
`12452`	`12455`	`if (!(type.flags & TypeFlags.Union)) {`
`12453`	`12456`	`return mapper(type);`
`12454`	`12457`	`}`