Filter origin types when filtering union types

src/compiler/checker.ts

@@ -21689,7 +21689,27 @@ namespace ts {
             if (type.flags & TypeFlags.Union) {
                 const types = (<UnionType>type).types;
                 const filtered = filter(types, f);
-                return filtered === types ? type : getUnionTypeFromSortedList(filtered, (<UnionType>type).objectFlags);
+                if (filtered === types) {
+                    return type;
+                }
+                const origin = (<UnionType>type).origin;
+                let newOrigin: Type | undefined;
+                if (origin && origin.flags & TypeFlags.Union) {
+                    // If the origin type is a (denormalized) union type, filter its non-union constituents. If that ends
+                    // up removing a smaller number of types than in the normalized constituent set (meaning some of the
+                    // filtered types are within nested unions in the origin), then we can't construct a new origin type.
+                    // Otherwise, if we have exactly one type left in the origin set, return that as the filtered type.
+                    // Otherwise, construct a new filtered origin type.
+                    const originTypes = (<UnionType>origin).types;
+                    const originFiltered = filter(originTypes, t => !!(t.flags & TypeFlags.Union) || f(t));
+                    if (originTypes.length - originFiltered.length === types.length - filtered.length) {
+                        if (originFiltered.length === 1) {
+                            return originFiltered[0];
+                        }
+                        newOrigin = createOriginUnionOrIntersectionType(TypeFlags.Union, originFiltered);
+                    }
+                }
+                return getUnionTypeFromSortedList(filtered, (<UnionType>type).objectFlags, /*aliasSymbol*/ undefined, /*aliasTypeArguments*/ undefined, newOrigin);
             }
             return type.flags & TypeFlags.Never || f(type) ? type : neverType;
         }

tests/baselines/reference/exhaustiveSwitchStatements1.errors.txt

@@ -1,5 +1,5 @@
 tests/cases/conformance/controlFlow/exhaustiveSwitchStatements1.ts(7,9): error TS7027: Unreachable code detected.
-tests/cases/conformance/controlFlow/exhaustiveSwitchStatements1.ts(235,5): error TS2367: This condition will always return 'false' since the types '"a" | "b"' and '"c"' have no overlap.
+tests/cases/conformance/controlFlow/exhaustiveSwitchStatements1.ts(235,5): error TS2367: This condition will always return 'false' since the types 'keyof O' and '"c"' have no overlap.
 
 
 ==== tests/cases/conformance/controlFlow/exhaustiveSwitchStatements1.ts (2 errors) ====
@@ -241,7 +241,7 @@ tests/cases/conformance/controlFlow/exhaustiveSwitchStatements1.ts(235,5): error
         }
         k === 'c';  // Error
         ~~~~~~~~~
-!!! error TS2367: This condition will always return 'false' since the types '"a" | "b"' and '"c"' have no overlap.
+!!! error TS2367: This condition will always return 'false' since the types 'keyof O' and '"c"' have no overlap.
         return o[k];
     }
 

tests/baselines/reference/exhaustiveSwitchStatements1.types

@@ -695,12 +695,12 @@ function ff(o: O, k: K) {
     }
     k === 'c';  // Error
 >k === 'c' : boolean
->k : "a" | "b"
+>k : keyof O
 >'c' : "c"
 
     return o[k];
 >o[k] : number
 >o : O
->k : "a" | "b"
+>k : keyof O
 }
 

tests/baselines/reference/jsxLibraryManagedAttributesUnusedGeneric.types

@@ -33,7 +33,7 @@ namespace jsx {
 >css : string
 
         export type LibraryManagedAttributes<C, P> = WithCSSProp<P>
->LibraryManagedAttributes : WithCSSProp<P>
+>LibraryManagedAttributes : LibraryManagedAttributes<C, P>
 
     }
 }

tests/baselines/reference/observableInferenceCanBeMade.types

@@ -52,6 +52,6 @@ function asObservable(input: string | ObservableInput<string>): Observable<strin
 >input : string
 >from(input) : Observable<string>
 >from : <O extends ObservableInput<any>>(input: O) => Observable<ObservedValueOf<O>>
->input : Subscribable<never> | Subscribable<string>
+>input : ObservableInput<string>
 }
 

tests/baselines/reference/partiallyDiscriminantedUnions.types

@@ -95,7 +95,7 @@ function fail(s: Shapes) {
         if (s.kind === "circle") {
 >s.kind === "circle" : boolean
 >s.kind : "square" | "circle"
->s : Square | Circle
+>s : Shape
 >kind : "square" | "circle"
 >"circle" : "circle"
 

tests/baselines/reference/spreadBooleanRespectsFreshness.types

@@ -27,6 +27,6 @@ foo1 = [...Array.isArray(foo2) ? foo2 : [foo2]];
 >isArray : (arg: any) => arg is any[]
 >foo2 : Foo
 >foo2 : FooArray
->[foo2] : (string | false)[]
->foo2 : string | false
+>[foo2] : FooBase[]
+>foo2 : FooBase
 

tests/baselines/reference/stringLiteralCheckedInIf02.types

@@ -29,7 +29,7 @@ function f(foo: T) {
 >foo : T
 
         return foo;
->foo : "a" | "b"
+>foo : S
     }
     else { 
         return foo[0];