Consistent narrowing to 'never' in conditional and switch statements (#39191)

* Allow unions and unit types to narrow to 'never'

* Remove odd check for TypeFlags.NotUnionOrUnit

* Accept new baselines

* Accept new API baselines

Author: ahejlsberg

src/compiler/checker.ts

@@ -20580,7 +20580,7 @@ namespace ts {
         }
 
         function createFlowType(type: Type, incomplete: boolean): FlowType {
-            return incomplete ? { flags: 0, type } : type;
+            return incomplete ? { flags: 0, type: type.flags & TypeFlags.Never ? silentNeverType : type } : type;
         }
 
         // An evolving array type tracks the element types that have so far been seen in an
@@ -21168,9 +21168,7 @@ namespace ts {
                 if (narrowedType === nonEvolvingType) {
                     return flowType;
                 }
-                const incomplete = isIncomplete(flowType);
-                const resultType = incomplete && narrowedType.flags & TypeFlags.Never ? silentNeverType : narrowedType;
-                return createFlowType(resultType, incomplete);
+                return createFlowType(narrowedType, isIncomplete(flowType));
             }
 
             function getTypeAtSwitchClause(flow: FlowSwitchClause): FlowType {
@@ -21508,15 +21506,11 @@ namespace ts {
                             assumeTrue ? TypeFacts.EQUndefined : TypeFacts.NEUndefined;
                     return getTypeWithFacts(type, facts);
                 }
-                if (type.flags & TypeFlags.NotUnionOrUnit) {
-                    return type;
-                }
                 if (assumeTrue) {
                     const filterFn: (t: Type) => boolean = operator === SyntaxKind.EqualsEqualsToken ?
                         (t => areTypesComparable(t, valueType) || isCoercibleUnderDoubleEquals(t, valueType)) :
                         t => areTypesComparable(t, valueType);
-                    const narrowedType = filterType(type, filterFn);
-                    return narrowedType.flags & TypeFlags.Never ? type : replacePrimitivesWithLiterals(narrowedType, valueType);
+                    return replacePrimitivesWithLiterals(filterType(type, filterFn), valueType);
                 }
                 if (isUnitType(valueType)) {
                     const regularType = getRegularTypeOfLiteralType(valueType);

src/compiler/types.ts

@@ -4861,7 +4861,6 @@ namespace ts {
         // 'Narrowable' types are types where narrowing actually narrows.
         // This *should* be every type other than null, undefined, void, and never
         Narrowable = Any | Unknown | StructuredOrInstantiable | StringLike | NumberLike | BigIntLike | BooleanLike | ESSymbol | UniqueESSymbol | NonPrimitive,
-        NotUnionOrUnit = Any | Unknown | ESSymbol | Object | NonPrimitive,
         /* @internal */
         NotPrimitiveUnion = Any | Unknown | Enum | Void | Never | StructuredOrInstantiable,
         // The following flags are aggregated during union and intersection type construction

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

@@ -2459,7 +2459,6 @@ declare namespace ts {
         Instantiable = 63176704,
         StructuredOrInstantiable = 66846720,
         Narrowable = 133970943,
-        NotUnionOrUnit = 67637251,
     }
     export type DestructuringPattern = BindingPattern | ObjectLiteralExpression | ArrayLiteralExpression;
     export interface Type {

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

@@ -2459,7 +2459,6 @@ declare namespace ts {
         Instantiable = 63176704,
         StructuredOrInstantiable = 66846720,
         Narrowable = 133970943,
-        NotUnionOrUnit = 67637251,
     }
     export type DestructuringPattern = BindingPattern | ObjectLiteralExpression | ArrayLiteralExpression;
     export interface Type {

tests/baselines/reference/discriminatedUnionTypes1.types

@@ -320,7 +320,7 @@ function f3(m: Message) {
 >"X" : "X"
 
         m;  // never
->m : Message
+>m : never
     }
 }
 

tests/baselines/reference/equalityWithIntersectionTypes01.errors.txt

@@ -1,14 +1,9 @@
 tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersectionTypes01.ts(17,5): error TS2367: This condition will always return 'false' since the types 'I1 & I3' and 'I2' have no overlap.
 tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersectionTypes01.ts(17,16): error TS2367: This condition will always return 'false' since the types 'I2' and 'I1 & I3' have no overlap.
 tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersectionTypes01.ts(19,10): error TS2367: This condition will always return 'true' since the types 'I1 & I3' and 'I2' have no overlap.
-tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersectionTypes01.ts(19,21): error TS2367: This condition will always return 'true' since the types 'I2' and 'I1 & I3' have no overlap.
-tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersectionTypes01.ts(21,10): error TS2367: This condition will always return 'false' since the types 'I1 & I3' and 'I2' have no overlap.
-tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersectionTypes01.ts(21,20): error TS2367: This condition will always return 'false' since the types 'I2' and 'I1 & I3' have no overlap.
-tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersectionTypes01.ts(23,10): error TS2367: This condition will always return 'true' since the types 'I1 & I3' and 'I2' have no overlap.
-tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersectionTypes01.ts(23,20): error TS2367: This condition will always return 'true' since the types 'I2' and 'I1 & I3' have no overlap.
 
 
-==== tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersectionTypes01.ts (8 errors) ====
+==== tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersectionTypes01.ts (3 errors) ====
     interface I1 {
         p1: number
     }
@@ -34,18 +29,8 @@ tests/cases/conformance/types/typeRelationships/comparable/equalityWithIntersect
     else if (y !== z || z !== y) {
              ~~~~~~~
 !!! error TS2367: This condition will always return 'true' since the types 'I1 & I3' and 'I2' have no overlap.
-                        ~~~~~~~
-!!! error TS2367: This condition will always return 'true' since the types 'I2' and 'I1 & I3' have no overlap.
     }
     else if (y == z || z == y) {
-             ~~~~~~
-!!! error TS2367: This condition will always return 'false' since the types 'I1 & I3' and 'I2' have no overlap.
-                       ~~~~~~
-!!! error TS2367: This condition will always return 'false' since the types 'I2' and 'I1 & I3' have no overlap.
     }
     else if (y != z || z != y) {
-             ~~~~~~
-!!! error TS2367: This condition will always return 'true' since the types 'I1 & I3' and 'I2' have no overlap.
-                       ~~~~~~
-!!! error TS2367: This condition will always return 'true' since the types 'I2' and 'I1 & I3' have no overlap.
     }

tests/baselines/reference/equalityWithIntersectionTypes01.types

@@ -47,24 +47,24 @@ else if (y !== z || z !== y) {
 >y : I1 & I3
 >z : I2
 >z !== y : boolean
->z : I2
->y : I1 & I3
+>z : never
+>y : never
 }
 else if (y == z || z == y) {
 >y == z || z == y : boolean
 >y == z : boolean
->y : I1 & I3
->z : I2
+>y : never
+>z : never
 >z == y : boolean
->z : I2
->y : I1 & I3
+>z : never
+>y : never
 }
 else if (y != z || z != y) {
 >y != z || z != y : boolean
 >y != z : boolean
->y : I1 & I3
->z : I2
+>y : never
+>z : never
 >z != y : boolean
->z : I2
->y : I1 & I3
+>z : never
+>y : never
 }