Allow unions and unit types to narrow to 'never'

Author: ahejlsberg

src/compiler/checker.ts

@@ -20229,7 +20229,7 @@ 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
@@ -20817,9 +20817,7 @@ 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 {
@@ -21164,8 +21162,7 @@ namespace ts {
                     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);