Fix getNarrowedType so that only strings and numbers are not processed

Author: islandryu

src/compiler/checker.ts

@@ -24735,20 +24735,15 @@ namespace ts {
                 // If the current type is a union type, remove all constituents that couldn't be instances of
                 // the candidate type. If one or more constituents remain, return a union of those.
                 if (type.flags & TypeFlags.Union) {
+                    let assignableType = filterType(type, t => isRelated(t, candidate));
                     if(candidate.flags & TypeFlags.Union){
-                        const assignableType = mapType(type, type => {
-                            if (!!(type.flags & TypeFlags.String) || !!(type.flags & TypeFlags.Number)) {
-                                return filterType(candidate, candidate => isTypeSubtypeOf(candidate, type));
-                            }
-                            else {
-                                return isRelated(type, candidate) ? type : neverType;
-                            }
+                        const unionAssignableType = mapType(type, type => {
+                            return filterType(candidate, candidate => isRelated(candidate, type));
                         });
-                        if (!(assignableType.flags & TypeFlags.Never)) {
-                            return assignableType;
+                        if (!isTypeSubsetOf(unionAssignableType, assignableType)) {
+                            assignableType = unionAssignableType;
                         }
                     }
-                    const assignableType = filterType(type, t => isRelated(t, candidate));
                     if (!(assignableType.flags & TypeFlags.Never)) {
                         return assignableType;
                     }