Revert "Merge pull request microsoft#7235 from weswigham/narrow-all-types"

This reverts commit ef0f6c8, reversing
changes made to 9f087cb.

Author: sandersn

src/compiler/checker.ts

@@ -7662,7 +7662,7 @@ namespace ts {
 
         function getFlowTypeOfReference(reference: Node, declaredType: Type, assumeInitialized: boolean, includeOuterFunctions: boolean) {
             let key: string;
-            if (!reference.flowNode || assumeInitialized && (declaredType.flags & TypeFlags.NotNarrowable)) {
+            if (!reference.flowNode || assumeInitialized && !(declaredType.flags & TypeFlags.Narrowable)) {
                 return declaredType;
             }
             const initialType = assumeInitialized ? declaredType : addNullableKind(declaredType, TypeFlags.Undefined);

src/compiler/types.ts

@@ -2218,13 +2218,7 @@ namespace ts {
         ObjectType = Class | Interface | Reference | Tuple | Anonymous,
         UnionOrIntersection = Union | Intersection,
         StructuredType = ObjectType | Union | Intersection,
-
-        // 'NotNarrowable' types are types where narrowing reverts to the original type, rather than actually narrow.
-        // This is never really correct - you can _always_ narrow to an intersection with that type, _but_ we keep
-        // Void as the only non-narrowable type, since it's a non-value type construct (representing a lack of a value)
-        // and, generally speaking, narrowing `void` should fail in some way, as it is nonsensical. (`void` narrowing
-        // to `void & T`, in a structural sense, is just narrowing to T, which we wouldn't allow under normal rules)
-        NotNarrowable = Void,
+        Narrowable = Any | ObjectType | Union | TypeParameter,
         /* @internal */
         RequiresWidening = ContainsUndefinedOrNull | ContainsObjectLiteral,
         /* @internal */