Refactor 'getNarrowableTypeForReference'.

Author: DanielRosenwasser

src/compiler/checker.ts

@@ -25338,13 +25338,24 @@ namespace ts {
             // control flow analysis an opportunity to narrow it further. For example, for a reference of a type
             // parameter type 'T extends string | undefined' with a contextual type 'string', we substitute
             // 'string | undefined' to give control flow analysis the opportunity to narrow to type 'string'.
+            if (checkMode && checkMode & CheckMode.Inferential) {
+                return type;
+            }
+
             let contextualType: Type | undefined;
-            const substituteConstraints = !(checkMode && checkMode & CheckMode.Inferential) &&
-                (isConstraintPosition(type, reference) || (contextualType = tryGetContextualTypeWithNoGenericTypes(reference, checkMode))) &&
-                (
-                    someType(type, isGenericTypeWithUnionConstraint) ||
-                    ((type.flags & TypeFlags.Instantiable) && contextualType && isEmptyObjectType(contextualType))
-                );
+            // If we aren't in a constraint position, or we can't find a contextual type, or the contextual type indicates
+            // that the type in question may be a direct inference source, then don't do anything special.
+            if (!isConstraintPosition(type, reference) && !(contextualType = tryGetContextualTypeWithNoGenericTypes(reference, checkMode))) {
+                return type;
+            }
+
+            const substituteConstraints =
+                // When we have a type parameter constrained to a union type, we can typically narrow to get better results.
+                someType(type, isGenericTypeWithUnionConstraint) ||
+                // When the contextual type is 'unknown', we may need to narrow for compatibility with non-null targets.
+                // This allows some parity with a constraint of '{} | null | undefined'.
+                (type.flags & TypeFlags.Instantiable) && contextualType && isEmptyObjectType(contextualType);
+
             return substituteConstraints ? mapType(type, t => t.flags & TypeFlags.Instantiable ? getBaseConstraintOfType(t) || unknownType : t) : type;
         }