Exempt bare 'boolean's from the check.

Author: DanielRosenwasser

src/compiler/checker.ts

@@ -17833,6 +17833,13 @@ namespace ts {
         }
 
         function typeCouldHaveTopLevelSingletonTypes(type: Type): boolean {
+            // Okay, yes, 'boolean' is a union of 'true | false', but that's not useful
+            // in error reporting scenarios. If you need to use this function but that detail matters,
+            // feel free to add a flag.
+            if (type.flags & TypeFlags.Boolean) {
+                return false;
+            }
+
             if (type.flags & TypeFlags.UnionOrIntersection) {
                 return !!forEach((type as IntersectionType).types, typeCouldHaveTopLevelSingletonTypes);
             }