Add more comments

Author: ahejlsberg

src/compiler/checker.ts

@@ -15516,6 +15516,10 @@ namespace ts {
                     }
                 }
                 else if (target.flags & TypeFlags.Intersection && some((<IntersectionType>target).types, t => !!getInferenceInfoForType(t))) {
+                    // We reduce intersection types only when they contain naked type parameters. For example, when
+                    // inferring from 'string[] & { extra: any }' to 'string[] & T' we want to remove string[] and
+                    // infer { extra: any } for T. But when inferring to 'string[] & Iterable<T>' we want to keep the
+                    // string[] on the source side and infer string for T.
                     if (source.flags & TypeFlags.Intersection) {
                         // Infer between identically matching source and target constituents and remove the matching types.
                         const [sources, targets] = inferFromMatchingTypes((<IntersectionType>source).types, (<IntersectionType>target).types, isTypeIdenticalTo);