Correct assignability for keyof types and type parameters

Author: ahejlsberg

src/compiler/checker.ts

@@ -6825,6 +6825,27 @@ namespace ts {
                     }
                 }
 
+                if (target.flags & TypeFlags.TypeParameter) {
+                    // Given a type parameter K with a constraint keyof T, a type S is
+                    // assignable to K if S is assignable to keyof T.
+                    let constraint = getConstraintOfTypeParameter(<TypeParameter>target);
+                    if (constraint && constraint.flags & TypeFlags.Index) {
+                        if (result = isRelatedTo(source, constraint, reportErrors)) {
+                            return result;
+                        }
+                    }
+                }
+                else if (target.flags & TypeFlags.Index) {
+                    // Given a type parameter T with a constraint C, a type S is assignable to
+                    // keyof T if S is assignable to keyof C.
+                    let constraint = getConstraintOfTypeParameter((<IndexType>target).type);
+                    if (constraint) {
+                        if (result = isRelatedTo(source, getIndexType(constraint), reportErrors)) {
+                            return result;
+                        }
+                    }
+                }
+
                 if (source.flags & TypeFlags.TypeParameter) {
                     let constraint = getConstraintOfTypeParameter(<TypeParameter>source);
 

src/compiler/types.ts

@@ -2709,7 +2709,7 @@ namespace ts {
         EnumLike = Enum | EnumLiteral,
         UnionOrIntersection = Union | Intersection,
         StructuredType = Object | Union | Intersection,
-        StructuredOrTypeParameter = StructuredType | TypeParameter,
+        StructuredOrTypeParameter = StructuredType | TypeParameter | Index,
 
         // 'Narrowable' types are types where narrowing actually narrows.
         // This *should* be every type other than null, undefined, void, and never