Instantiate type parameters with themselves as their 'this' type when relating.

src/compiler/checker.ts

@@ -5595,9 +5595,14 @@ namespace ts {
 
                 if (source.flags & TypeFlags.TypeParameter) {
                     let constraint = getConstraintOfTypeParameter(<TypeParameter>source);
+
                     if (!constraint || constraint.flags & TypeFlags.Any) {
                         constraint = emptyObjectType;
                     }
+
+                    // The constraint may need to be further instantiated with its 'this' type.
+                    constraint = getTypeWithThisArgument(constraint);
+
                     // Report constraint errors only if the constraint is not the empty object type
                     const reportConstraintErrors = reportErrors && constraint !== emptyObjectType;
                     if (result = isRelatedTo(constraint, target, reportConstraintErrors)) {