Handle type variables in isTypeDerivedFrom

src/compiler/checker.ts

@@ -8868,13 +8868,15 @@ namespace ts {
         // An object type S is considered to be derived from an object type T if
         // S is a union type and every constituent of S is derived from T,
         // T is a union type and S is derived from at least one constituent of T, or
+        // S is a type variable with a base constraint that is derived from T,
         // T is one of the global types Object and Function and S is a subtype of T, or
         // T occurs directly or indirectly in an 'extends' clause of S.
         // Note that this check ignores type parameters and only considers the
         // inheritance hierarchy.
         function isTypeDerivedFrom(source: Type, target: Type): boolean {
             return source.flags & TypeFlags.Union ? every((<UnionType>source).types, t => isTypeDerivedFrom(t, target)) :
                 target.flags & TypeFlags.Union ? some((<UnionType>target).types, t => isTypeDerivedFrom(source, t)) :
+                source.flags & TypeFlags.TypeVariable ? isTypeDerivedFrom(getBaseConstraintOfType(source) || emptyObjectType, target) :
                 target === globalObjectType || target === globalFunctionType ? isTypeSubtypeOf(source, target) :
                 hasBaseType(source, getTargetType(target));
             }