Allow extending types referenced through interfaces

Suggestion

Allow things like the following:

interface I extends HTMLElementTagNameMap['abbr'] {}

Currently, the following error message is given: “An interface can only extend an identifier/qualified-name with optional type arguments.” Which I don’t even understand.

I believe it requires no further clarification or justification, but please let me know if that is the case…

[coreh]

As long as you assign the type a name it works, and it does the proper check already:

interface Bar {}
interface Baz {}

// This doesn't work
interface Foo extends (Bar & Baz) {}

// This works
type _tmp = Bar & Baz;
interface Foo extends _tmp {};

// This fails with a meaningful error message:
// > An interface can only extend an object type or intersection of object types with statically known members. ts(2312)
type _tmp2 = Bar | Baz;
interface Foo extends _tmp2 {};

So IMO we should definitely support the anonymous/expression version of this for orthogonality/consistency

[RyanCavanaugh]

In principle it’s doable - we can detect when the resolved entity is a legal extends target