Unable to define SVG elements conflicting with HTML

[falsandtru]

TypeScript Version: master

Expected behavior:

interface SVGElementTagNameMap {
    'a': SVGAElement;
}
interface ElementTagNameMap extends HTMLElementTagNameMap, SVGElementTagNameMap {
}

But deriving from both HTMLElementTagNameMap and SVGElementTagNameMap still throw an error by conflicts. To resolve this, ElementTagNameMap has to be defined using Diff type as follows.

interface ElementTagNameMap extends HTMLElementTagNameMap, Diff<SVGElementTagNameMap, HTMLElementTagNameMap> {
}

type Diff<T, U> = Pick<T, DiffKey<keyof T, keyof U>>;
type DiffKey<T extends string, U extends string> = (
  & { [P in T]: P; }
  & { [P in U]: never; }
  & { [x: string]: never; }
)[T];

However, defining ElementTagNameMap is originally wrong. It should be removed later.

Actual behavior:

interface ElementTagNameMap {
    'a': SVGAElement; // error, conflicts with 'a': HTMLAnchorElement;
}

related: #20134, #14783

[mhegazy]

We should not include Diff in the library. For ones that conflict having them default to the HTMLElement seems like a reasonable compromise.

[falsandtru]

We can resolve this issue without Diff type by removing ElementTagNameMap: microsoft/TypeScript-DOM-lib-generator#326