Print better errors for unusual void, null and undefined types

[dcodeIO]

Fixes #558 by making

export function test1(x: void): void {}

export function test2(x: undefined): void {}

export function test3(x: null): void {}

export function test4(a: string | undefined): void {}

now diagnosing

ERROR TS1110: Type expected.

export function test1(x: void): void {}
                         ~~~~
in input.ts(1,26)

ERROR TS2304: Cannot find name 'undefined'.

export function test2(x: undefined): void {}
                         ~~~~~~~~~
in input.ts(3,26)

ERROR TS2304: Cannot find name 'null'.

export function test3(x: null): void {}
                         ~~~~
in input.ts(5,26)

ERROR TS1005: 'null' expected.

export function test4(a: string | undefined): void {}
                                  ~~~~~~~~~
in input.ts(7,35)

with test4 diagnosed early on parser level.

[MaxGraey]

Could it also handle this cases?

let x: void;
let y = <void>1;
let z: undefined;
let w = <undefined>1;

export function test5(a: string): void | undefined {}
                                         ~~~~~~~~~

export function test6(a: string): void | null {}
                                  ~~~~~~~~~~~

[MaxGraey]

Overall LGTM

[dcodeIO]

The new given examples diagnose

let x: void; // TS1110: Type expected.
let y = <void>1; // TS2322: Type 'void' is not assignable to type '<auto>'.
let z: undefined; // TS2304: Cannot find name 'undefined'.
let w = <undefined>1; // TS2304: Cannot find name 'undefined'.

while a void | something return type seems to exit early on void, leading to TS2391: Function implementation is missing or not immediately following the declaration kind of errors. Going to see if I can do something about these as well.

[dcodeIO]

Last commit changes the parsing of union return types to always occur, so

export function test6(a: string): void | null {}

now diagnoses

ERROR AS204: Basic type 'void' cannot be nullable.

export function test6(a: string): void | null {}
                                  ~~~~
in input.ts(1,35)

and

export function test5(a: string): void | undefined {}

yields

ERROR TS1005: 'null' expected.

export function test5(a: string): void | undefined {}
                                         ~~~~~~~~~
in input.ts(1,42)

[MaxGraey]

Great! Well done!