Gracefully parse and error on unparenthesized function/constructor types

[DanielRosenwasser]

Motivated by #39546

type Foo = string | () => string;

type InstOrConst<T> = T | new () => T;

Playground Link

We made a decision to disallow function and constructor shorthand types in these contexts, but as a result we have really poor error recovery.

Exported type alias 'Foo' has or is using private name ''.(4081)
Type expected.(1110)
';' expected.(1005)
'string' only refers to a type, but is being used as a value here.(2693)
Cannot find name 'new'.(2304)
Exported type alias 'InstOrConst' has or is using private name 'new'.(4081)
';' expected.(1005)
Cannot find name 'T'.(2304)

We should be able to issue an error in these contexts:

Function type notation must be parenthesized when used in a union type.
Constructor type notation must be parenthesized when used in a union type.

[uhyo]

I'll try this one tomorrow 🙂

[DanielRosenwasser]

Thank you @uhyo!