Allow generic attributes

[kerams]

Implements fsharp/fslang-suggestions#965.

[vzarytovskii]

Amazing!

[baronfel]

This feels like a Trivia node that @nojaf might have opinions on?

[kerams]

Yes, SynType.App, SynType.LongIdentApp and SynExpr.TypeApp use those 3 range fields too.

[nojaf]

SynType.App, SynType.LongIdentApp and SynExpr.TypeApp pre-date SynTrivia, I think for a new concept it make sense to have these as trivia.
It might also make sense to introduce a new type for TypeName : SynLongIdent.

[kerams]

fsharp/src/Compiler/TypedTree/TypedTree.fsi

Lines 3102 to 3106 in fd6eb11

	[<NoEquality; NoComparison; StructuredFormatDisplay("{DebugText}")>]
	type Attrib =
	| Attrib of
	tyconRef: TyconRef *
	kind: AttribKind *

This is a problem since we're working with TType now. Do I store both? And what about things like remapAttribImpl? Is it safe to replace just the tycon in TType_app?

Ah, and most importantly, typed tree pickling works with attribute tycons.

I'm also currently forced to do [<Foo<string> >] because the parser doesn't like >>]. How would I go about sorting that out?

[auduchinok]

It seems to duplicate info from SynType.App, SynType.LongIdentApp and SynExpr.TypeApp. Maybe we could extract these field into a separate type, like SynTypeArgumentList or something?

[kerams]

So the question is whether we extract all 4 as SynTypeArgumentList or just the ranges as SynTypeArgumentTrivia, or

type SynTypeArgumentTrivia = {
      LessRange: range option
      CommaRanges: range list
      GreaterRange: range option
}

type SynTypeArgumentList = {
    TypeArgs: SynType list
    Trivia: SynTypeArgumentTrivia
}