Cannot instantiate T from `=>` in generic

[bluenote10]

There seems to be an issue with future's => and generics:

import sequtils, future

proc f*[T](x: T) =
  let other = @[1, 2, 3]
  var mapped = toSeq(0 ..< other.len).map(i => (other[i], i))

f(1)

Produces: lib/system.nim(3549, 14) Error: cannot instantiate: 'T'

The issue disappears:

• when f is not generic
• when using other expressions like i => (i, i)
• when writing the expression verbosely toSeq(0 ..< other.len).map(proc(i: int): (int, int) = (other[i], i))

[timotheecour]

#8679 will fix this

[bluenote10]

I think I reported this twice because I keep forgetting about the limitations ;).

Closing in favor of #7816