UFCS/MCS broken for routines accepting iterables (via untyped params) eg toSeq

[nothratal]

The toSeq-template of sequtils has no support for UFCS when using it together with the keys-iterator of tables.

Example

import tables
import sequtils

var tab = initTable[string, seq[int]]()

tab["monday"] = @[1,2,3,4,5,6]
tab["tuesday"] = @[5,7,2,5,2]
tab["wednesday"] = @[1,2,3,4,5,6]
tab["thursday"] = @[1,2,3,4,5,6]
tab["friday"] = @[1,2,3,4,5,6]
tab["saturday"] = @[1,2,3,4,5,6]
tab["sunday"] = @[1,2,3,4,5,6]

echo tab.keys.toSeq

Current Output

/usercode/in.nim(15, 9) Error: undeclared field: 'keys' 
  found 'tables.keys(t: TableRef[keys.A, keys.B]) [declared in /playground/nim/lib/pure/collections/tables.nim(1127, 10)]' of kind 'iterator'
  found 'tables.keys(t: Table[keys.A, keys.B]) [declared in /playground/nim/lib/pure/collections/tables.nim(694, 10)]' of kind 'iterator'
  found 'tables.keys(t: OrderedTable[keys.A, keys.B]) [declared in /playground/nim/lib/pure/collections/tables.nim(1674, 10)]' of kind 'iterator'
  found 'tables.keys(t: CountTableRef[keys.A]) [declared in /playground/nim/lib/pure/collections/tables.nim(2724, 10)]' of kind 'iterator'
  found 'tables.keys(t: OrderedTableRef[keys.A, keys.B]) [declared in /playground/nim/lib/pure/collections/tables.nim(2068, 10)]' of kind 'iterator'
  found 'tables.keys(t: CountTable[keys.A]) [declared in /playground/nim/lib/pure/collections/tables.nim(2459, 10)]' of kind 'iterator'

Expected Output

Expecting it to compile

Additional Information

• Using echo toSeq(tab.keys) works correctly:
  @["wednesday", "thursday", "saturday", "friday", "tuesday", "monday", "sunday"]
• Tested it on Linux & Windows & play.nim-lang.org:

> nim --version
Nim Compiler Version 1.0.6 [Windows: amd64]

$ nim --version
Nim Compiler Version 1.0.6 [Linux: amd64]

[andreaferretti]

Related

nim-lang/RFCs#512
#9219
https://nim-lang.org/docs/manual.html#templates-limitations-of-the-method-call-syntax

[timotheecour]

it's not specific to toSeq, it's a sigmatch compiler limitation:

template fun(a: untyped) = discard
iterator iter(): int = yield 10
fun(iter()) # ok
# iter().fun() # CT error

IMO it's fixable (I had a POC for it IIRC).
one way would be allowing passing an iterator as iterator {.inline.} instead of untyped, eg:

template fun(a: iterator {.inline.}) = discard
iterator iter(): int = yield 10
iter().fun()

[denise-amiga]

I understand that this problem is related to the same parser limitation

import tables

let txt = "zbabaacbaabz"
var ct_fine = toCountTable[char](txt)
var ct_fail = txt.toCountTable[char]()   # <--- no compile

[beef331]

Except there is a solution for that txt.toCountTable[:char]()