Compiler crashed, expr(skTemplate); unknown symbol

[al6x]

Description

Code below would crash the compiler

import std/[sequtils, tables]

var codes: Table[string, uint16]
template encode_trigram*(s: string): uint16 =
  codes.mget_or_put(s, codes.len.uint16)

echo @[""].map(encode_trigram)

Nim Version

Nim Compiler Version 1.6.12 [MacOSX: amd64]
Compiled at 2023-03-10
Copyright (c) 2006-2023 by Andreas Rumpf

active boot switches: -d:release -d:nimUseLinenoise

Current Output

Error: internal error: expr(skTemplate); unknown symbol

Expected Output

No response

Possible Solution

No response

Additional Information

No response

[metagn]

encode_trigram has a normal proc type proc (s: string): uint16, the information that it's a template is hidden too deep in the type to check during the type relation.

One might think to disable this, but this shockingly allows some stuff we can't do otherwise:

proc foo(x: static proc (a: int)) =
  x((echo "abc"; 123))
  
template bar(a: int) =
  echo a
  echo a

foo(bar)

Output:

abc
123
abc
123

Maybe we can add a new template type to separate from the normal proc one (just tyProc with tfTemplate like tfIterator) and require its values to be compile-time only (like NimNode in theory, but I don't think there is such an enforcement for NimNode, it just acts like ref object at runtime). Related #17617, nim-lang/RFCs#212

[beef331]

I'd say a counter point is:

proc foo(x: static proc (a: int)) =
  echo "hello"
  x(123)
  echo "good bye"
  
template bar(a: int) =
  return

foo(bar)

[metagn]

proc foo(x: static template (a: int)) =
  echo "hello"
  x(123)
  echo "good bye"
  
template bar(a: int) =
  return

foo(bar)

Is this reasonable?

[beef331]

It's more reasonable to me at least, arguably though x: template would suffice since templates are explicitly static. This follows down to iterator map[T, Y](i: iterable[T], fn: proc(x: T): Y): Y working the same way.