Invalid generic type deduction in mapIt

[arnetheduck]

Description

import sequtils

type F[T, E] = object

proc start(v: int): F[void, (ValueError,)] = discard
proc stop(v: int): F[void, tuple[]] = discard

var a = mapIt([1, 2, 3], start(it))

assert $type(a) == "seq[F[system.void, (ValueError,)]]", $type(a)

var b = mapIt([1, 2, 3], stop(it))

assert $type(b) == "seq[F[system.void, tuple[]]]", $type(b)

The expectation is that the b type is derived from stop, yet the start type gets reused..

Nim Version

1.6, devel

Current Output

Error: unhandled exception: testit2.nim(14, 8) `$type(b) == "seq[F[system.void, tuple[]]]"` seq[F[system.void, (ValueError,)]] [AssertionDefect]

Expected Output

No response

Possible Solution

No response

Additional Information

No response

[arnetheduck]

Notably, if a and its assert is removed, the correct type is deduced for b - the presence of the deduction for a breaks b.

[SirOlaf]

Looks like a variation of #22479

[arnetheduck]

ah, right but without destructor this time..

The type system just does not see a generic body without generic parameters as a different type.

this applies though - also in the real bug of which this is a reproduction

[arnetheduck]

fwiw, we've seen this bug in many different places though we never teased out a repro as simple as here - the showstopper label on 22479 feels appropritate

[arnetheduck]

workaround:

import sequtils

type F[T, E] = object
  dummy: array[0, E]

proc start(v: int): F[void, (ValueError,)] = discard
proc stop(v: int): F[void, tuple[]] = discard

var a = mapIt([1, 2, 3], start(it))

assert $type(a) == "seq[F[system.void, (ValueError,)]]", $type(a)

var b = mapIt([1, 2, 3], stop(it))

assert $type(b) == "seq[F[system.void, tuple[]]]", $type(b)

[Araq]

Reduced example:

template mapIt*(x: untyped): untyped =
  type OutType = typeof(x, typeOfProc)
  newSeq[OutType](5)

type F[T, E] = object

proc start(v: int): F[void, (ValueError,)] = discard
proc stop(v: int): F[void, tuple[]] = discard

var a = mapIt(start(9))
assert $type(a) == "seq[F[system.void, (ValueError,)]]", $type(a)

var b = mapIt(stop(9))
assert $type(b) == "seq[F[system.void, tuple[]]]", $type(b)