Generic overload is incorrectly matched

[ire4ever1190]

Nim Version

Nim Compiler Version 2.3.1 [Linux: amd64]
Compiled at 2026-01-10
Copyright (c) 2006-2026 by Andreas Rumpf

git hash: 01eedd9
active boot switches: -d:release

Description

Reproducible case is this

import std/options

type
  Combinator*[T] = proc (): Option[T] {.closure.}
    ## Parser that optionally returns data

proc fin*(): Option[bool] = some(true)

# This is the overload that should be matched
proc `*`*[T](left: Combinator[T], right: Combinator[bool]): Combinator[bool] =
  return proc (): Option[bool] =
    echo right().get()

# Instead this one is selected
proc `*`*[T](left: Combinator[T], right: Combinator[seq[T]]): Combinator[seq[T]] =
  return proc (): Option[seq[T]] =
    echo right().get()

proc foo(): Option[string] = discard

# The type must be Combinator[string], if its the normal proc type then the type checking
# correctly fails
# let bar = foo
let bar: Combinator[string] = foo

let t = bar * fin
assert type(t()) is Option[bool], "Wrong overload selected"
echo t()

For some reason it is selecting the second overload instead of the first which caused a nil access error since it thought right was returning a string. Behavior only happens when I haven't explicitly set the type to be Combinator[string], making the type be proc (): Option[string] makes the code work correctly

Current Output

/home/user/Documents/projects/nort/test.nim(26) test
/home/user/.choosenim/toolchains/nim-hashdevel/lib/std/assertions.nim(45) failedAssertImpl
/home/user/.choosenim/toolchains/nim-hashdevel/lib/std/assertions.nim(40) raiseAssert
/home/user/.choosenim/toolchains/nim-hashdevel/lib/system/fatal.nim(53) sysFatal
Error: unhandled exception: /home/user/Documents/projects/nort/test.nim(26, 1) `type(t()) is Option[bool]` Wrong overload selected [AssertionDefect]
Error: execution of an external program failed: '/home/user/.cache/nim/test_d/test_951333475FBBAD2DA50919906415803BCCFC2541'

# When I remove the assertion
Traceback (most recent call last)
/home/user/Documents/projects/nort/test.nim(27) test
/home/user/Documents/projects/nort/test.nim(17) :anonymous
/home/user/.choosenim/toolchains/nim-hashdevel/lib/system/dollars.nim(133) $
/home/user/.choosenim/toolchains/nim-hashdevel/lib/system/dollars.nim(116) collectionToString
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
Error: execution of an external program failed: '/home/user/.cache/nim/test_d/test_951333475FBBAD2DA50919906415803BCCFC2541'

Expected Output

true
none(bool)

Known Workarounds

No response

Additional Information

No response

[ire4ever1190]

!nim c

import std/options

type
  Combinator*[T] = proc (): Option[T] {.closure.}
    ## Parser that optionally returns data

proc fin*(): Option[bool] = some(true)

# This is the overload that should be matched
proc `*`*[T](left: Combinator[T], right: Combinator[bool]): Combinator[bool] =
  return proc (): Option[bool] =
    echo right().get()

# Instead this one is selected
proc `*`*[T](left: Combinator[T], right: Combinator[seq[T]]): Combinator[seq[T]] =
  return proc (): Option[seq[T]] =
    echo right().get()

proc foo(): Option[string] = discard

# The type must be Combinator[string], if its the normal proc type then it selects the right overload
#let bar = foo
let bar: Combinator[string] = foo

let t = bar * fin
static: assert type(t()) is Option[bool], "Wrong overload selected"
echo t()