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fixes 25826; var type class parameter matches lvalues #25827

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fixes 25826; var type class parameter matches lvalues #25827

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7 changes: 6 additions & 1 deletion compiler/sigmatch.nim
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Original file line number Diff line number Diff line change
Expand Up @@ -1907,6 +1907,11 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
return isNone
if doBind: put(c, f, a)
return isGeneric
elif targetKind == tyVar:
# always matches.
# The argument is checked if it is lvalue or not outside of this proc.
if doBind: put(c, f, if aOrig.kind == tyVar: aOrig else: makeVarType(c.c, a))
return isGeneric
else:
return isNone
of tyUserTypeClassInst, tyUserTypeClass:
Expand Down Expand Up @@ -2863,7 +2868,7 @@ proc matchesAux(c: PContext, n, nOrig: PNode, m: var TCandidate, marker: var Int
else:
noMatch()

if formal.typ.kind in {tyVar}:
if formal.typ.kind in {tyVar} or (formal.typ.kind == tyBuiltInTypeClass and formal.typ[0].kind == tyVar):
let argConverter = if arg.kind == nkHiddenDeref: arg[0] else: arg
if argConverter.kind == nkHiddenCallConv:
if argConverter.typ.kind notin {tyVar}:
Expand Down
23 changes: 23 additions & 0 deletions tests/typerel/tvar_type_class_param.nim
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@@ -0,0 +1,23 @@
# issue #25826

proc foo(x: var) = x = 123
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@Araq Araq May 29, 2026

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I don't remember the part of my spec that says "you can write var without any type at all"...

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@demotomohiro demotomohiro May 29, 2026

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var type class is listed in Nim manual:
https://nim-lang.org/docs/manual.html#generics-type-classes
But it is not clear how it should work:
#9443 (comment)


block:
var a = 0
foo(a)
doAssert a == 123

proc bar(x: var int) =
foo(x)

block:
var a = 0
bar(a)
doAssert a == 123

proc baz(x: var int): var int = x

block:
var a = 0
foo(baz(a))
doAssert a == 123
10 changes: 10 additions & 0 deletions tests/typerel/tvar_type_class_param_fail.nim
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@@ -0,0 +1,10 @@
discard """
action: "reject"
errormsg: "type mismatch: got <int>"
"""
# issue #25826

proc foo(x: var) = x = 123

let a = 0
foo(a)
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