Overloading based on ‘var T’ / ‘out T’

If the formal parameter f is of type var T (or out T) in addition to the ordinary type checking, the argument is checked to be an l-value. var T (or out T) matches better than just T then.

proc sayHi(x: int): string =
  # matches a non-var int
  result = $x
proc sayHi(x: var int): string =
  # matches a var int
  result = $(x + 10)
proc sayHello(x: int) =
  var m = x # a mutable version of x
  echo sayHi(x) # matches the non-var version of sayHi
  echo sayHi(m) # matches the var version of sayHi
sayHello(3) # 3
            # 13

An l-value matches var T and out T equally well, hence the following is ambiguous:

proc p(x: out string) = x = ""
proc p(x: var string) = x = ""
var v: string
p(v) # ambiguous