result variable is not hygienic in templates

[krux02]

When a template introduces a new variable, it is normally hidden in the context where the templates expands into, to prevent name clashes. The problem is, that when a template generates a function header, this function has this implicit result variable that is not hidden.

Best seen in this example

template foobar(targ: untyped): untyped =
  ## this template should only put ``arg`` in a context where a result
  ## variable exist.  The variables ``hidden`` I have put here to
  ## create a comparison of how a variables shold behave.
  proc foo(arg: bool): bool =
    result = arg
    # this variable is correctly hidden with the hygenic template.
    let hidden = "mystring"
    targ

  # Well this part of the template is pretty pointless.  I just put it
  # here, so the generated function is actually used.
  if foo(true):
    echo "bar"

  if foo(false):
    echo "bara"

proc bananas(): int =
  result = 123 # <-- should bind to this, but it doesn't ----,
  var hidden = 111 #                                          \
  foobar: #                                                   |
    # this binds correctly to the local variable.             |
    hidden += 123 #                                           |
    # this ``result`` binds to the result:bool defined in the |
    # template.  This violates hygenic macros.                /
    result += 123  # ----------------------------------------´

compilation output:

scratch.nim(20, 3) template/generic instantiation from here
scratch.nim(24, 12) Error: type mismatch: got <bool, int literal(123)>
but expected one of: 
proc `+=`[T: SomeOrdinal | uint | uint64](x: var T; y: T)
  first type mismatch at position: 2
  required type: T: int or int8
  but expression '123' is of type: int literal(123)
proc `+=`[T: float | float32 | float64](x: var T; y: T)
  first type mismatch at position: 1
  required type: var T: float or float32
  but expression 'result' is of type: bool

expression: result += 123

Compilation exited abnormally with code 1 at Mon May  7 13:58:34

This causes a problem in my ast pattern matching library:
krux02/ast-pattern-matching#6

[mratsim]

Maybe related to this: #7323

[krux02]

@mratsim The cause for the problem you link to is a different one. The quote do notation tries to bind local variables and functions that are in scope, including the result variable, and this basically goes wrong all the time.

[mratsim]

Forgot about this when I created #8208, it's not just the result variable, all variables are unhygienic when the template is used inside a proc.

[innerop]

Wondering, any progress on this?

[krux02]

I just tested it, it is still broken.