Error: invalid type for const: T prevents default initializing at compile time when T has a reference field

[timotheecour]

proc defaultImpl[T]():T=
  var a: T
  return a

type App = object
  a: ref int

proc foo(): auto=
  let val = defaultImpl[App]()
  return val

proc test(): auto=
  when defined(case1):
    # ok
    const val = defaultImpl[ref int]()
    echo repr(val)

  when defined(case2):
    # Error: invalid type for const: App
    const val = defaultImpl[App]()

  when defined(case3):
    # ok
    let val = defaultImpl[App]()
    echo repr(val)

  when defined(case4):
    # Error: invalid type for const: App
    const val = foo()

  ## EDIT: case5 is unrelated => see https://github.com/nim-lang/Nim/issues/8733
  when defined(case5):
    # Error: cannot evaluate at compile time: val
    var val{.compileTime.} = defaultImpl[App]()
    echo repr(val)

test()

/cc @Araq
This is a blocker for doing compile time reflection (and doing many things that rely on it). Any chance this could be fixed?

• it also limits fix #8485 ; make default public instead of redefined everywhere; use better implementation #8490

• EDIT it's also inconsistent with the fact that this works:

const foo = defaultImpl[seq[seq[string]]]()

(seq and string are reference types)

[GULPF]

I'm very surprised that const a: ref int = nil compiles. IMO const of a ref type doesn't make any sense. Note that the limitation is specifically for const, not for compile time:

# Works fine
static:
  var a: ref int
  new(a)
  a[] = 1
  echo a[]

[andreaferretti]

I think this makes sense, right? You cannot have a constant reference, the actual address will be unknown until runtime. You can do this in the case of nil, although I am not sure how useful this is

[timotheecour]

Note that the limitation is specifically for const, not for compile time:

when static: var a: int is inside a proc, it gives compile error.

[timotheecour]

You cannot have a constant reference, the actual address will be unknown until runtime. You can do this in the case of nil, although I am not sure how useful this is

It's useful for generic programming. And yes, the value should obviously be nil, but it doesn't compile.

[Araq]

This is a blocker for doing compile time reflection (and doing many things that rely on it). Any chance this could be fixed?

No, it makes no sense to support every type T for a const section. And it doesn't block anybody from writing good software no matter how many grandiose nouns you use to describe the issue.

[timotheecour]

the problem is when a type T contains a (sub-sub-...) field which has a ref. That field will (currently) prevent T from being in const section, even if we're not dereferencing that field.

[timotheecour]

Note that the limitation is specifically for const, not for compile time

EDIT: just added a new test case (see case5) with var val{.compileTime.} = defaultImpl[App]() ; that also fails

[GULPF]

@timotheecour I think case5 is unrelated. The following also fails to compile with the same error:

proc foo =
  var val {.compileTime.} = 1

It looks like {.compileTime.} is broken inside procs. The following example with {.compileTime.} and default works:

proc defaultImpl[T]():T=
  var a: T
  return a

type App = object
  a: ref int

var val {.compileTime.} = defaultImpl[App]()

static:
  echo val

[timotheecour]

@GULPF you're right, case5 is unrelated, moved it to #8733

[Araq]

"Error: invalid type for const: App" is a good error message and works as designed. Closing.