[BUG] `fields` on tuple of tuples returns a non-tuple depending on whether it's var or const

[timotheecour]

/cc @Araq

this is a reduction from a function that computes a join of tuples, or other processing on tuples; it's not a contrived example/

import macros
import typetraits

proc doInit[T]():auto=
  var a: T
  return a

proc fun2[T](tup1:T)=
  const tup0=doInit[T]()

  # var tup=tup0 #ok
  const tup=tup0 #causes bug

  assert tup is tuple
  assert tup[0] is tuple
  for ai in tup.fields:
    assert ai is tuple, "BUG2"

# const c=(foo:(bar1: 0.0))
const c=(foo:(bar1:"foo1"))
fun2(c)

• at runtime, gives: Error: unhandled exception: false BUG2 [AssertionError]

• when replacing const c=(foo:(bar1:"foo1")) by const c=(foo:(bar1: 0.0)), it gives CT error: Error: type mismatch: got <float64, int literal(0)>

• those problems go away if we use var tup=tup0 instead of const tup=tup0

• those problems go away if we use const c=(foo:(bar1: 0, bar2:0), foo2:(bar1: 0, bar2:0)) instead of const c=(foo:(bar1: 0.0))

# D20180412T184337
import macros
import typetraits

proc doInit[T]():auto=
  var a: T
  return a

proc fun[T](tup1:T)=
  const tup2=doInit[T]()
  echo "tup1:", tup1
  echo "tup2:", tup2

  # var tup=tup2 # works
  const tup=tup2 # fails

  assert tup[1] is tuple

  for ai in tup.fields:
    echo "ai:" & ai.type.name & " tuple?" & $(ai is tuple)
    assert ai is tuple, " BUG!"
  echo "----"

let a=(age:1, score: 2.5)

let b0=()
let b1=(bar1:"foo1", bar2:"foo2")
let b2=(bar1:"foo1")

fun((a,b0))
fun((a,b1))
fun((a,b2))

let c=(a,b2)
echo c[0] is tuple
echo c[1] is tuple

tup1:(Field0: (age: 1, score: 2.5), Field1: ())
tup2:(Field0: (age: 0, score: 0.0), Field1: ())
ai:(int, float64) tuple?true
ai:tuple[] tuple?true
----
tup1:(Field0: (age: 1, score: 2.5), Field1: (bar1: "foo1", bar2: "foo2"))
tup2:(Field0: (age: 0, score: 0.0), Field1: (bar1: "", bar2: ""))
ai:(int, float64) tuple?true
ai:(string, string) tuple?true
----
tup1:(Field0: (age: 1, score: 2.5), Field1: (bar1: "foo1"))
tup2:(Field0: (age: 0, score: 0.0), Field1: (bar1: ""))
ai:(int, float64) tuple?true
ai:string tuple?false
/Users/timothee/git_clone/nim/timn/bugs/t18_tuple_for_fields.nim(33) t18_tuple_for_fields
/Users/timothee/git_clone/nim/timn/bugs/t18_tuple_for_fields.nim(21) fun
/Users/timothee/.choosenim/toolchains/nim-#devel/lib/system.nim(3785) failedAssertImpl
/Users/timothee/.choosenim/toolchains/nim-#devel/lib/system.nim(3778) raiseAssert
/Users/timothee/.choosenim/toolchains/nim-#devel/lib/system.nim(2840) sysFatal
Error: unhandled exception: false  BUG! [AssertionError]
Error: execution of an external program failed: './app '

[timotheecour]

NOTE: implementing https://forum.nim-lang.org/t/3750/1#23343 ([RFC] for index, name in Tup.fieldsIndexed to get index at compile time of a tuple type) would allow circumventing this bug

[timotheecour]

if accepted, https://github.com/nim-lang/Nim/pull/7597 would allow a simple workaround