Generic template parameter order determines whether compilation succeeds

[tersec]

A template can't bind to generic types in a specific combination involving array and int64 when parameter are orders in one way, but not when they're reversed.

Example

type
  Foo*[maxLen: static int64; T] = object
    data*: array[maxLen, T]

  Baz*[maxLen: static int64; T] = object
    data*: array[maxLen, T]

# test.nim(12, 42) Error: cannot instantiate Foo
# got: <static[type T](T), type U>  (Nim 1.4)
# got: <typedesc[T], typedesc[U]>   (Nim current head)
# but expected: <maxLen, T>
template bar[T, U](x: array[T, U], y: Foo[T, U]) = discard # comment this line and it compiles

template bar[T, U](x: array[T, U], y: array[T, U]) = discard
template bar[T, U](x: Foo[T, U], y: array[T, U]) = discard
template bar[T, U](x: Foo[T, U], y: Foo[T, U]) = discard

# These all compile fine; it appears to relate to taking array before Foo
template bar[T, U](x: Baz[T, U], y: Foo[T, U]) = discard
template bar[T, U](x: Baz[T, U], y: Baz[T, U]) = discard
template bar[T, U](x: Foo[T, U], y: Baz[T, U]) = discard
template bar[T, U](x: Foo[T, U], y: Foo[T, U]) = discard

Current Output

$ Nim/bin/nim --version
Nim Compiler Version 1.5.1 [Linux: amd64]
Compiled at 2021-03-04
Copyright (c) 2006-2021 by Andreas Rumpf

git hash: c180de60a8ac5459d1e553152132fcfc6cd2fe6f
active boot switches: -d:release
$ Nim/bin/nim c test.nim
Hint: used config file 'Nim/config/nim.cfg' [Conf]
Hint: used config file 'Nim/config/config.nims' [Conf]
....
test.nim(12, 42) Error: cannot instantiate Foo
got: <typedesc[T], typedesc[U]>
but expected: <maxLen, T>

Expected Output

A successful compilation, or at least some error message explaining why the apparently inconsistent situation makes sense.

Additional Information

I've checked that this also occurs in

$ nim --version
Nim Compiler Version 1.4.2 [Linux: amd64]
Compiled at 2020-12-02
Copyright (c) 2006-2020 by Andreas Rumpf

active boot switches: -d:release
$ nim c test.nim
Hint: used config file '/etc/nim/nim.cfg' [Conf]
Hint: used config file '/etc/nim/config.nims' [Conf]
....
test.nim(12, 42) Error: cannot instantiate Foo
got: <type T, type U>
but expected: <maxLen, T>

and

$ ./env.sh nim --version
Nim Compiler Version 1.2.6 [Linux: amd64]
Compiled at 2021-02-27
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: bf320ed172f74f60fd274338e82bdc9ce3520dd9
active boot switches: -d:release
$ ./env.sh nim c ../test.nim 
test.nim(12, 42) Error: cannot instantiate Foo
got: <static[type T](T), type U>
but expected: <maxLen, T>

[metagn]

Still errors but since #24005 you can change Foo[T, U] to Foo[x.len, U] for it to work. The T inferred in array[T, U] is a range type, not a static type, so it can't be applied to Foo, but array types still accept static ints for their first parameter. So another workaround would be a wrapper type for Foo that turns the first parameter from a range to a static int which should also work in 2.0.