Invalid codegen when unpacking tuple via template

[arnetheduck]

Description

type A = object
  field: tuple[a, b, c: seq[int]]

func value(v: var A): var tuple[a, b, c: seq[int]] =
  v.field
template get(v: A): tuple[a, b, c: seq[int]] = v.value

var v = A(field: (@[1], @[2], @[3]))
var (a, b, c) = v.get()

echo a, b, c

this generates invalid C code where the var:ness of the returned value is not taken into into account because of the non-var tuple.

The result is memory corruption / crashes due to dereferencing mismatches and invalid pointer casts.

Nim Version

1.6.12

Current Output

[arnetheduck@praeceps nimbus-eth2]$ nim c -r "/tmp/testit.nim"
Hint: used config file '/home/arnetheduck/status/nimbus-eth2/vendor/nimbus-build-system/vendor/Nim/config/nim.cfg' [Conf]
Hint: used config file '/home/arnetheduck/status/nimbus-eth2/vendor/nimbus-build-system/vendor/Nim/config/config.nims' [Conf]
Hint: used config file '/home/arnetheduck/.config/nim/nim.cfg' [Conf]
..........................................................
CC: ../home/arnetheduck/status/nimbus-eth2/vendor/nimbus-build-system/vendor/Nim/lib/system/dollars.nim
CC: ../home/arnetheduck/status/nimbus-eth2/vendor/nimbus-build-system/vendor/Nim/lib/system.nim
CC: testit.nim
Hint:  [Link]
Hint: gc: refc; opt: none (DEBUG BUILD, `-d:release` generates faster code)
26667 lines; 0.362s; 31.699MiB peakmem; proj: /tmp/testit.nim; out: /tmp/testit [SuccessX]
Hint: /tmp/testit  [Exec]
out of memory

### Expected Output

_No response_

### Possible Solution

_No response_

### Additional Information

_No response_

[metagn]

Smaller case:

type A = object
  field: tuple[a: int]

func value(v: var A): var tuple[a: int] =
  v.field
template get(v: A): tuple[a: int] = v.value

var v = A(field: (a: 1))
echo get(v)[0]

error: 'T2_' is a pointer; did you mean to use '->'?
  T1_[0] = dollar___systemZdollars_u8(T2_.Field0);
                                         ^
                                         ->

[tersec]

A compile-time error from the C compiler is different than a runtime error which the C compiler allowed though, or?

[ringabout]

Smaller case:

I'm debugging this case, probably nkHiddenSubConv is swallowed somewhere.

[ringabout]

A compile-time error from the C compiler is different than a runtime error which the C compiler allowed though, or?

True, they are different issues, I have created a new issue for the second case.

[ringabout]

After half day's industrious work, now It's obvious that templates implicitly convert types even though it doesn't check the types at all. A dirty workaround that disables implicit conversions for templates works like a charm, solving two issues with one stone.