cgen incorrectly considers type to have C array semantics

[shayanhabibi]

Example

type
  int128 {.importc:"__int128".} = distinct array[2, uint64]
  hint128 = object
    hi, lo: int64

proc atomicAddFetch[int128](p: ptr int128, v: int128, mo = ATOMIC_SEQ_CST): int128 {.importc:"__atomic_add_fetch", nodecl.}

var atm = hint128(hi: 1'i64)
var res = cast[hint128](atomicAddFetch[int128](
  cast[ptr int128](addr atm), cast[int128](hint128(lo: 1'i64))
))
echo repr atm
echo repr res

provides the following compilation error: operand type 'NU64 (*)[2]' {aka 'long long unsigned int (*)[2]'} is incompatible with argument 1 of '__atomic_add_fetch'

see the following cgen

\\ ...
	_A2u64* _2;
	__int128 _3;
\\ ...
	nimCopyMem((void*)_5, (NIM_CONST void*)__atomic_add_fetch(_2, _3, __ATOMIC_SEQ_CST), 16);
\\ ...

This is corrected by changing the int128 definition to the following:

type
  int128 {.importc:"__int128".} = object
    t,b: int64

[zerbina]

There are two problems with the generated C code:

1. the mentioned compilation error, caused by _2 being of type _A2u64* instead of __int128*
2. the __int128 values are cast into pointers and then used as the source and destination of a memcpy, which is nonsense

Both problems stem from int128 being considered as having C array semantics by cgen (mapType returns ctArray for the int128).

---

Whether this behaviour is correct (as in, cgen does what it's told) is not entirely clear. On the surface, this could be considered a bug, but it's less clear when taking a deeper look. I think the more fundamental question that needs to be answered first is what type T {.importc: N.} = distinct U actually means.

The current meaning could be summarized as:

• T is a type imported from C
• T acts as a distinct type from U in the type system, but both have the same underlying storage type, and conversions between them preserves lvalueness (i.e., var x: T; U(x) = default(U) is valid)

According to this meaning, I'd say that the current cgen behaviour is correct. However, I'm not sure whether that's good and/or useful. In case it is, type T {.importc: N.} = distinct U should be always be an error, because a type cannot be imported (exact storage and target-language semantics are unknown) and at have the same time have the same storage type as some other type.

[saem]

According to this meaning, I'd say that the current cgen behaviour is correct. However, I'm not sure whether that's good and/or useful. In case it is, type T {.importc: N.} = distinct U should be always be an error, because a type cannot be imported (exact storage and target-language semantics are unknown) and at have the same time have the same storage type as some other type.

As I raised in matrix, my thinking is that it should be an error, because:

my present understanding is that importc is an escape hatch of sorts. you're basically saying "this thing is unrepresentable in skull" so it's kinda weird that we'd know anything/much about the guts of the type. so having much more than object, or possibly some other primitive type, seems odd.