importcpp types don't work well at module scope

[timotheecour]

• requiresInit should forbid (during sempass) var a: A or var a = default(A) if A is a C++ struct/class with no default constructor; but currently it only gives a cgen error; that's the lower priority bug since the code is invalid anyway
• the more annoying bug is that var a = A(args) doesn't work at module scope due to current cgen implementation

Example

{.emit:"""
#include <stdio.h>
struct A{
  A(int a){ printf("ctor\n"); }
  ~A(){ printf("dtor\n"); }
};
""".}
type A {.importcpp: "A", requiresInit.} = object
proc initA(a: cint): A {.importcpp: "A(@)", constructor.}

proc fun() =
  var a1 = initA(12) # ok: this works
  # var a2: A # BUG: this should give semphase error but give cgen error
  # var a3 = default(A) # BUG: ditto
fun()

var a4 = initA(12) # BUG: this doesn't work but should

Current Output

@mt10772.nim.cpp:63:17: error: no matching constructor for initialization of 'A'

Expected Output

works

Possible Solution

cgen should instead use pointers for all global importcpp variables (regardless of whether they have default ctor or not):

instead of generating:

/* section: NIM_merge_VARS */
N_LIB_PRIVATE A a__kEcttXN5qSDQ71dflcCVww;
N_LIB_PRIVATE N_NIMCALL(void, NimMainModule)(void) {{
  a__kEcttXN5qSDQ71dflcCVww = A(((int) 12));
}}

the compiler should generate:

/* section: NIM_merge_VARS */
N_LIB_PRIVATE A* a__kEcttXN5qSDQ71dflcCVww;
N_LIB_PRIVATE N_NIMCALL(void, NimMainModule)(void) {{
  *a__kEcttXN5qSDQ71dflcCVww = A(((int) 12));
}}

Additional Information

• originally reported in 5 bugs with importcpp exceptions #14369 but moved it here as suggested here 5 bugs with importcpp exceptions #14369 (comment)
• bc42e9a 1.3.5