Codegen bug with proc with union arguments with default values

[ghost]

This code errors at C compilation step

Example

proc myMegaProc(a: string, b: seq[string] | seq[bool] = @[]) = 
  when a is string:
    discard
  elif b is seq[string]:
    for x in b:
      echo x

myMegaProc("hello")

Current Output

CC: hello.nim
In file included from /home/dian/.cache/nim/hello_d/@mhello.nim.c:9:
/home/dian/.cache/nim/hello_d/@mhello.nim.c:82:31: error: conflicting types for 'myMegaProc__lWBnHp6d1v1cWhDKZfWHiQ'
   82 | N_LIB_PRIVATE N_NIMCALL(void, myMegaProc__lWBnHp6d1v1cWhDKZfWHiQ)(NimStringDesc* a, tySequence__77YVzYb2AOu2vP0iI0b8Dw* b) {
      |                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/dian/.nim/lib/nimbase.h:250:44: note: in definition of macro 'N_NIMCALL'
  250 | #  define N_NIMCALL(rettype, name) rettype name /* no modifier */
      |                                            ^~~~
/home/dian/.cache/nim/hello_d/@mhello.nim.c:44:31: note: previous declaration of 'myMegaProc__lWBnHp6d1v1cWhDKZfWHiQ' was here
   44 | N_LIB_PRIVATE N_NIMCALL(void, myMegaProc__lWBnHp6d1v1cWhDKZfWHiQ)(NimStringDesc* a, TGenericSeq* b);
      |                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/dian/.nim/lib/nimbase.h:250:44: note: in definition of macro 'N_NIMCALL'
  250 | #  define N_NIMCALL(rettype, name) rettype name /* no modifier */
      |                                            ^~~~
Error: execution of an external compiler program 'gcc -c  -w   -I/home/dian/.nim/lib -I/home/dian/Projects/skidfix -o /home/dian/.cache/nim/hello_d/@mhello.nim.c.o /home/dian/.cache/nim/hello_d/@mhello.nim.c' failed with exit code: 1

Expected Output

Compiler error message

Additional Information

• I consider this to be low-priority because the code is not valid Nim anyway

$ nim -v
Nim Compiler Version 1.3.1 [Linux: amd64]
Compiled at 2020-04-09
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: cc3f5d21b249fe8f96f61369e53e794bf34e65a5
active boot switches: -d:release

[metagn]

Similar to #4858 but with empty collection literals instead of int literal

[metagn]

Now gives

Error: internal error: cannot map the empty seq type to a C type

It should still give a proper error though