Fixing C's Biggest Mistake

[Iain Buclaw]

On Thursday, 22 December 2022 at 21:18:04 UTC, Walter Bright 
wrote:
> On 12/22/2022 2:21 AM, Iain Buclaw wrote:
>> Which no one will ever use. :-)
>
> They were very useful for the ImportC test suite.

It's still a DMD folly.  GDC has been able to compile C files 
since day 0 (19 years ago?) as it delegates foreign sources to 
the other compilers it was built with - the reverse also works, 
GCC can compile D sources because it uses the same mechanism.  So 
you could say that C and C++ have been capable of compiling D 
sources since 2019. ;-)

```
$ gccgo -fno-druntime compiler/test/runnable/bcraii.d -o go-bcraii
$ ./go-bcraii
S.this()
S.~this()
inside
```

None of these tests where dmd pretends to be a C compiler even 
work outside of dmd.

```
$ gdc compiler/test/compilable/cimport.c
compiler/test/compilable/cimport.c:3:1: error: unknown type name 
‘__import’
     3 | __import core.stdc.stdarg;
       | ^~~~~~~~
compiler/test/compilable/cimport.c:3:14: error: expected ‘=’, 
‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘.’ token
     3 | __import core.stdc.stdarg;
       |              ^
compiler/test/compilable/cimport.c:4:1: error: unknown type name 
‘__import’
     4 | __import imports.impcimport;
       | ^~~~~~~~
compiler/test/compilable/cimport.c:4:17: error: expected ‘=’, 
‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘.’ token
     4 | __import imports.impcimport;
       |                 ^
compiler/test/compilable/cimport.c: In function ‘foo’:
compiler/test/compilable/cimport.c:8:5: error: unknown type name 
‘va_list’
     8 |     va_list x;
       |     ^~~~~~~
compiler/test/compilable/cimport.c:1:1: note: ‘va_list’ is 
defined in header ‘<stdarg.h>’; did you forget to ‘#include 
<stdarg.h>’?
   +++ |+#include <stdarg.h>
     1 | // https://issues.dlang.org/show_bug.cgi?id=22666
compiler/test/compilable/cimport.c:9:16: error: ‘A’ undeclared 
(first use in this function)
     9 |     return 1 + A;
       |                ^
compiler/test/compilable/cimport.c:9:16: note: each undeclared 
identifier is reported only once for each function it appears in
```

DMD becoming a C compiler is a side-effect of importC, but just 
because you can doesn't mean you should.