This code does not compile using mingw-w64 gcc in MSYS2 (and probably not in any version of gcc)

does_not_compile.txt

$ cat somefile.h
struct a;

struct a *init();
void setd(struct a*p, int dd);
int getd(struct a*p);
void cleanup(struct a*p);


$ cat somefile.c
struct a {
  int d;
};

struct a *init(){
    return malloc(sizeof(struct a));
}

void setd(struct a *p, int dd){
    p->d= dd;
}

int getd(struct a *p){
    return p->d;
}

void mfree(struct a *p){
    free(p);
}

$ cat myFile.c
#include "someFile.h"

int main(){
  struct a* d[2];
  d[0]= init();
  d[1]= init();
  struct a* pp = d[0];
  pp+=1; // thisis pointer arithmetic.
  //getd(pp); will return d[1]'s value.

}

$ gcc -v
Using built-in specs.
COLLECT_GCC=D:\msys64\mingw64\bin\gcc.exe
COLLECT_LTO_WRAPPER=D:/msys64/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/7.2.0/lto-wrapper.exe
Target: x86_64-w64-mingw32
Configured with: ../gcc-7.2.0/configure --prefix=/mingw64 --with-local-prefix=/mingw64/local --build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --with-native-system-header-dir=/mingw64/x86_64-w64-mingw32/include --libexecdir=/mingw64/lib --enable-bootstrap --with-arch=x86-64 --with-tune=generic --enable-languages=c,lto,c++,objc,obj-c++,fortran,ada --enable-shared --enable-static --enable-libatomic --enable-threads=posix --enable-graphite --enable-fully-dynamic-string --enable-libstdcxx-time=yes --disable-libstdcxx-pch --disable-libstdcxx-debug --disable-isl-version-check --enable-lto --enable-libgomp --disable-multilib --enable-checking=release --disable-rpath --disable-win32-registry --disable-nls --disable-werror --disable-symvers --with-libiconv --with-system-zlib --with-gmp=/mingw64 --with-mpfr=/mingw64 --with-mpc=/mingw64 --with-isl=/mingw64 --with-pkgversion='Rev1, Built by MSYS2 project' --with-bugurl=https://sourceforge.net/projects/msys2 --with-gnu-as --with-gnu-ld
Thread model: posix
gcc version 7.2.0 (Rev1, Built by MSYS2 project)

$ gcc myFile.c somefile.c -o testrun -Wall
myFile.c: In function 'main':
myFile.c:8:5: error: invalid use of undefined type 'struct a'
   pp+=1; // thisis pointer arithmetic.
     ^~
somefile.c: In function 'init':
somefile.c:6:12: warning: implicit declaration of function 'malloc' [-Wimplicit-function-declaration]
     return malloc(sizeof(struct a));
            ^~~~~~
somefile.c:6:12: warning: incompatible implicit declaration of built-in function 'malloc'
somefile.c:6:12: note: include '<stdlib.h>' or provide a declaration of 'malloc'
somefile.c: In function 'mfree':
somefile.c:18:5: warning: implicit declaration of function 'free' [-Wimplicit-function-declaration]
     free(p);
     ^~~~
somefile.c:18:5: warning: incompatible implicit declaration of built-in function 'free'
somefile.c:18:5: note: include '<stdlib.h>' or provide a declaration of 'free'
david@URANIUM MINGW64 ~/Documents/scraps/crap
$