This is a small snippet from a C++ library blend2d that can be used in C.

Here's the header file lib.h:

---
#ifdef __cplusplus
  #define BL_DEFINE_CONST static constexpr
  #define BL_DEFINE_ENUM(NAME) enum NAME : uint32_t
  #define BL_FORCE_ENUM_UINT32(ENUM_VALUE_PREFIX)
#else
  #define BL_DEFINE_CONST static const
  #define BL_DEFINE_ENUM(NAME) typedef enum NAME NAME; enum NAME
  #define BL_FORCE_ENUM_UINT32(ENUM_VALUE_PREFIX) ,ENUM_VALUE_PREFIX##_FORCE_UINT = 0xFFFFFFFFu
#endif

BL_DEFINE_ENUM(BLObjectType) {
  BL_OBJECT_TYPE_RGBA = 0,
  BL_OBJECT_TYPE_RGBA32 = 1,
};

int blArrayInit(BLObjectType arrayType);
---

Running the VC preprocessor:

---
cl /P /TC /EP /Filib.c lib.h
---

Generates this code:

---
typedef enum BLObjectType BLObjectType; enum BLObjectType {
  BL_OBJECT_TYPE_RGBA = 0,
  BL_OBJECT_TYPE_RGBA32 = 1,
};

int blArrayInit(BLObjectType arrayType);
---

This cannot be imported with DMD:

---
$ dmd -o- lib.c

lib.c(5): Error: cannot have parameter of opaque type `BLObjectType` by value
---

Moving the type def /after/ the enum definition fixes this as expected.

But it looks like existing C compilers (other than DMD's) seem to support this syntax.

Both VC and GCC work with these typedefs. I'm not sure what they do underneath the hood to make this work.