Possible preprocessor issues with empty token concatenation

[godlikepanos]

Imagine the following code:

#define MACRO(tokenA, tokenB, tokenC) tokenA##tokenB##tokenC
#define MACRO2(filled, none) MACRO(filled, C, none)

#define NONE()
MACRO2(A, NONE())

In gcc's and clang's preprocessor the outpout is AC. glslang in the other hand outputs ACtokenC.

A slight twist of the above:

#define MACRO(tokenA, tokenB, tokenC) tokenA##tokenB##tokenC
#define MACRO2(filled, none) MACRO(none, filled, C)

#define NONE()
MACRO2(A, NONE())

In gcc's and clang's preprocessor the outpout is AC once again. glslang in the other hand throws some errors:

ERROR: pp_test.frag:5: '##' : unexpected location 
ERROR: pp_test.frag:5: '##' : unexpected location

The following also fails on glslang:

#define MACRO(tokenA, tokenB, tokenC) tokenA##tokenB##tokenC
#define MACRO2(filled, none) MACRO(none, filled, C)

MACRO2(A, )

I'm not sure which behavior is correct but maybe you can verify. Thank you.

[johnkslang]

So, the C++ specification (normative for GLSL preprocessing) says

If (before argument substitution) any argument consists of no preprocessing tokens, the behavior is undefined.

So, generally, for portability, you'd want to set things up without using arguments that have no tokens in them.

Now, if you want to identify a specific behavior that HLSL does define and is important to use, I could look and see whether that is difficult to support or not.

[godlikepanos]

If (before argument substitution) any argument consists of no preprocessing tokens, the behavior is undefined.

I'm using GLSL so the spec quote and the explanation covers me.

DXC seems to have the same behavior as GCC and clang (as expected) but I don't care. If anyone does maybe they can reopen the ticket.

Closing for now. Thank you!