Is it legal to use the #line directive in a macro argument?

Question

I noticed that this program compiles with gcc:

#define X(A) A

int x = X(
#line 3 "test1.c"
        0
        );

However, Visual Studio fails to compile it:

main.cpp
main.cpp(6): error C2121: '#': invalid character: possibly the result of a macro expansion
main.cpp(6): error C2065: 'line': undeclared identifier
main.cpp(6): error C2143: syntax error: missing ';' before 'constant'
main.cpp(6): error C2059: syntax error: 'constant'

I wonder: is the program even legal, or is it silently relying on undefined (or implementation defined) behaviour which happens to make gcc accept the code?

Answer 1

It's invalid. The behaviour is undefined, so no diagnostic is required, and implementations are allowed to accept the code.

From the C standard, 6.10.3 Macro replacement:

11 [...] If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives, the behavior is undefined.

The C++ standard contains these exact same words in 16.3 Macro replacement.