C11's static_assert

Helpful, yes.

But shouldn’t I be able to concatenate string literals (like everywhere else) for the message to be output? (Like I need to be able to do for some of my checking macros)

static_assert(2 == 3, "oh" " " "dear");

just errors with ‘oh’ on Norcroft cc5.91 whereas clang, gcc, MSVC are fine (see Compiler Explorer).

Oh hang on, it gets verse… It doesn’t like being placed at the end of a block.

static int foo(void) { _Static_assert(2 == 3, "Oh" " " "dear!"); }

"Serious Error: <command> expected but found '}'"

shouldn’t I be able to concatenate string literals (like everywhere else) for the message to be output?

Yes:

String Literals 

...

Semantics

4 In translation phase 6, the multibyte character sequences specified by any 
  sequence of adjacent character and wide string literal tokens are concatenated
  into a single multibyte character sequence. If any of the tokens are wide 
  string literal tokens, the resulting multibyte character sequence is treated
  as a wide string literal; otherwise, it is treated as a character string literal.

The standard also provides some example (excerpt):

EXAMPLE This pair of adjacent character string literals
"\x12" "3"

produces a single character string literal containing the two characters whose 
values are '\x12' and '3', because escape sequences are converted into single 
members of the execution character set just prior to adjacent string literal 
concatenation.

and

printf("x" "1" "= %d, x" "2" "= %s", x1, x2);

after concatenation

printf("x1= %d, x2= %s", x1, x2);

Indeedy. So why, he muses, is their processing in static_assert a bit wierd?

humm, this works however, so not sure what is going on with your issue:

#include <stdio.h>

int main(void)
{
   char test[100] = "This" " " "is" " " "a long" " " "string!" "\0";

   printf("%s\n", test);

   return 0;
}

Indeedy. So why, he muses, is their processing in static_assert a bit wierd?

either that or maybe it’s a macro? (no idea tbh, haven’t checked, so guessing)