Bison 1.875 generates code which contains something like this:-

yyerrlab1:
 __attribute__ ((__unused__))

Now this works for C compilation (ie. tells the compiler yyerrlab1 might not be
used, so stops the compiler generating a warning), but fails to compile for C++.

The following test code is similar to what bison does. It compiles for C, but
not for C++:-

void fred()
{
   int x;
   
yyerrlab1:
 __attribute__ ((__unused__))

 x = 42;
 
}
p.cc: In function `void fred()':
p.cc:8: error: ISO C++ forbids declaration of `x' with no type
p.cc:8: error: redeclaration of `int x'
p.cc:3: error: `int x' previously declared here
p.cc:8: error: declaration of `int x'
p.cc:3: error: conflicts with previous declaration `int x'


The documentation "Attribute Syntax" info node appears to say this should work:-

"   An attribute specifier list may appear after the colon following a
label, other than a `case' or `default' label.  The only attribute it
makes sense to use after a label is `unused'.  This feature is intended
for code generated by programs which contains labels that may be unused
but which is compiled with `-Wall'.  It would not normally be
appropriate to use in it human-written code, though it could be useful
in cases where the code that jumps to the label is contained within an
`#ifdef' conditional.
"