Created attachment 42767 [details]
zip file with all the files to reproduce

on x86 `int` instruction needs to receive `imm8`. g++ allows to pass an input operand in inline assembly as "register" too. Example:

template<unsigned char int_no> void int_test_pass() {
  asm("int %0"
    :
    : "N"(int_no)
    :);
}

called with:
int_test_pass<10>();

results in assembly (cutting out garbage):
  _Z13int_test_passILh10EEvv:
  	int $10

but:
template<unsigned char int_no> void int_test_fail() {
  asm("int %0"
    :
    : "r"(int_no)
    :);
}

quietly results in assembly like this. no errors:
  _Z13int_test_failILh10EEvv:
	movl	$10, %eax
	int %al

which is illigal. I've tested this on:
g++-7 (Ubuntu 7.2.0-1ubuntu1~16.04) 7.2.0                                             
g++-6 (Ubuntu/Linaro 6.3.0-18ubuntu2~16.04) 6.3.0 20170519                            
g++-5 (Ubuntu 5.4.1-2ubuntu1~16.04) 5.4.1 20160904                                    
g++-4.9 (Ubuntu 4.9.4-2ubuntu1~16.04) 4.9.4                                           
g++-4.8 (Ubuntu 4.8.5-4ubuntu2) 4.8.5                                                 
g++-4.7 (Ubuntu/Linaro 4.7.4-3ubuntu12) 4.7.4                                         
g++-4.6 (Ubuntu/Linaro 4.6.4-6ubuntu6) 4.6.4                                          

on all above, this program compiles with no errors on -Wall -Werror.
Clang on the other hands breaks compilation with an error:
  gcc_imm8_bug.cpp:9:7: error: invalid operand for instruction
    asm("int %0"
        ^
  <inline asm>:1:6: note: instantiated into assembly here
          int %al
            ^~~
  1 error generated.