Warning when comparing references to bool and int with MSVC 2015

Question

The following code produces a warning with MSVC (2015 Update 3) - with /W4:

const bool& a = true;
const int& b = 1;
if(a == b)

C4805: '==': unsafe mix of type 'const bool' and type 'const int' in operation

but without the references it compiles cleanly.

const bool a = true;
const int b = 1;
if(a == b)

WHY?

EDIT:

just tested without const as well

bool a = true;
int b = 1;
if(a == b)

and the warning reappeared...

EDIT 2:

Compiling in Debug... I did have to silence C4127: conditional expression is constant in the const noref case though...

EDIT 3:

here are the disassemblies for the 3 cases:

const ref

0113BA92  in          al,dx  
0113BA93  sub         esp,24h  
0113BA96  mov         eax,0CCCCCCCCh  
0113BA9B  mov         dword ptr [ebp-24h],eax  
0113BA9E  mov         dword ptr [ebp-20h],eax  
0113BAA1  mov         dword ptr [ebp-1Ch],eax  
0113BAA4  mov         dword ptr [ebp-18h],eax  
0113BAA7  mov         dword ptr [b],eax  
0113BAAA  mov         dword ptr [ebp-10h],eax  
0113BAAD  mov         dword ptr [ebp-0Ch],eax  
0113BAB0  mov         dword ptr [ebp-8],eax  
0113BAB3  mov         dword ptr [a],eax  
    const bool& a = true;
0113BAB6  mov         byte ptr [ebp-9],1  
0113BABA  lea         eax,[ebp-9]  
0113BABD  mov         dword ptr [a],eax  
    const int& b = 1;
0113BAC0  mov         dword ptr [ebp-1Ch],1  
0113BAC7  lea         ecx,[ebp-1Ch]  
0113BACA  mov         dword ptr [b],ecx  
    if(a == b)
0113BACD  mov         edx,dword ptr [a]  
0113BAD0  movzx       eax,byte ptr [edx]  
0113BAD3  mov         ecx,dword ptr [b]  
0113BAD6  cmp         eax,dword ptr [ecx]  
0113BAD8  jne         DOCTEST_ANON_FUNC_2+5Fh (0113BAEFh)  
        throw 5;
0113BADA  mov         dword ptr [ebp-24h],5  
0113BAE1  push        offset __TI1H (0117318Ch)  
0113BAE6  lea         edx,[ebp-24h]  
0113BAE9  push        edx  
0113BAEA  call        __CxxThrowException@8 (01164B04h)  

const only

0137BA92  in          al,dx  
0137BA93  sub         esp,0Ch  
0137BA96  mov         dword ptr [ebp-0Ch],0CCCCCCCCh  
0137BA9D  mov         dword ptr [b],0CCCCCCCCh  
0137BAA4  mov         dword ptr [ebp-4],0CCCCCCCCh  
    const bool a = true;
0137BAAB  mov         byte ptr [a],1  
    const int b = 1;
0137BAAF  mov         dword ptr [b],1  
    if(a == b)
0137BAB6  mov         eax,1  
0137BABB  test        eax,eax  
0137BABD  je          DOCTEST_ANON_FUNC_2+44h (0137BAD4h)  
        throw 5;
0137BABF  mov         dword ptr [ebp-0Ch],5  
0137BAC6  push        offset __TI1H (013B318Ch)  
0137BACB  lea         ecx,[ebp-0Ch]  
0137BACE  push        ecx  
0137BACF  call        __CxxThrowException@8 (013A4B04h)  

no const no ref

0012BA92  in          al,dx  
0012BA93  sub         esp,0Ch  
0012BA96  mov         dword ptr [ebp-0Ch],0CCCCCCCCh  
0012BA9D  mov         dword ptr [b],0CCCCCCCCh  
0012BAA4  mov         dword ptr [ebp-4],0CCCCCCCCh  
    bool a = true;
0012BAAB  mov         byte ptr [a],1  
    int b = 1;
0012BAAF  mov         dword ptr [b],1  
    if(a == b)
0012BAB6  movzx       eax,byte ptr [a]  
0012BABA  cmp         eax,dword ptr [b]  
0012BABD  jne         DOCTEST_ANON_FUNC_2+44h (012BAD4h)  
        throw 5;
0012BABF  mov         dword ptr [ebp-0Ch],5  
0012BAC6  push        offset __TI1H (016318Ch)  
0012BACB  lea         ecx,[ebp-0Ch]  
0012BACE  push        ecx  
0012BACF  call        __CxxThrowException@8 (0154B04h)  

So it seems there is awlays a jump for the if statement so it isn't optimized and not warning-diagnosed in the const case (debug config)...

Answer 1

This is what happens with GCC 6.1 for the following code:

int ref(int num) {
   const bool& a = true;
   const int& b = 1;
   return a == b;
}

int noref(int num) {
   const bool a = true;
   const int b = 1;
   return a == b;
}


int noref_noconst(int num) {
   bool a = true;
   int b = 1;
   return a == b;
}

Assembly output:

ref(int):
        pushq   %rbp
        movq    %rsp, %rbp
        movl    %edi, -36(%rbp)
        movl    $1, %eax
        movb    %al, -21(%rbp)
        leaq    -21(%rbp), %rax
        movq    %rax, -8(%rbp)
        movl    $1, %eax
        movl    %eax, -20(%rbp)
        leaq    -20(%rbp), %rax
        movq    %rax, -16(%rbp)
        movq    -8(%rbp), %rax
        movzbl  (%rax), %eax
        movzbl  %al, %edx
        movq    -16(%rbp), %rax
        movl    (%rax), %eax
        cmpl    %eax, %edx
        sete    %al
        movzbl  %al, %eax
        popq    %rbp
        ret
noref(int):
        pushq   %rbp
        movq    %rsp, %rbp
        movl    %edi, -20(%rbp)
        movb    $1, -1(%rbp)
        movl    $1, -8(%rbp)
        movl    $1, %eax
        popq    %rbp
        ret
noref_noconst(int):
        pushq   %rbp
        movq    %rsp, %rbp
        movl    %edi, -20(%rbp)
        movb    $1, -1(%rbp)
        movl    $1, -8(%rbp)
        movzbl  -1(%rbp), %eax
        cmpl    -8(%rbp), %eax
        sete    %al
        movzbl  %al, %eax
        popq    %rbp
        ret

The easiest thing to note here about the assembly is that in the case of noref there isn't even a jump; it's just a simple return 1.

The same thing is probably happening in MSVC, which then slips past the detection mechanism for warnings, and you just don't get one.

EDIT:

Let's look at your assembly, and the const only version in particular:

    if(a == b)
0137BAB6  mov         eax,1  
0137BABB  test        eax,eax  
0137BABD  je          DOCTEST_ANON_FUNC_2+44h (0137BAD4h)  

While it is true that the if statement (je) is there, we can see that it doesn't actually compare the variables--which is the precise operation that triggers the warning. It simply puts 1 in a register and then compares the register with itself.