[cfe-users] -Wunreachable-code warnings can no longer be suppressed?

[Chris Peterson via cfe-users]

On 8/6/16 9:50 AM, David Blaikie wrote:
> Could you provide a small example that fails (warns/doesn't suppress) with 3.9/ToT but succeeds (successfully suppresses the warning) with earlier?

Thanks for testing, David. Below is a standalone test case that 
reproduces the insuppressible warning even with Xcode's clang (Apple 
LLVM version 7.3.0 (clang-703.0.31)). So there is a latent clang bug 
plus a regression that caused Firefox's previously-suppressible warning 
to no longer be suppressible.

The warning looks like it might only happen when the boolean condition 
involves an expression with a templated type. In the test case below, a 
templated local variable gets copy-constructed to form an argument to a 
boolean helper-function.

I filed clang bug 28918 for this issue:

https://llvm.org/bugs/show_bug.cgi?id=28918

$ clang++ -Wunreachable-code test.cpp
test.cpp:23:5: warning: code will never be executed [-Wunreachable-code]
     printf("Does clang warn about this code being unreachable?\n");
     ^~~~~~
1 warning generated.

thanks,
chris


// Compile me like so:
//   clang++ -Wunreachable-code test.cpp
// and see if I produce a build warning.
//
// Note that if you change aSomeVec to be a non-templated type, then
// the build warning goes away.

#include "stdio.h"
#include <vector>

// Note: aSomeVec must be passed by value to trigger the problem, it seems.
// Changing to std::vector<int>& reference silences the warning.
static bool
FuncThatTakesATemplatedArg(std::vector<int> /* aSomeVec */)
{
   return false;
}

int main()
{
   std::vector<int> myVec;
   if ((false) &&
       !FuncThatTakesATemplatedArg(myVec)) {
     printf("Does clang warn about this code being unreachable?\n");
     return 1;
   }

   return 0;
}
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