Clang warning in dubious case: function 'foo' could be declared with attribute 'noreturn'?

I have been playing around a bit with the [[noreturn]] attribute, which I'm trying to get a handle of and make use of (I understand [[noreturn]] is a C++11 standard attribute and __attribute__((noreturn)) is a GCC/Clang extension). As part of this, I enabled the Clang warning -Wmissing-noreturn.

> clang++ -v
Ubuntu clang version 3.7.1-svn253742-1~exp1 (branches/release_37) (based on LLVM 3.7.1)
Target: x86_64-pc-linux-gnu
Thread model: posix

foo.cpp:

enum bar
{
  A = 1,
  B,
  C
};

void foo()
{
  switch (bar())
  {
    case A:
    case B:
    case C:
    default:
      break;
  }
}

int main()
{
  foo();
  return 0;
}

Then compile:

> clang++ foo.cpp -o foo -Wmissing-noreturn -std=c++14
foo.cpp:9:1: warning: function 'foo' could be declared with attribute 'noreturn'
      [-Wmissing-noreturn]
{
^
1 warning generated.

It appears to me that it would return! What's going on here? Is this a compiler bug?

If you remove the "= 1" from A, then it compiles fine without a warning.

If I do make the foo() function [[noreturn]] void foo(), then it does crash with a segmentation fault.