[cfe-dev] constexpr difference between gcc and clang

[David Wood]

The following compiles fine in macports gcc 4.7, but does not in clang svn
161307.  I believe the key is the const reference in combination with the
?: operator.

#include <stdexcept>

struct A {
  constexpr A(int a) : value(a) {}

  constexpr int get() { return value; }

private:
  int value;
};

constexpr A someFn(const A& a) {
  return a.get() == 0 ? throw std::exception() : a;
}

int main(int argc, char** argv) {
  constexpr A a(4);
  static_assert( someFn(a).get() == 4, "error" );
}

Some digging on the internets reveals
http://stackoverflow.com/questions/5605142/stdmax-and-stdmin-not-constexpr,
which seems to point to lvalues, glvalues and memory allocation, although I
did not follow it very well.  There is also the particularly interesting
comment :

The C++ committee have suggested that it was intended to be possible for
function invocation substitution to produce an lvalue referring to a
temporary. g++ behaves that way, but clang currently implements the
standard as written.

So, I have two questions.  1) Is the way clang handles this in fact
conformant to the spec, and gcc is lax? 2) Can someone take another crack
at explaining why it is that this doesn't work?

Thanks
--David
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