Order of destructor call of temporary and __cxa_throw

[strimo378]

I might have found an issue with the order of destructor call of a temporary and the call to the __cxa_throw function within a throw statement.

In the following code, the Clang frontend first calls __cxa_throw and afterwards a::~a. See https://godbolt.org/z/jbx74s1Ge

struct a {
  ~a() noexcept;
};
struct b {
  b(a) noexcept;
};
void c() {throw b(a()); }

GCC generates it in opposite order which is more logical to me. ICC generates it in the same order as Clang. I don't know what is correct from the C++ standard, maybe it is undefined.

[strimo378]

Maybe the problem is related to #50865

[strimo378]

Okey, it looks like that clang is correct and GCC not. I found the following text in the C++ standard [class.temporary]:

When an implementation introduces a temporary object of a class that has a non-trivial constructor (15.1,
15.8) , it shall ensure that a constructor is called for the temporary object. Similarly, the destructor shall be
called for a temporary with a non-trivial destructor (15.4) . Temporary objects are destroyed as the last step
in evaluating the full-expression (4.6) that (lexically) contains the point where they were created. This is true
even if that evaluation ends in throwing an exception. The value computations and side effects of destroying
a temporary object are associated only with the full-expression, not with any specific subexpression.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[shafik]

It does look like that is what the standard says

CC @MaskRay who might be able to give some insights here as well.