Different types of `decltype((const int)a)` and `decltype((const int)1)`

Question

// g++ 7.3
template<typename T>
struct td;

int main()
{
  int a = 1;

  td<decltype((const int)a)> t1;
  td<decltype((const int)1)> t2;

  return 0;
}

Below is the output of compilation:

error: aggregate ‘td<int> t1’ has incomplete type and cannot be defined
error: aggregate ‘td<const int> t2’ has incomplete type and cannot be defined

So, why are the types of decltype((const int)a) and decltype((const int)1) different?

Answer 1

The specifiers decltype((const int)a) and decltype((const int)1) both resolve to int. This is because there are no const prvalues of non-class type, as covered in C++17 [expr]:

If a prvalue initially has the type cv T, where T is a cv-unqualified non-class, non-array type, the type of the expression is adjusted to T prior to any further analysis.

Your output might just be a bug in the diagnostic message. To confirm a compiler bug you could write some code whose behaviour differs depending on the decltype result, e.g.:

decltype((const int)1) x;  x = 5;

which should compile successfully.