[Bug c++/66042] New: Implicitly converts lvalue to rvalue when returning reference parameter in function template

["david at doublewise dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66042

            Bug ID: 66042
           Summary: Implicitly converts lvalue to rvalue when returning
                    reference parameter in function template
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david at doublewise dot net
  Target Milestone: ---

#include <type_traits>

template<typename T>
auto compiles(T && t) -> T && {
        static_assert(std::is_same<T, int>::value, "Incorrect type deduced for
T.");
        return t;
}
static_assert(std::is_same<int &&, decltype(compiles(0))>::value, "shouldn't
compile");

auto fails(int && t) -> int && {
        return t;
}
static_assert(std::is_same<int &&, decltype(fails(0))>::value, "doesn't
compile");


Neither function should compile. In the function template case, T is deduced as
int, so the parameter type is int &&. Local variables and function parameters
taken by value can be implicitly moved into the return value, but not function
parameters taken by rvalue reference (t is an lvalue, even though decltype(t)
== int &&).

The non-template function correctly honors this behavior.

If the return type of compiles is changed to decltype(auto) or auto &&, then
the function correctly returns int & and the static_assert fires.