在模板模板参数的情况下，decltype（auto）可转发的参考非类型模板参数是否可转发

Question

Yet another decltype(auto) template template-parameter question. This time minimal code I was able to create to reproduce the error looks like this:

template <template <decltype(auto)> class TT, decltype(auto) V>
void foo(TT<V>) {
};

template <decltype(auto)>
struct Bar{};

int x;

int main() {
    foo(Bar<(x)>{});
}

This in [clang] results in:

prog.cc:11:5: error: no matching function for call to 'foo'
    foo(Bar<(x)>{});
    ^~~
prog.cc:2:6: note: candidate template ignored: substitution failure [with TT = Bar]: non-type template argument is not a constant expression
void foo(TT<V>) {
     ^
1 error generated.

[gcc] accepts the code.

To my understanding the code is well-formed and clang is buggy in its interpretation, but need the confirmation before submitting a bug to lvvm. Am I right?

Answer 1

According to error clang does not have a problem with deducing template template-parameter which is also standard compliant - [temp.arg.template]/3 (empasis mine):

A template-argument matches a template template-parameter P when P is at least as specialized as the template-argument A . If P contains a parameter pack, then A also matches P if each of A's template parameters matches the corresponding template parameter in the template-parameter-list of P. Two template parameters match if they are of the same kind (type, non-type, template), for non-type template-parameters, their types are equivalent ([temp.over.link]) , and for template template-parameters, each of their corresponding template-parameters matches, recursively. When P's template-parameter-list contains a template parameter pack, the template parameter pack will match zero or more template parameters or template parameter packs in the template-parameter-list of A with the same type and form as the template parameter pack in P (ignoring whether those template parameters are template parameter packs)

Now lets make sure the Bar<(x)>{} should be deduced as reference. This is covered by [dcl.type.auto.deduct]/5 and [dcl.type.simple]/4 .

Finally lets check if we actually can use the reference to the variable with linkage as a template argument [temp.arg.nontype]/2 :

A template-argument for a non-type template-parameter shall be a converted constant expression of the type of the template-parameter. For a non-type template-parameter of reference or pointer type, the value of the constant expression shall not refer to (or for a pointer type, shall not be the address of) :

• a subobject,
• a temporary object,
• a string literal,
• the result of a typeid expression, or
• a predefined ­ func__ variable.

[ Note: If the template-argument represents a set of overloaded functions (or a pointer or member pointer to such), the matching function is selected from the set ([over.over]). — end note ]

The deduced argument fulfils the requirements. This makes the code well-formed and suggests clang's bug.

Filed bug 34690