Can auto placeholder be used to deduce function result in non-type template parameter?

Question

Consider simple example:

template 
struct Foo {
    decltype(X()) x;
};

int bar();

int main() {
    static_cast(Foo

        

Answer 1

Yes, auto may be used inside a compound type ([temp.param]/4.6, [dcl.type.auto.deduct]). I believe that gcc is in error in your second example: your explicitly specified T of int is substituted before performing deduction ([temp.deduct]/2.3, /5, and /6, referenced by [dcl.type.auto.deduct]/2.3 and /4).