g++ c++17 class template argument deduction not working in a very specific case

Question

I have the following code:

template <class T>
class lit {
public:
    lit(T l) : val(l) {}
    T val;
};

template <class T>
class cat {
public:
    cat(lit<T> const& a, lit<T> const& b) : a(a), b(b) {}
    lit<T> const& a;
    lit<T> const& b;
};

template <class T>
cat<T> operator+(lit<T> const& a, lit<T> const& b) {
    return cat(a, b);
}

int main() {
    auto r1 = cat((lit      ('b')),  lit('d')); // compiles
    auto r2 =     (lit      ('b')) + lit('d') ; // doesn't compile
    auto r3 =      lit      ('b')  + lit('d') ; // compiles
    auto r4 =     (lit      ('b'))            ; // compiles
    auto r5 =     (lit<char>('b')) + lit('d') ; // compiles
}

This compiles fine with clang (as I would expect), but gcc produces the following error:

prog.cc: In function 'int main()':
prog.cc:23:20: error: missing template arguments after 'lit'
     auto r2 =     (lit      ('b')) + lit('d') ; // doesn't compile
                    ^~~
prog.cc:2:7: note: 'template<class T> class lit' declared here
 class lit : public ExpressionBuilder<T> {
       ^~~

It seems to not be able to figure out the class template deduction from the constructor only in one very specific case (r2). I am assuming gcc is wrong, but can someone explain why it would fail only in this very specific case?

Example here: https://wandbox.org/permlink/jQCOhXFFQekS17Y1

Answer 1

This is a brand new feature in C++17, and therefore brand new in GCC. The pattern you've observed — or lack thereof — looks very much like a compiler bug. The way that it is triggered apparently randomly also fits that pattern.

Delving further into the exact hows and whys is a tedious job for the GCC devs, not for a Stack Overflow answer, as it's likely to be extraordinarily complex… but the correct approach now is to raise a bug and watch what happens. (OP has now done that, as bug 87709.)

Related examples do already exist on Bugzilla.