Ambiguity error in C++17 (template template parameters and default arguments issue)

Question

I have code which is differently interpreted by g++ with the c++14 and c++17 standard flags:

#include 
#include 

        

Answer 1

The behavior changed since C++17.

Before C++17, the code works because std::vector has two template parameters (the 2nd one has the default argument std::allocator<T>), while the template template parameter Vector is declared to have only one, they don't match then the 2nd func won't be considered.

Since C++17 (CWG 150), the default template arguments are allowed for a template template argument to match a template template parameter with fewer template parameters. That means both func become valid candidates and then leads to ambiguity.

template<class T> class A { /* ... */ };
template<class T, class U = T> class B { /* ... */ };

template<template<class> class P> class X { /* ... */ };

X<A> xa; // OK
X<B> xb; // OK in C++17 after CWG 150
         // Error earlier: not an exact match