[c++11] clang allows questionable CTAD for template base of derived class with default argument

[Rush10233]

The following code is accepted by clang but rejected by GCC,MSVC and ICC with the option of -std=c++11:

template<typename,typename=int> struct Base { };

struct Derived : Base<void> { };

template<template<typename> class TT>
void func (TT<void>) { }

int main () {
  func (Derived { });
}

https://timsong-cpp.github.io/cppwp/std11/temp.deduct.call#4 says that "If P is a class and P has the form simple-template-id, then the transformed A can be a derived class of the deduced A." So the class Derived passed in line 9 can be deduced to its base type.

But the parameter requested by func is a template class with 1 argument, while class Base has 2(including 1 with default value), which seems incompatible.

It's interesting that gcc accepts that code under -std=c++20, but other compilers don't. It seems that more standard docs need to be consulted.

https://godbolt.org/z/G7fenEsYY

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (Rush10233)

The following code is accepted by clang but rejected by GCC,MSVC and ICC with the option of `-std=c++11`:

template<typename,typename=int> struct Base { };

struct Derived : Base<void> { };

template<template<typename> class TT>
void func (TT<void>) { }

int main () {
  func (Derived { });
}

[https://timsong-cpp.github.io/cppwp/std11/temp.deduct.call#4](https://timsong-cpp.github.io/cppwp/std11/temp.deduct.call#4) says that "If P is a class and P has the form simple-template-id, then the transformed A can be a derived class of the deduced A." So the class Derived passed in line 9 can be deduced to its base type.

But the parameter requested by func is a template class with 1 argument, while class Base has 2(including 1 with default value), which seems incompatible.

It's interesting that gcc accepts that code under -std=c++20, but other compilers don't. It seems that more standard docs need to be consulted.

https://godbolt.org/z/G7fenEsYY

[MitalAshok]

I think this is CWG150/P0522R0, which was controlled by -frelaxed-template-template-args, which became on by default in #89807

@mizvekov Should this be accepted in C++11/14:

template<typename,typename=int> struct Base { };

template<template<typename> class TT>
void func (TT<void>) { }

int main () {
  func (Base<void> { });
}

? It appears the core issue only made changes to C++17