Clang allows template constructor to have different template argument list with that of class definition with default argument

[wangbo15]

The following invalid code is not rejected by clang for incompatible template parameter list between ctor and class definition:

template <class TClass, int NON=0> 
class T 
{ 
public: 
  T<TClass>(int f) {}
}; 

int main(){
    T<int> t(10);
}

EDG and GCC reject it. The diagnostic of GCC is weird though.

EDG rejects it with the following diagnostic, which seems more reasonable.

"<source>", line 5: error: type used as constructor name does not match type "T<TClass, NON>"
    T<TClass>(int f) {}
    ^

1 error detected in the compilation of "<source>".

Please see https://godbolt.org/z/s3KK85qxo

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Bo Wang (wangbo15)

The following invalid code is not rejected by clang for incompatible template parameter list between ctor and class definition:

template <class TClass, int NON=0> 
class T 
{ 
public: 
  T<TClass>(int f) {}
}; 

int main(){
    T<int> t(10);
}

EDG and GCC reject it. The diagnostic of GCC is weird though.

EDG rejects it with the following diagnostic, which seems more reasonable.

"<source>", line 5: error: type used as constructor name does not match type "T<TClass, NON>"
    T<TClass>(int f) {}
    ^

1 error detected in the compilation of "<source>".

Please see https://godbolt.org/z/s3KK85qxo

[zyn0217]

Might be related https://cplusplus.github.io/CWG/issues/1922.html

[chrchr-github]

FYI, MSVC also rejects: https://godbolt.org/z/xeerW83G5

[zygoloid]

Seems like Clang isn't doing any checking of the template argument list, at all. Eg, this is accepted:

template<typename> struct T { 
  T<"hello">() {}
}; 

[zygoloid]

Note also that this syntax became invalid in C++20 onwards. Clang also doesn't reject in C++20 onwards but should.