Template function error in clang but not gcc

Question

I've got the following code that has a template function that is not called at any point:

#include <exception>

class MyException : public std::exception {
protected:
        MyException( const char* name, const char* reason = nullptr ){
        }
};

template <typename T>
void doSomething( T& temp ){
        throw MyException( "reason" );
}

int main(int argc, char** argv){
}

When using g++, the code compiles just fine, however using clang the compilation fails with the following error:

$ clang++ -std=c++11 -o clang-test test.cpp
test.cpp:11:8: error: calling a protected constructor of class 'MyException'
        throw MyException( "reason" );
              ^
test.cpp:5:2: note: declared protected here
        MyException( const char* name, const char* reason = nullptr ){
        ^
1 error generated.

If I call the function doSomething , g++ will then fail with a similar error:

test.cpp:11:8: error: ‘MyException::MyException(const char*, const char*)’ is protected within this context

So it seems that g++ is not bothering to parse the template function if it is not called, while clang is. Are both g++ and clang valid in this case(eg it is compiler-specific/undefined), or is one of these compilers not aligned with the specification?

Answer 1

C++17 [temp.res]/8 says:

The program is ill-formed, no diagnostic required, if:

• (8.1) no valid specialization can be generated for a template or a substatement of a constexpr if statement within a template and the template is not instantiated, or
• [...]
• (8.3) a hypothetical instantiation of a template immediately following its definition would be ill-formed due to a construct that does not depend on a template parameter
• [...]

Otherwise, no diagnostic shall be issued for a template for which a valid specialization can be generated.

I'm never too sure about template instantiation stuff, but I think your program falls foul of both 8.1 and 8.3. Every instantiation of the template will be ill-formed due to the access check failing, and there are no valid specializations (ie no T which makes it valid).

So the program is ill-formed NDR and therefore neither compiler is bugged.