error in unused template method
c++, clang, gcc, non-static, templates
Solution
Your program is incorrect, and both compilers are right as the standard does not require a diagnostic from a conforming compiler (letting gcc ignore it). A template for which there can be no valid instantiation (specialization in standard jargon) is incorrect even if that template is never instantiated.
In your case, the name `B::foo()` inside `A<T>::A(int)` is a non-dependent name, so it needs to be resolved during the first phase lookup, and it can only refer to the `B` class defined above. Because it is not a `static` member function, but a non-static one, the code is incorrect, regardless of the type `T` used to instantiate the `A<T>` template and the program is ill-formed.
The relevant quote is from 14.6 [temp.res]/8:
Knowing which names are type names allows the syntax of every template definition to be checked. No diagnostic shall be issued for a template definition for which a valid specialization can be generated. If no valid specialization can be generated for a template definition, and that template is not instantiated, the template definition is ill-formed, no diagnostic required.
Problem
``` struct B { int a; void foo() {a = 5;} }; template <typename T> struct A { A(int i) { B::foo(); } A(double d) {} }; int main() { A<int> a(5.0); } ``` gcc 4.7.2 compiles it without errors. clang 3.4svn complains: ``` $ clang -Wall -Wextra test.cpp test.cpp:10:16: error: call to non-static member function without an object argument A(int i) { B::foo(); } ~~~^~~ ``` Of course the code is wrong, but which compiler is conformant with the standard? It's also strange that clang doesn't print any 'in instantiation' note like gcc does if you use 5 instead of 5.0: ``` $ gcc test.cpp test.cpp: In instantiation of ‘A<T>::A(int) [with T = int]’: test.cpp:15:12: required from here test.cpp:9:13: error: cannot call member function ‘void B::foo()’ without object ```