What if I do in another TU is_complete<incomplete, __LINE__> at the same line number and when incomplete is actually complete?
You can always add some more misc template parameters:
void foo1() { struct unique_foo1_tag; cout << is_complete<incomplete, __LINE__, unique_foo1_tag>::value << is_complete<incomplete_template<0> , __LINE__, unique_foo1_tag>::value << is_complete<incomplete_template<1> , __LINE__, unique_foo1_tag>::value << is_complete<complete, __LINE__, unique_foo1_tag>::value << is_complete<complete_abstract, __LINE__, unique_foo1_tag>::value << endl; }
And you can also use __COUNTER__ on MSVC.
__COUNTER__ won't do what you want across multiple TU's. The problem runs deeper: Translation Units a.cpp and b.cpp each instantiate a function foo<T> which uses is_complete<U>, but U is complete in TU a.cpp, and incomplete in b.cpp (because they include different headers). You now have an insidious ODR violation which would be very hard to diagnose and track down - which version of foo<T> is the correct one? John.