Two phase lookup - explanation needed

Question

What does it mean that compiler is using two phase lookup in order to compile template class?

Answer 1

Templates are compiled (atleast) twice:

1. Without Instantiation the template code itself is checked for syntax.
   Eg: Any syntax errors such as ; etc.

2. At the time of instantiation(when the exact type is known), the template code is checked again to ensure all calls are valid for that particular type.
   Eg: The template might in turn call to functions which might not be present for that particular type.

This is called as Two Phase Lookup.