In attempting to use function template argument deduction to select among a number of overloaded function templates and nontemplate functions, the compiler may attempt a specialization that fails on one or more of them.
template <typename T> void f( T );
template <typename T> void f( T * );
f( 1024 ); // instantiates first f
Even though substitution of the nonzero integer for T * in the second f function template would have been incorrect, the attempted substitution does not give rise to an error provided that a correct substitution is found. In this case, the first f is instantiated, and there is no error. Thus, we have the "substitution failure is not an error" concept, dubbed SFINAE by Vandevoorde and Josuttis.