Consider (the name of the file is hello.cpp) this code; the idea is to engineer a safe casting of numeric types without loss or overflow. (I'm porting some code from MSVC to g++).
#include <cstdint>#include <iostream>template< typename T/*the desired type*/, typename/*the source type*/ Y> T integral_cast(const Y& y){ static_assert(false, "undefined integral cast");}// Specialisation to convert std::uint32_t to doubletemplate<>inline double integral_cast(const std::uint32_t& y){ double ret = static_cast<double>(y); return ret;}int main(){ std::uint32_t a = 20; double f = integral_cast<double>(a); // uses the specialisation std::cout << f;}When I compile with gcc 8.3 by typing g++ -o hello hello.cpp I get the error error: static assertion failed: undefined integral cast.
This means that g++ is always compiling the unused template code.
Note that MSVC compiles this (which is nice since it allows me to spot any integral cast specialisations that I haven't considered).
Clearly I'm missing something. But what?