4.9 introduced unconstrained generic functions as an extension: https://gcc.gnu.org/gcc-4.9/changes.html "G++ supports unconstrained generic functions as specified by §4.1.2 and §5.1.1 of N3889: Concepts Lite Specification" And in that spec, 5.1.1 says: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3889.pdf 15 "A generic function is denoted by function declarator having auto or a concept-name as part of the type-specifier in its parameter-declaration-clause" 17 "All placeholder types introduced using the same concept-name have the same invented template parameter." If I write a generic function using two 'auto' type-specifiers, they seem to become the same invented template parameter as if they were concept names instead.: auto add(auto a, auto b) { return a + b; } add(3, 4.5) gives me 7 add(3.5, 4) gives me 7.5 sample program: #include <iostream> auto add_1(auto a, auto b) { return a + b;} auto add_2 = [](auto a, auto b) { return a + b;}; int main() { std::cout << "a1: " << add_1(3.5, 4) << "\n" << "a2: " << add_1(3, 4.5) << "\n" << "a3: " << add_2(3.5, 4) << "\n" << "a4: " << add_2(3, 4.5) << "\n"; } This gives me: a1: 7.5 a2: 7 a3: 7.5 a4: 7.5 This is with a fedora 22/x86-64 machine. > g++ -v Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/5.1.1/lto-wrapper Target: x86_64-redhat-linux Configured with: ../configure --enable-bootstrap --enable-languages=c,c++,objc,obj-c++,fortran,ada,go,lto --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared --enable-threads=posix --enable-checking=release --enable-multilib --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --enable-plugin --enable-initfini-array --disable-libgcj --with-default-libstdcxx-abi=c++98 --with-isl --enable-libmpx --enable-gnu-indirect-function --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux Thread model: posix gcc version 5.1.1 20150422 (Red Hat 5.1.1-1) (GCC) Should I open a bug report or try again with a recent snapshot? thanks, -Kenny
