Bug 19945 – In betterC strange linker error can occur when importing alias of template struct

I reproduced this problem with DMD v2.086.0 and DMD v2.085.1 and LDC 1.15.0. I did not test other versions of DMD or LDC. ### 1. This works with -betterC. --- extern(C) void main() { import std.random : LinearCongruentialEngine; enum A = 48_271, B = 0, C = 2_147_483_647; alias MyMinstdRand = LinearCongruentialEngine!(uint, A, B, C); MyMinstdRand gen; auto n = gen.front; } --- ### 2. This works with -betterC. --- extern(C) void main() { import std.random : LinearCongruentialEngine, MinstdRand; enum A = 48_271, B = 0, C = 2_147_483_647; alias MyMinstdRand = LinearCongruentialEngine!(uint, A, B, C); static assert(is(MyMinstdRand == MinstdRand), "definitions are identical"); } --- ### 3. But there is a linker error if they are combined when using -betterC. --- extern(C) void main() { import std.random : LinearCongruentialEngine, MinstdRand; enum A = 48_271, B = 0, C = 2_147_483_647; alias MyMinstdRand = LinearCongruentialEngine!(uint, A, B, C); static assert(is(MyMinstdRand == MinstdRand), "definitions are identical"); MyMinstdRand gen; // error: undefined reference to '_D3std6random__T24LinearCongruentialEngineTkVki48271Vki0Vki2147483647ZQCc6__initZ' auto n = gen.front; //error: undefined reference to '_D3std6random__T24LinearCongruentialEngineTkVki48271Vki0Vki2147483647ZQCc5frontMxFNaNbNdNiNfZk' } --- ### 4. The same error occurs with -betterC when trying to use MindstdRand directly. --- extern(C) void main() { import std.random : MinstdRand; MinstdRand gen; // error: undefined reference to '_D3std6random__T24LinearCongruentialEngineTkVki48271Vki0Vki2147483647ZQCc6__initZ' auto n = gen.front; // error: undefined reference to '_D3std6random__T24LinearCongruentialEngineTkVki48271Vki0Vki2147483647ZQCc5frontMxFNaNbNdNiNfZk' } ---

The -betterC is for programs that do not use the D runtime library. At link time, the D runtime library is not searched by the linker, only the C runtime library is. Your example program is using std.random, which is part of the D runtime library. The undefined symbol: _D3std6random__T24LinearCongruentialEngineTkVki48271Vki0Vki2147483647ZQCc5frontMxFNaNbNdNiNfZk looks like std.random.LinearCongruentialEngine.front() is not found, which makes sense because that function is in the D runtime library which is not linked in. _D3std6random__T24LinearCongruentialEngineTkVki48271Vki0Vki2147483647ZQCc6__initZ is the static initializer for std.random.LinearCongruentialEngine and is also not found because it is in the D runtime library. If the compile is able to inline everything it needs from the D runtime library, it will work because it then won't need to look in the D runtime library to find it. You'll know when it doesn't work by the appearance of the undefined symbols. I'm going to mark this as invalid because using the D runtime library in -betterC is not officially supported (even though it may work) since the whole point of -betterC is to not use the D runtime library.

Walter is probably correct that we cannot fix this. However, it should work due to LinearCongruentialEngine being templated. For dmd if -allinst makes this code work, then it is indeed a template emission bug.