int foo()() { return 1; } void main() { import std.algorithm: tee; [1].tee!foo; } dmd2.067alpha gives: ...\dmd2\src\phobos\std\range\package.d(8194,22): Error: template instance fun!int does not match template declaration foo()() Assertion failure: 'false' on line 154 in file 'mtype.c'