There's a wanting implementation of a sumtype in the standard library (https://dlang.org/phobos/std_variant.html#.Algebraic), and a much better one as a package: https://code.dlang.org/packages/sumtype
The docs on static if may shed some more light: https://dlang.org/spec/version.html#staticif
Template arguments need to be known at compile time, and the extra set of parens is how templates parameters are declared in D.
Template declarations in D take 2 parameter lists. The first is the template parameters the second the runtime parameter: in auto whatDoesItReturn(int i)() { static if (i == 0) { return int.init; } else { return string.init; } }
we have (int i) as template parameter and () as an empty runtime parameter. In C++ syntax whatDoesItReturn<int i>()
at instanciation the syntax is different:
whatDoesItReturn!0() will instantiate a function returning an int
whatDoesItReturn!42() will instantiate a function returning a string.