My template tuple code does not compile

[Victor Porton]

Compilation of unittest at the bottom of this file fails with an 
error. What is my error?

https://github.com/vporton/struct-params-dlang/blob/c32cfde60dbb03cb80a4a8aeb8185f5c86705790/source/struct_params.d

It is very important both for this useful little D project and my 
bigger research project. Please help.

source/struct_params.d(74,30): Error: found `,` when expecting 
`.` following int
source/struct_params.d(74,32): Error: found `"x"` when expecting 
identifier following `int`.
source/struct_params.d(74,44): Error: found `,` when expecting 
`.` following float
source/struct_params.d(74,46): Error: found `"y"` when expecting 
identifier following `float`.