Type sequence instantiation is a special declaration syntax, documented here: https://dlang.org/articles/ctarguments.html#type-seq-instantiation It should be documented in the language specification, most likely in the section about template sequence parameters [1] or in the section about declarations. [2] [1] https://dlang.org/spec/template.html#variadic-templates [2] https://dlang.org/spec/declaration.html
The section on "Sequence Parameters" now includes a paragraph documenting this feature: > A TypeSeq can similarly be used to declare variables. Parameters or variables declared with a TypeSeq are called an lvalue sequence.