Mysterious identifier in std.traits.ParameterStorageClassTuple

[Jens]

On Tuesday, 24 July 2018 at 21:05:05 UTC, Jean-Louis Leroy wrote:
> What is `PT` here: 
> https://github.com/dlang/phobos/blob/master/std/traits.d#L1224
> Where does it come from?

`PT` is an identifier chosen by the programmer (see 
https://dlang.org/spec/expression.html#is_expression; 
specifically the 7th form) to denote the type of the parameters 
of a function as a tuple in the mentioned code.
Add a `pragma(msg, PT)` (maybe needs to be `pragma(msg, 
PT.stringof)`; I didn't check) and see what it outputs.

Jens