A Discussion of Tuple Syntax

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This is the reason I had originally thought Kenji's DIP was about 
run-time tuples. If it's just syntactic sugar over 
std.typecons.Tuple (plus some extra destructuring/pattern 
matching stuff), it would require no ABI changes and no changes 
to TypeTuple semantics. The one thing it wouldn't do is unify 
Tuple and TypeTuple.

However, with compiler support, we can make this situation 
better. TypeTuple stays as-is, while Tuple becomes a value 
instead of a type, which has its own literal syntax. This means 
that typeof(tuple(1, "a")) is not Tuple!(int, string), as in the 
struct, but TypeTuple!(int, string), as in the compiler tuple.

You could still do all the stuff that can be done with TypeTuple 
currently, but it should be strongly discouraged to mix types and 
values within TypeTuple, or even limited by the compiler. This 
creates a clear distinction between runtime tuples and TypeTuple. 
It becomes much easier to reason about their semantics, as we can 
think of TypeTuple as a type, just like int, string, double, 
etc., and Tuple as a value, just like 1, "a", 2.3, etc. The only 
difference between TypeTuple and regular types is that there are 
a few special static operations that can be used to manipulate 
it. Otherwise it behaves as a type.