[ynik]

You misunderstood. N, M are supposed to be integers (const generics); in your example code you've made them types. Also, your `type Output = Foo<<N as Add<M>>::Output>;` just means "multiplication has the same return type as addition". But desired is that multiplying a Foo<4> with a Foo<3> results in a Foo<7>.

Rust decided that it's important to not have instantiation-time compiler errors, but this makes computation with const generics complicated: you're not allowed to write Foo<{N+M}> because N+M might overflow, even if it never actually overflows for the generic arguments used in the program.

[duped]

aiui this isn't inherently limited by instantiation-time error messages and is available on nightly today with the generic_const_exprs feature. It's in the pipeline.

[Ar-Curunir]

I believe this kind of stuff is being worked on.

[quotemstr]

Instantiation time errors unlock so much metaprogramming potential that Rust is going to be forced to allow them sooner or later.