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The problem is combining the "const generic" and "expression" part. If your "compile time constants" can actually be complex expressions, you arguably end up with the same kind of generality as dependent types.

This is true even for expressions that are only evaluated in a compile-time context, since dependently-typed languages do "everything" at compile time anyway, they don't have a phase distinction where you can talk about "runtime" being separate.

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galangalalgol 4 hours ago | parent [-]

Ah, yeah! I get it now. So c++ is a dependently typed language. That is hilarious. I want lisp syntax in c++29. That said, too many features are blocked on const generic expressions, so I think they are going to have to bite that off. There is already talk about migrating proceduralacros to be something more like normal rust, this moght fit in with that.

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Rusky an hour ago | parent [-]

C++ is not a dependently typed language, for the same reason that templates do not emit errors until after they are instantiated. All non-type template parameters get fully evaluated at instantiation time so they can be checked concretely.

A truly dependently typed language performs these checks before instantiation time, by evaluating those expressions abstractly. Code that is polymorphic over values is checked for all possible instantiations, and thus its types can actually depend on values that will not be known until runtime.

The classic example is a dynamic array whose type includes its size- you can write something like `concat(vector<int, N>, vector<int, M>) -> vector<int, N + M>` and call this on e.g. arrays you have read from a file or over the network. The compiler doesn't care what N and M are, exactly- it only cares that `concat` always produces a result with the length `N + M`.

	▲	galangalalgol 41 minutes ago \| parent [-]
		In c++ it does care what N and M are at compile time, at least the optimizer does for autovectorization and unrolling. Would that not be the case with const generic expressions?