There are a ton of functions in std.algorithm that can be customized with a predicate. At ton of them can also be used as predicates themselves. Here is a cool example:

assert(equal!equal([[1, 2], [3, 4]], iota(1, 5).chunks(2)));

The problem with this piece of code is that:
The inner "equal" only works because it itself doesn't have a predicate.
The outer "equal" works because it takes arguments.

However, neither of this works:
alias fun = equal!equal; //Nope
equal!(equal!equal))(3Dmatrix1, 3Dmatrix2); //Nope

It is *not* possible to declare an algorithm function with a predicate, unless you also specify the argument types. Long story short, they are not eagerly specialize-able.

"equal" (I think) is the most obvious offender, but so are any/all:
import std.ascii : isWhite;
assert( all!(any!isWhite)(["a a", "b b"])); //Nope
assert(!any!(all!isWhite)(["a a", "b b"])); //Nope

Any function that takes a pred (+an argument) and returns a bool is faulty. Here is a (probably incomplete) list:
-equal
-canFind
-any
-all
-cmp