I suggest to add isComparable to std.traits, to be used as template constraint for (both user defined and standard library) sorting/searching functions. It looks for a opCmp or in general for template isComparable(T) { enum bool isComparable = is(typeof({ return T.init < T.init; })); } version (unittest) { static assert(isComparable!int); static assert(isComparable!string); static assert(!isComparable!creal); static struct Foo {} static assert(!isComparable!Foo); static struct Bar { bool opCmp(Bar) { return true; } } static assert(isComparable!Bar); } I think using this constraint will improve the error messages of functions like std.algorithm.sort too, etc: import std.algorithm; static struct Foo {} static struct Bar { bool opCmp(Bar) { return true; } } void main() { auto a1 = [Foo(), Foo()]; sort(a1); auto a2 = [Bar(), Bar()]; sort(a2); } With DMD 2.054 it gives: ...\dmd\src\phobos\std\algorithm.d(6438): Error: static assert "Invalid predicate passed to sort: a < b" test.d(8): instantiated from here: sort!("a < b",cast(SwapStrategy)0,Foo[])
Note that `std.algorithm.sort` can accept arbitrary predicate, and you can use something like Complex!double[] cmplxarr; ... sort!"a.re < b.re"(cmplxarr); If the purpose is to improve the error message of `std.algorithm.sort`, `isComparable` won't work.
There are actually at least two possible isComparable's that would be helpful in signature constraints. One checks if "a<b" compiles, the other checks if "a==b" compiles. I notice that some Phobos code uses is(typeof(a) : typeof(b)) in signature constraints, only to write if(a==b) in the function body, which is fallacious: just because a is implicitly convertible to b does not necessarily mean == is defined between them. More generally, code should really test for specific operations they need to use, instead of just assuming that is(A : B) implies A.init <op> B.init is valid. To that end, maybe we need isValidBinaryOp instead: template isValidBinaryOp(T1, string op, T2) { alias isValidBinaryOp = is(mixin("T1.init" ~ op ~ "T2.init") : bool); } then we could write signature constraints of the form: auto myFunc(T,U)(T t, U u) if (isValidBinaryOp!(T, "<", U)) { ... }
Commits pushed to master at https://github.com/dlang/phobos https://github.com/dlang/phobos/commit/24bfb294b7523307d10aaaf41ad2021c6cb38501 Fix Issue 6384 - add std.traits.isComparable https://github.com/dlang/phobos/commit/a6ef870042c2c569c31173645f01a2ec682fc283 Merge pull request #5566 from RazvanN7/Issue_6384 Fix Issue 6384 - add std.traits.isComparable merged-on-behalf-of: Andrei Alexandrescu <andralex@users.noreply.github.com>
Commits pushed to stable at https://github.com/dlang/phobos https://github.com/dlang/phobos/commit/24bfb294b7523307d10aaaf41ad2021c6cb38501 Fix Issue 6384 - add std.traits.isComparable https://github.com/dlang/phobos/commit/a6ef870042c2c569c31173645f01a2ec682fc283 Merge pull request #5566 from RazvanN7/Issue_6384
Commits pushed to dmd-cxx at https://github.com/dlang/phobos https://github.com/dlang/phobos/commit/24bfb294b7523307d10aaaf41ad2021c6cb38501 Fix Issue 6384 - add std.traits.isComparable https://github.com/dlang/phobos/commit/a6ef870042c2c569c31173645f01a2ec682fc283 Merge pull request #5566 from RazvanN7/Issue_6384