import std.traits: isCallable; struct S { int opCall(T)(T) { return 0; } } static assert(isCallable!(S.init)); --- The above static assert should pass, but instead fails.
It's impossible to distinguish between the above case and this: struct S { enum opCall(T) = true; } Clearly, the latter is *not* callable. To illustrate the situation perhaps more clearly: struct S { template opCall(T) { static if (is(T == int)) { void opCall(T t) {} } else { enum opCall = 14; } } } The above is perfectly valid code, but should isCallable!S return true?
Seems like this is an issue with the interface, then, not the implementation; the question "is S.init callable" is undecidable, so there's not much use trying to pretend we can answer it. One possible solution would be a trait like `isCallableWith!(S.init, int)`, that requires the user to specify the argument type(s). That way, we could just check with __traits(compiles) whether the call is valid or not.
(In reply to Paul Backus from comment #2) > One possible solution would be a trait like `isCallableWith!(S.init, int)`, > that requires the user to specify the argument type(s). That way, we could > just check with __traits(compiles) whether the call is valid or not. That does not take into account rvalue-ness of the arguments: void foo(ref int) {} store static if (isCallableWith!(typeof(1))) foo(1); // error isCallableWith would have to accept the actual arguments, (type, rvalue-ness) pairs, or both. And if one implements isCallableWith for functions, he should also implement isInstantiatableWith for templates. And that is a case for general __traits(matchFunction/matchTemplate, symbol, args) that would return the overloads callable/instantiatable with args. isCallableWith/isInstantiatableWith could be built on that.
(In reply to Max Samukha from comment #3) > isCallableWith!(typeof(1)) Typo: isCallableWith!(foo, typeof(1))
(In reply to Max Samukha from comment #3) > That does not take into account rvalue-ness of the arguments: > > void foo(ref int) {} > store > static if (isCallableWith!(typeof(1))) > foo(1); // error > > isCallableWith would have to accept the actual arguments, (type, > rvalue-ness) pairs, or both. Then perhaps the best solution is to add a note to the documentation of isCallable highlighting its shortcomings, and encourage everyone to instead use __traits(compiles) directly with the actual function call they want to make.
*** Issue 21137 has been marked as a duplicate of this issue. ***
It's impossible to solve this in the general case, and even the special cases can't be solved in library code, but a __traits could check if the only member of the template is a callable (potentially also if all possible members would be callables).
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