Bug 14756 – cannot deduce function with template constraint

The following code compiled with dmd v2.067, but results in an error for dmd v2.068.0-b1: // copied from std.traits enum bool isInstanceOf(alias S, T) = is(T == S!Args, Args...); struct Test(size_t id) { static void from(Other)(Other other) if(isInstanceOf!(Test, Other)) { } } void main() { Test!(1) test; Test!(2).from(test); } Here is the error message: test.d(14): Error: template test.Test!2u.Test.from cannot deduce function from argument types !()(Test!1u), candidates are: test.d(6): test.Test!2u.Test.from(Other)(Other other) if (isInstanceOf!(Test, Other)) If the template constraint uses the is-expression directly, the error goes away.

(In reply to Tim from comment #0) This behavior change is an intentional bug fix. See issue 14290. > enum bool isInstanceOf(alias S, T) = is(T == S!Args, Args...); > > struct Test(size_t id) > { > static void from(Other)(Other other) > if (isInstanceOf!(Test, Other)) In here, 'Test' represents an instantiated struct, not a template. By fixing issue 14290, is-expression won't match to the instantiation form S!Args when S is a struct. > If the template constraint uses the is-expression directly, the error goes > away. When you rewrite the condition as follows: static void from(Other)(Other other) if (is(Other == Test!Args, Args...)) 'Test' is still a struct, but in here, the is-expression can recognize it may also represent the outer template Test(size_t id) because it's accessible through the lexical scope. To fix the issue, you need to pass explicitly the template 'Test' to the isInstanceOf. struct Test(size_t id) { static void from(Other)(Other other) if (isInstanceOf!(.Test, Other)) // <-- Use Module Scope Operator (http://dlang.org/module) {} }

*** Issue 15042 has been marked as a duplicate of this issue. ***