Bug 4816 – template constraint and __traits(compiles, ...) don't work properly together with a delegate

The following refuses to compile import std.stdio; void callFunc(alias func, T...)(T args) if(__traits(compiles, func(args))) { func(args); } void main() { auto func = (){writeln(4);}; callFunc!(func)(); } Rather, it gives you the error message d.d(12): Error: template d.callFunc(alias func,T...) if (__traits(compiles,func(args))) does not match any function template declaration d.d(12): Error: template d.callFunc(alias func,T...) if (__traits(compiles,func(args))) cannot deduce template function from argument types !(func)() d.d(12): Error: template instance errors instantiating template If I remove the template constraint, then it compiles fine. If I declare func as a nested function and pass that, it works fine. If I use __traits(compiles, func()) in main(), it returns true. However, the template constraint fails as long as you pass it a delegate.

This code won't compile (D v2.050) either. I think it is the same bug. import std.stdio; string process(alias callback)(string s) if (is(typeof(callback(s)))) { return s; } void main() { void fun(string s) { write(s); } writeln(process!fun("Hello world!")); // OK enum dlg1 = (string s) { write(s); }; writeln(process!dlg1("Hello world!")); // OK auto dlg2 = (string s) { write(s); }; writeln(process!dlg2("Hello world!")); // ERROR } Error message: test.d(18): Error: template instance process!(dlg2) does not match template declaration process(alias callback) if (is(typeof(callback(s))))

Both code in comment#0 and #1 works in 2.061head(2284bb9).