Bug 9179 – Invalid template instantiation attempt should result in a readable error message

template Pred(T) { enum Pred = true; } void func(T)(T t) if (Pred(T)) { } void main() { func(1); } Errors: test.d(9): Error: template test.Pred does not match any function template declaration. Candidates are: test.d(3): test.Pred(T) test.d(9): Error: template test.Pred(T) cannot deduce template function from argument types !()(int) test.d(15): Error: template test.func does not match any function template declaration. Candidates are: test.d(8): test.func(T)(T t) if (Pred(T)) test.d(15): Error: template test.func(T)(T t) if (Pred(T)) cannot deduce template function from argument types !()(int) The problem is Pred was used as a function instead of as a template, the fix is: void func(T)(T t) if (Pred!(T)) // note the !() syntax { } There is no other way the Pred template can be called because it's not a function template, therefore we should print out a better error message such as: test.d(9): Error: Need to use !() syntax to instantiate template test.Pred

*** Issue 11444 has been marked as a duplicate of this issue. ***

This particular case is well diagnosized nowadays: >/tmp/temp_7FAAE5E75B70.d(7,14): Error: cannot pass type `int` as a function argument >/tmp/temp_7FAAE5E75B70.d(13,9): Error: template `temp_7FAAE5E75B70.func` cannot > deduce function from argument types `!()(int)`, candidates are: >/tmp/temp_7FAAE5E75B70.d(6,6): `func(T)(T t)`