An issue with templates with non-existant symbols

[Andrej Mitrovic]

void foo(T)(T t) if(is(X == struct)) { }

void main()
{
    foo(4);
}

This prints out:

test.d(9): Error: template test.foo(T) if (is(X == struct)) does not
match any function template declaration
test.d(9): Error: template test.foo(T) if (is(X == struct)) cannot
deduce template function
from argument types !()(int)

which is your typical template instantiation error. The real issue is
that the symbol "X" doesn't actually exist.

I've had a template I was refactoring and I've accidentally mistyped
the name of the type, e.g.:

void foo(T)(T r) if (is(T == struct) || is(t == class))  // t should have been T

I was hoping for a better error message here. Can the compiler check
that `is` has an expression with valid symbols in it?