immutable(char) a = 's';
bool isWhitespace = -1 == countUntil(whitespace, a);

gui\guisystem\editbox.d(139): Error: template std.algorithm.countUntil(alias pred = "a == b",R1,R2) if (is(typeof(startsWith!(pred)(haystack,needle)))) does not match any function template declaration
gui\guisystem\editbox.d(139): Error: template std.algorithm.countUntil(alias pred = "a == b",R1,R2) if (is(typeof(startsWith!(pred)(haystack,needle)))) cannot deduce template function from argument types !("a==b")(immutable(char[6u]),immutable(char))
gui\guisystem\editbox.d(139): Error: template instance errors instantiating template