Templates, implicit conversions and const

[Regan Heath]

The 2nd thing which irritated me a little over the weekend was how 
templates and const can interact.  For example:

bool contains(T)(T[] array, T value)
{
	foreach(v; array) if (v == value) return true;
	return false;
}

void foo(const(char) c) {}

void main()
{
	string tests = "test";
	char[] testc = tests.dup;
	
	tests.contains(cast(const(char))'a');
	contains!(const(char))(tests, 'a');
	testc.contains('a');	
	foo('a');
}



The cast in the first contains call is required, without it we get the 
error:

C:\D\src\tmp\tplconst.d(14): template tplconst.contains(T) cannot deduce 
template function from argument types (const(char)[],char)

The available templates are:
(const(char)[],const(char))
(char[],char)

And looking at the template argument deduction rules I can see why it 
cannot find a match.  T is found to be both 'const(char)' and 'char'.

But, I wonder if it should then apply the standard function overloading 
rule:

"In D, function overloading is simple. It matches exactly, it matches 
with implicit conversions, or it does not match. If there is more than 
one match, it is an error."

If it performed implicit conversion on 'char' it could get 'const(char)' 
and would therefore match the template:

(const(char)[],const(char))

It cannot implcitly convert 'const(char)[]' to 'char[]' so it will never 
match the other option.

Thoughts?

Regan