simple template constraint - compile error.

[WhatMeWorry via Digitalmars-d-learn]

On Sunday, 7 June 2015 at 23:13:14 UTC, anonymous wrote:
> On Sunday, 7 June 2015 at 23:08:02 UTC, WhatMeWorry wrote:
>> However, when I try to add a simple constraint to the function 
>> like so:
>>
>> int arrayByteSize(T)(T[] someArray) if (isDynamicArray(T))
>
> You forgot an exclamation mark here. Make that: 
> isDynamicArray!(T)

Thanks, but I still can't get out of the woods. I've distilled
everything down to the following:

------------------------------------------------------------------

int arrayByteSize(T)(T[] someArray)
{
     ubyte[] arr = cast(ubyte[]) someArray;
     return arr.length;
}

int i = arrayByteSize(dynamicArray);  // works with above 
function.

------------------------------------------------------------------

However, if I just add a constraint to the function template

int arrayByteSize(T)(T[] someArray)
     if (isDynamicArray!(T))
{
     ubyte[] arr = cast(ubyte[]) someArray;
     return arr.length;
}

int i = arrayByteSize(dynamicArray); // Same call - all hell 
breaks loose.

Error arrayByteSize(T)(T[] someArray) if (isDynamicArray!T)
Error: template compilelinkshaders.arrayByteSize cannot deduce 
function from argument types !()(float[]), candidates are:

Why would a working function call stop working just because a 
constraint was added to the function?