Template Parameter Deduction

[Paul D Anderson via Digitalmars-d-learn]

On Wednesday, 11 March 2015 at 23:04:15 UTC, Ali Çehreli wrote:
> On 03/11/2015 03:44 PM, Paul D Anderson wrote:
>> This used to work in D2.065:
>>
>> given
>>
>> 1) public T mul(T)(in T x, in T y,
>>     Context context = T.context) if (isDecimal!T)
>>     // one template parameter for the two input values
>>
>> and
>>
>> 2) public T mul(T, U)(in T x, U n, Context context = T.context)
>>     if (isDecimal!T && isIntegral!U)
>>     // two different template parameters for the two input 
>> values
>>
>> then
>>
>> 3) dec9 arg1 = dec9("1.20");
>>    long arg2 = 3;
>>    result = mul(arg1, arg2);
>>    // correctly deduced function
>>
>> But now (D2.066.1) either 1) has to be changed to
>>
>> 1) public T mul(T, U)(in T x, U y, Context context = T.context)
>>     if (isDecimal!T && isDecimal!U)
>>     // two identical template parameters for the two input 
>> values
>>
>> or 3) has to be changed to
>>
>> 3) dec9 arg1 = dec9("1.20");
>>    long arg2 = 3;
>>    result = mul!(dec9,long)(arg1, arg2);
>>    // template parameters have to be made explicit
>>
>> Is this expecded behavior?
>>
>> Paul
>
> Hint: It makes it much simpler to work with complete code. The 
> following code took a while for me to put together:
>
> template isDecimal(T)
> {
>     enum isDecimal = true;
> }
>
> template isIntegral(T)
> {
>     enum isIntegral = true;
> }
>
> public T mul(T)(in T x, in T y,
>     Context context = T.context) if (isDecimal!T)
>     // one template parameter for the two input values
> {
>     return x;
> }
>
> alias Context = int;
>
> public T mul(T, U)(in T x, U n, Context context = T.context)
>     if (isDecimal!T && isIntegral!U)
>     // two different template parameters for the two input 
> values
> {
>     return x;
> }
>
> struct dec9
> {
>     string s;
>     enum context = 42;
> }
>
> void main()
> {
>     dec9 arg1 = dec9("1.20");
>     long arg2 = 3;
>     dec9 result = mul(arg1, arg2);
>    // correctly deduced function
> }
>
> Yes, it fails with 2.066.1 but compiles fine with git head.
>
> Ali

Thanks to you and John for taking the time to work this out. I 
didn't mean for anyone to rebuild the example -- I was just 
hoping there was a quick answer based on known language changes. 
I should have included complete example code.

At any rate, it looks like there is a hiccup in the template 
parameter deduction code and that it is being fixed. And it's 
easy to work around. I'll dig a little deeper to see if I can 
find a related bug report.

Paul