Template Parameter Deduction

[Ali Çehreli via Digitalmars-d-learn]

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