Need a std::numeric_limits<T>::lowest() equivalent

[Dominikus Dittes Scherkl via Digitalmars-d]

On Saturday, 21 January 2017 at 15:55:35 UTC, Xavier Bigand wrote:
> I don't see any other use case than for initialized maths 
> struct to an invalid state, and because it is generally in 
> template that working with integers and floats it is easier to 
> have same properties (when it have the same meaning).

I use

/// consider T.max to be NaN of unsigned types
/// and T.min to be NaN of signed types
@property bool isNaN(T)(const(T) x) pure @safe @nogc nothrow 
if(isIntegral!T)
{
    static if(isSigned!T)
       return x == T.min;
    else // unsigned
       return x == T.max;
}

/// add a property to numeric types that can be used as return 
value if a result is out of bounds
template invalid(T) if(isNumeric!T)
{
    static if(isFloatingPoint!T)
       @property enum invalid = T.init;
    else static if(isSigned!T)
       @property enum invalid = T.min; // 0x80..00
    else // unsigned
       @property enum invalid = T.max; // 0xFF..FF
}

/// returns the save (not invalid) minimum value for the given 
type
template smin(T) if(isNumeric!T)
{
    static if(isFloatingPoint!T)
       @property enum smin = -T.max; // T.min is the smallest 
representable positive value!!
    else static if(isSigned!T)
       @property enum smin = T(T.min+1);
    else // unsigned
       @property enum smin = T.min; // 0
}

/// returns the save (not invalid) maximum value for the given 
type
template smax(T) if(isNumeric!T)
{
    static if(isUnsigned!T)
       @property enum smax = T(T.max-1u);
    else
       @property enum smax = T.max;
}