Think implicit promotion is a good idea?

src/core/Sk4x_portable.h

 // It is important _not_ to put header guards here.
 // This file will be intentionally included three times.
 
 #if defined(SK4X_PREAMBLE)
 
 #elif defined(SK4X_PRIVATE)
     typedef T Type;
     typedef T Vector[4];
 
     Vector fVec;
 
     template <int m, int a, int s, int k>
     static Sk4x Shuffle(const Sk4x&, const Sk4x&);
 
     void set(const T vals[4]) { for (int i = 0; i < 4; i++) { fVec[i] = vals[i]; } }
 
 #else
 
 #define M(...) template <typename T> __VA_ARGS__ Sk4x<T>::
 
 M() Sk4x() {}
+M() Sk4x(T v) { fVec[0] = fVec[1] = fVec[2] = fVec[3] = v; }
 M() Sk4x(T a, T b, T c, T d) { fVec[0] = a; fVec[1] = b; fVec[2] = c; fVec[3] = d; }
 
 M()              Sk4x(const Sk4x<T>& other) { this->set(other.fVec);               }
 M(Sk4x<T>&) operator=(const Sk4x<T>& other) { this->set(other.fVec); return *this; }
 
 M(Sk4x<T>) Load       (const T vals[4]) { Sk4x r; r.set(vals); return r; }
 M(Sk4x<T>) LoadAligned(const T vals[4]) { return Load(vals);             }
 
 M(void) store       (T vals[4]) const { for (int i = 0; i < 4; i++) { vals[i] = fVec[i]; } }
 M(void) storeAligned(T vals[4]) const { this->store(vals); }
@@ skipping 60 matching lines @@
                 k < 4 ? x.fVec[k] : y.fVec[k-4]);
 }
 
 M(Sk4x<T>) zwxy() const                             { return Shuffle<2,3,0,1>(*this, *this); }
 M(Sk4x<T>) XYAB(const Sk4x& xyzw, const Sk4x& abcd) { return Shuffle<0,1,4,5>( xyzw,  abcd); }
 M(Sk4x<T>) ZWCD(const Sk4x& xyzw, const Sk4x& abcd) { return Shuffle<2,3,6,7>( xyzw,  abcd); }
 
 #undef M
 
 #endif