[Simd] Implement broadcast construction function.

dune/common/simd/defaults.hh

@@ -18,6 +18,7 @@
 #include <dune/common/rangeutilities.hh>
 #include <dune/common/simd/base.hh>
 #include <dune/common/simd/interface.hh>
+#include <dune/common/typelist.hh>
 #include <dune/common/typetraits.hh>
 
 namespace Dune {
@@ -134,6 +135,13 @@ namespace Dune {
         return Simd::mask(v1) && Simd::mask(v2);
       }
 
+      //! implements Simd::broadcast<V>()
+      template<class V, class S>
+      auto broadcast(ADLTag<0>, MetaType<V>, S s)
+      {
+        return V(Simd::Scalar<V>(s));
+      }
+
       //! @} Overloadable and default functions
       //! @} Group SIMDAbstract
     } // namespace Overloads

dune/common/simd/interface.hh

@@ -18,6 +18,7 @@
 #include <utility>
 
 #include <dune/common/simd/base.hh>
+#include <dune/common/typelist.hh>
 
 namespace Dune {
 
@@ -413,6 +414,23 @@ namespace Dune {
       return maskAnd(Overloads::ADLTag<7>{}, v1, v2);
     }
 
+    //! Broadcast a scalar to a vector explicitly
+    /**
+     * Implemented by `Overloads::broadcast()`
+     *
+     * This is useful because the syntax for broadcasting can vary wildly
+     * between implementations.
+     *
+     * \note One of the few functions that explicitly take a template
+     *       argument (`V` in this case).
+     */
+    template<class V, class S>
+    constexpr V broadcast(S s)
+    {
+      return broadcast(Overloads::ADLTag<7>{}, MetaType<std::decay_t<V> >{},
+                       std::move(s));
+    }
+
     //! @}
 
     /** @name Syntactic Sugar