Add a type-dependent false expression

Signed-off-by: Dan Bailey <danbailey@ilm.com>

Author: danrbailey

openvdb/openvdb/Types.h

@@ -446,6 +446,26 @@ template<typename FromType, typename ToType> struct CopyConstness<const FromType
 /// @endcond
 
 
+////////////////////////////////////////
+
+/// @brief A type-dependent expression that always evaluates to false.
+/// This is used as a work-around for the much discussed problem of wanting to do:
+/// if constexpr (expr) { }
+/// else { static_assert(false); }
+///
+/// At present, C++ evaluates static_assert(false) regardless of the outcome of the
+/// compile-time expression. By using a type-dependent expression, the static assert
+/// is only evaluated during instantiation:
+///
+/// static_assert(openvdb::AlwaysFalseValue<T>);
+///
+/// This results in the desired outcome of generating a compile-time static assert
+/// only if the first clause evaluates to false.
+
+template<class>
+inline constexpr bool AlwaysFalseValue = false;
+
+
 ////////////////////////////////////////