[SYCL] Make swizzle::operator vec available for 1-elem swizzles (#17870)

KhronosGroup/SYCL-Docs#800

Author: aelovikov-intel

sycl/include/sycl/vector.hpp

@@ -309,8 +309,13 @@ template <typename Self> class ConversionToVecMixin {
 
 public:
   operator vec_ty() const {
-    vec_ty res{*static_cast<const Self *>(this)};
-    return res;
+    auto &self = *static_cast<const Self *>(this);
+    if constexpr (vec_ty::size() == 1)
+      // Avoid recursion by explicitly going through `vec(const DataT &)` ctor.
+      return vec_ty{static_cast<typename vec_ty::element_type>(self)};
+    else
+      // Uses `vec`'s variadic ctor.
+      return vec_ty{self};
   }
 };
 
@@ -398,9 +403,8 @@ class __SYCL_EBO Swizzle
       public ApplyIf<sizeof...(Indexes) == 1,
                      ScalarConversionOperatorsMixIn<
                          Swizzle<IsConstVec, DataT, VecSize, Indexes...>>>,
-      public ApplyIf<sizeof...(Indexes) != 1,
-                     ConversionToVecMixin<
-                         Swizzle<IsConstVec, DataT, VecSize, Indexes...>>>,
+      public ConversionToVecMixin<
+          Swizzle<IsConstVec, DataT, VecSize, Indexes...>>,
       public NamedSwizzlesMixinBoth<
           Swizzle<IsConstVec, DataT, VecSize, Indexes...>> {
   using Base = SwizzleBase<Swizzle<IsConstVec, DataT, VecSize, Indexes...>>;

sycl/test/basic_tests/vectors/cxx_conversions.cpp

@@ -53,7 +53,7 @@ using sw_double_2 = decltype(std::declval<vec<double, 4>>().swizzle<1, 2>());
 static_assert(                  std::is_invocable_v<decltype(f_half_v1), half>);
 static_assert(EXCEPT_IN_PREVIEW std::is_invocable_v<decltype(f_half_v1), float>);
 static_assert(EXCEPT_IN_PREVIEW std::is_invocable_v<decltype(f_half_v1), double>);
-static_assert(EXCEPT_IN_PREVIEW std::is_invocable_v<decltype(f_half_v1), sw_half_1>);
+static_assert(                  std::is_invocable_v<decltype(f_half_v1), sw_half_1>);
 static_assert(EXCEPT_IN_PREVIEW std::is_invocable_v<decltype(f_half_v1), sw_float_1>);
 static_assert(EXCEPT_IN_PREVIEW std::is_invocable_v<decltype(f_half_v1), sw_double_1>);
 static_assert(                  std::is_invocable_v<decltype(f_half_v1), vec<half, 1>>);
@@ -64,7 +64,7 @@ static_assert(EXCEPT_IN_PREVIEW std::is_invocable_v<decltype(f_float_v1), half>)
 static_assert(                  std::is_invocable_v<decltype(f_float_v1), float>);
 static_assert(                  std::is_invocable_v<decltype(f_float_v1), double>);
 static_assert(EXCEPT_IN_PREVIEW std::is_invocable_v<decltype(f_float_v1), sw_half_1>);
-static_assert(EXCEPT_IN_PREVIEW std::is_invocable_v<decltype(f_float_v1), sw_float_1>);
+static_assert(                  std::is_invocable_v<decltype(f_float_v1), sw_float_1>);
 static_assert(EXCEPT_IN_PREVIEW std::is_invocable_v<decltype(f_float_v1), sw_double_1>);
 static_assert(EXCEPT_IN_PREVIEW std::is_invocable_v<decltype(f_float_v1), vec<half, 1>>);
 static_assert(                  std::is_invocable_v<decltype(f_float_v1), vec<float, 1>>);