[SYCL] Augment known_identity for std::complex and std::plus. (#8425)

This PR proposes to augment sycl::has_known_identity to return true for
std::complex and std::plus operator. This will have two benefits:

1. It enables support for complex numbers in sycl::reduction without the
user having to explicitly pass identity.
2. sycl::known_identity can now be used to simplify the implementation
of group algorithms (See PR #5394).

Also, this PR addresses the Github issue #5477.
Test Case PR: intel/llvm-test-suite#1609

Author: uditagarwal97

sycl/doc/extensions/proposed/sycl_ext_oneapi_complex_algorithms.asciidoc

@@ -125,6 +125,18 @@ NOTE: `sycl::bit_and`, `sycl::bit_or`, `sycl::bit_xor`, `sycl::logical_and`,
 because their behaviors are defined in terms of operators that `std::complex`
 does not implement.
 
+=== Known Identities
+
+This extension adds the following known identities, augmenting https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#table.identities[Table 123] in the core SYCL specification:
+[cols="5,30,70"]
+[grid="rows"]
+[options="header"]
+|========================================
+|Operator|Available When|Identity
+|`sycl::plus`|`std::is_same_v<std::remove_cv_t<AccumulatorT>, std::complex<float>> \|\|
+std::is_same_v<std::remove_cv_t<AccumulatorT>, std::complex<double>>`|`AccumulatorT{}`
+|========================================
+
 == Issues
 
 None.

sycl/include/sycl/known_identity.hpp

@@ -63,14 +63,22 @@ using IsLogicalOR =
     bool_constant<std::is_same<BinaryOperation, sycl::logical_or<T>>::value ||
                   std::is_same<BinaryOperation, sycl::logical_or<void>>::value>;
 
+template <typename T>
+using isComplex =
+    bool_constant<std::is_same<T, std::complex<float>>::value ||
+                  std::is_same<T, std::complex<double>>::value>;
+
 // Identity = 0
 template <typename T, class BinaryOperation>
 using IsZeroIdentityOp =
     bool_constant<(is_geninteger<T>::value &&
                    (IsPlus<T, BinaryOperation>::value ||
                     IsBitOR<T, BinaryOperation>::value ||
                     IsBitXOR<T, BinaryOperation>::value)) ||
-                  (is_genfloat<T>::value && IsPlus<T, BinaryOperation>::value)>;
+                  (is_genfloat<T>::value &&
+                    IsPlus<T, BinaryOperation>::value) ||
+                  (isComplex<T>::value &&
+                    IsPlus<T, BinaryOperation>::value)>;
 
 // Identity = 1
 template <typename T, class BinaryOperation>