Add type() interface for paddle::variant (#42943)

* Add type() interface for variant

* Fix CI errors

paddle/utils/CMakeLists.txt

 add_subdirectory(string)
-cc_test(small_vector_test SRCS small_vector_test.cc DEPS gtest gflags)
+
 cc_test(array_ref_test SRCS array_ref_test.cc DEPS gtest gflags)
+cc_test(small_vector_test SRCS small_vector_test.cc DEPS gtest gflags)
+cc_test(variant_test SRCS variant_test.cc DEPS gtest)

paddle/utils/variant.h

@@ -2199,6 +2199,18 @@ class impl : public copy_assignment<traits<Ts...>> {
     }
   }
 
+  inline const std::type_info &type() const {
+    return visitation::alt::visit_alt_at(
+        this->index(),
+#ifdef MPARK_GENERIC_LAMBDAS
+        [](auto &alt) -> const std::type_info & { return typeid(alt.value); }
+#else
+        typer {}
+#endif
+        ,
+        *this);
+  }
+
  private:
 #ifndef MPARK_GENERIC_LAMBDAS
   struct swapper {
@@ -2208,6 +2220,13 @@ class impl : public copy_assignment<traits<Ts...>> {
       swap(this_alt.value, that_alt.value);
     }
   };
+
+  struct typer {
+    template <typename Alt>
+    inline const std::type_info &operator()(Alt &alt) const {
+      return typeid(alt.value);
+    }
+  };
 #endif
 
   inline constexpr bool move_nothrow() const {
@@ -2432,6 +2451,8 @@ class variant {
     impl_.swap(that.impl_);
   }
 
+  inline const std::type_info &type() noexcept { return impl_.type(); }
+
  private:
   detail::impl<Ts...> impl_;
 

paddle/utils/variant_test.cc

+// Copyright (c) 2022 PaddlePaddle Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "paddle/utils/variant.h"
+#include "gtest/gtest.h"
+#include "paddle/phi/core/enforce.h"
+
+TEST(interface_test, type) {
+  using phi::enforce::demangle;
+
+  paddle::variant<bool, int, float> var;
+
+  var = true;
+  EXPECT_EQ(demangle(var.type().name()), "bool");
+
+  var = 0;
+  EXPECT_EQ(demangle(var.type().name()), "int");
+
+  var = 0.f;
+  EXPECT_EQ(demangle(var.type().name()), "float");
+}