VS fix for ambiguous union assignment operator.

Change-Id: I1c37db1ced462fd558d3e893a501341f3eca6379

samples/monster_generated.h

@@ -78,8 +78,8 @@ struct EquipmentUnion {
     type(Equipment_NONE), value(nullptr)
     { std::swap(type, u.type); std::swap(value, u.value); }
   EquipmentUnion(const EquipmentUnion &) FLATBUFFERS_NOEXCEPT;
-  EquipmentUnion &operator=(EquipmentUnion u) FLATBUFFERS_NOEXCEPT
-    { std::swap(type, u.type); std::swap(value, u.value); return *this; }
+  EquipmentUnion &operator=(const EquipmentUnion &u) FLATBUFFERS_NOEXCEPT
+    { EquipmentUnion t(u); std::swap(type, t.type); std::swap(value, t.value); return *this; }
   EquipmentUnion &operator=(EquipmentUnion &&u) FLATBUFFERS_NOEXCEPT
     { std::swap(type, u.type); std::swap(value, u.value); return *this; }
   ~EquipmentUnion() { Reset(); }

src/idl_gen_cpp.cpp

@@ -688,8 +688,8 @@ class CppGenerator : public BaseGenerator {
       code_ += "    type({{NONE}}), value(nullptr)";
       code_ += "    { std::swap(type, u.type); std::swap(value, u.value); }";
       code_ += "  {{NAME}}Union(const {{NAME}}Union &) FLATBUFFERS_NOEXCEPT;";
-      code_ += "  {{NAME}}Union &operator=({{NAME}}Union u) FLATBUFFERS_NOEXCEPT";
-      code_ += "    { std::swap(type, u.type); std::swap(value, u.value); return *this; }";
+      code_ += "  {{NAME}}Union &operator=(const {{NAME}}Union &u) FLATBUFFERS_NOEXCEPT";
+      code_ += "    { {{NAME}}Union t(u); std::swap(type, t.type); std::swap(value, t.value); return *this; }";
       code_ += "  {{NAME}}Union &operator=({{NAME}}Union &&u) FLATBUFFERS_NOEXCEPT";
       code_ += "    { std::swap(type, u.type); std::swap(value, u.value); return *this; }";
       code_ += "  ~{{NAME}}Union() { Reset(); }";

tests/monster_test_generated.h

@@ -109,8 +109,8 @@ struct AnyUnion {
     type(Any_NONE), value(nullptr)
     { std::swap(type, u.type); std::swap(value, u.value); }
   AnyUnion(const AnyUnion &) FLATBUFFERS_NOEXCEPT;
-  AnyUnion &operator=(AnyUnion u) FLATBUFFERS_NOEXCEPT
-    { std::swap(type, u.type); std::swap(value, u.value); return *this; }
+  AnyUnion &operator=(const AnyUnion &u) FLATBUFFERS_NOEXCEPT
+    { AnyUnion t(u); std::swap(type, t.type); std::swap(value, t.value); return *this; }
   AnyUnion &operator=(AnyUnion &&u) FLATBUFFERS_NOEXCEPT
     { std::swap(type, u.type); std::swap(value, u.value); return *this; }
   ~AnyUnion() { Reset(); }

tests/union_vector/union_vector_generated.h

@@ -56,8 +56,8 @@ struct CharacterUnion {
     type(Character_NONE), value(nullptr)
     { std::swap(type, u.type); std::swap(value, u.value); }
   CharacterUnion(const CharacterUnion &) FLATBUFFERS_NOEXCEPT;
-  CharacterUnion &operator=(CharacterUnion u) FLATBUFFERS_NOEXCEPT
-    { std::swap(type, u.type); std::swap(value, u.value); return *this; }
+  CharacterUnion &operator=(const CharacterUnion &u) FLATBUFFERS_NOEXCEPT
+    { CharacterUnion t(u); std::swap(type, t.type); std::swap(value, t.value); return *this; }
   CharacterUnion &operator=(CharacterUnion &&u) FLATBUFFERS_NOEXCEPT
     { std::swap(type, u.type); std::swap(value, u.value); return *this; }
   ~CharacterUnion() { Reset(); }