Add conformance test for enum alias of the same name with different case

conformance/conformance_test_impl.cc

diff --git a/conformance/conformance_test_impl.cc b/conformance/conformance_test_impl.cc
index a884dea..0597cda 100644
--- a/conformance/conformance_test_impl.cc
+++ b/conformance/conformance_test_impl.cc
@@ -1373,6 +1373,23 @@
       "EnumField", REQUIRED,
       R"({"optionalNestedEnum": "FOO"})",
       "optional_nested_enum: FOO");
+  // Enum fields with alias
+  RunValidJsonTest(
+      "EnumFieldWithAlias", REQUIRED,
+      R"({"optionalAliasedEnum": "ALIAS_BAZ"})",
+      "optional_aliased_enum: ALIAS_BAZ");
+  RunValidJsonTest(
+      "EnumFieldWithAliasUseAlias", REQUIRED,
+      R"({"optionalAliasedEnum": "QUX"})",
+      "optional_aliased_enum: ALIAS_BAZ");
+  RunValidJsonTest(
+      "EnumFieldWithAliasLowerCase", REQUIRED,
+      R"({"optionalAliasedEnum": "qux"})",
+      "optional_aliased_enum: ALIAS_BAZ");
+  RunValidJsonTest(
+      "EnumFieldWithAliasDifferentCase", REQUIRED,
+      R"({"optionalAliasedEnum": "bAz"})",
+      "optional_aliased_enum: ALIAS_BAZ");
   // Enum values must be represented as strings.
   ExpectParseFailureForJson(
       "EnumFieldNotQuoted", REQUIRED,

src/google/protobuf/test_messages_proto3.proto

diff --git a/src/google/protobuf/test_messages_proto3.proto b/src/google/protobuf/test_messages_proto3.proto
index 4f295aa..e47856d 100644
--- a/src/google/protobuf/test_messages_proto3.proto
+++ b/src/google/protobuf/test_messages_proto3.proto
@@ -73,6 +73,17 @@
     NEG = -1;  // Intentionally negative.
   }
 
+  enum AliasedEnum {
+    option allow_alias = true;
+
+    ALIAS_FOO = 0;
+    ALIAS_BAR = 1;
+    ALIAS_BAZ = 2;
+    QUX = 2;
+    qux = 2;
+    bAz = 2;
+  }
+
   // Singular
   int32 optional_int32    =  1;
   int64 optional_int64    =  2;
@@ -95,6 +106,7 @@
 
   NestedEnum                           optional_nested_enum     = 21;
   ForeignEnum                          optional_foreign_enum    = 22;
+  AliasedEnum                          optional_aliased_enum    = 23;
 
   string optional_string_piece = 24 [ctype=STRING_PIECE];
   string optional_cord = 25 [ctype=CORD];