make unions with type aliases more usable (#5019)

Some generic C++ and Rust code is not generated when unions use type
aliases because of potential ambiguity. Actually check for this
ambiguity and only disable offending code only if it is found.

tests/monster_test_generated.ts

@@ -21,6 +21,28 @@ export enum Any{
   MyGame_Example2_Monster= 3
 }};
 
+/**
+ * @enum
+ */
+export namespace MyGame.Example{
+export enum AnyUniqueAliases{
+  NONE= 0,
+  M= 1,
+  T= 2,
+  M2= 3
+}};
+
+/**
+ * @enum
+ */
+export namespace MyGame.Example{
+export enum AnyAmbiguousAliases{
+  NONE= 0,
+  M1= 1,
+  M2= 2,
+  M3= 3
+}};
+
 /**
  * @constructor
  */
@@ -1593,11 +1615,75 @@ vectorOfNonOwningReferencesLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 };
 
+/**
+ * @returns MyGame.Example.AnyUniqueAliases
+ */
+anyUniqueType():MyGame.Example.AnyUniqueAliases {
+  var offset = this.bb!.__offset(this.bb_pos, 90);
+  return offset ? /**  */ (this.bb!.readUint8(this.bb_pos + offset)) : MyGame.Example.AnyUniqueAliases.NONE;
+};
+
+/**
+ * @param MyGame.Example.AnyUniqueAliases value
+ * @returns boolean
+ */
+mutate_any_unique_type(value:MyGame.Example.AnyUniqueAliases):boolean {
+  var offset = this.bb!.__offset(this.bb_pos, 90);
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb!.writeUint8(this.bb_pos + offset, value);
+  return true;
+};
+
+/**
+ * @param flatbuffers.Table obj
+ * @returns ?flatbuffers.Table
+ */
+anyUnique<T extends flatbuffers.Table>(obj:T):T|null {
+  var offset = this.bb!.__offset(this.bb_pos, 92);
+  return offset ? this.bb!.__union(obj, this.bb_pos + offset) : null;
+};
+
+/**
+ * @returns MyGame.Example.AnyAmbiguousAliases
+ */
+anyAmbiguousType():MyGame.Example.AnyAmbiguousAliases {
+  var offset = this.bb!.__offset(this.bb_pos, 94);
+  return offset ? /**  */ (this.bb!.readUint8(this.bb_pos + offset)) : MyGame.Example.AnyAmbiguousAliases.NONE;
+};
+
+/**
+ * @param MyGame.Example.AnyAmbiguousAliases value
+ * @returns boolean
+ */
+mutate_any_ambiguous_type(value:MyGame.Example.AnyAmbiguousAliases):boolean {
+  var offset = this.bb!.__offset(this.bb_pos, 94);
+
+  if (offset === 0) {
+    return false;
+  }
+
+  this.bb!.writeUint8(this.bb_pos + offset, value);
+  return true;
+};
+
+/**
+ * @param flatbuffers.Table obj
+ * @returns ?flatbuffers.Table
+ */
+anyAmbiguous<T extends flatbuffers.Table>(obj:T):T|null {
+  var offset = this.bb!.__offset(this.bb_pos, 96);
+  return offset ? this.bb!.__union(obj, this.bb_pos + offset) : null;
+};
+
 /**
  * @param flatbuffers.Builder builder
  */
 static startMonster(builder:flatbuffers.Builder) {
-  builder.startObject(43);
+  builder.startObject(47);
 };
 
 /**
@@ -2254,6 +2340,38 @@ static startVectorOfNonOwningReferencesVector(builder:flatbuffers.Builder, numEl
   builder.startVector(8, numElems, 8);
 };
 
+/**
+ * @param flatbuffers.Builder builder
+ * @param MyGame.Example.AnyUniqueAliases anyUniqueType
+ */
+static addAnyUniqueType(builder:flatbuffers.Builder, anyUniqueType:MyGame.Example.AnyUniqueAliases) {
+  builder.addFieldInt8(43, anyUniqueType, MyGame.Example.AnyUniqueAliases.NONE);
+};
+
+/**
+ * @param flatbuffers.Builder builder
+ * @param flatbuffers.Offset anyUniqueOffset
+ */
+static addAnyUnique(builder:flatbuffers.Builder, anyUniqueOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(44, anyUniqueOffset, 0);
+};
+
+/**
+ * @param flatbuffers.Builder builder
+ * @param MyGame.Example.AnyAmbiguousAliases anyAmbiguousType
+ */
+static addAnyAmbiguousType(builder:flatbuffers.Builder, anyAmbiguousType:MyGame.Example.AnyAmbiguousAliases) {
+  builder.addFieldInt8(45, anyAmbiguousType, MyGame.Example.AnyAmbiguousAliases.NONE);
+};
+
+/**
+ * @param flatbuffers.Builder builder
+ * @param flatbuffers.Offset anyAmbiguousOffset
+ */
+static addAnyAmbiguous(builder:flatbuffers.Builder, anyAmbiguousOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(46, anyAmbiguousOffset, 0);
+};
+
 /**
  * @param flatbuffers.Builder builder
  * @returns flatbuffers.Offset