Add field-initializing factory static methods to generated mojo unions.

mojo/public/tools/bindings/generators/mojom_cpp_generator.py

Index: mojo/public/tools/bindings/generators/mojom_cpp_generator.py
diff --git a/mojo/public/tools/bindings/generators/mojom_cpp_generator.py b/mojo/public/tools/bindings/generators/mojom_cpp_generator.py
index 38d222b136995a93b668e607594989d1f2771b41..b1a78b2018df0cfe92dd0717b97073b6cbd0bfc7 100644
--- a/mojo/public/tools/bindings/generators/mojom_cpp_generator.py
+++ b/mojo/public/tools/bindings/generators/mojom_cpp_generator.py
@@ -4,6 +4,8 @@
 
 """Generates C++ source files from a mojom.Module."""
 
+import collections
+
 import mojom.generate.generator as generator
 import mojom.generate.module as mojom
 import mojom.generate.pack as pack
@@ -179,6 +181,24 @@ def IsNativeOnlyKind(kind):
       kind.native_only
 
 
+def IsDistinctUnionKind(union, field):
+  """Returns whether a field in a union has a distinct type from other fields.
+
+  The fields of a union for which this returns True may form an overload set
+  without ambiguity. Due to the way integral and floating point type promotion
+  works, integral and floating point types are considered not distinct.
+  """
+  if (mojom.IsIntegralKind(field.kind) or mojom.IsFloatKind(field.kind) or
+      mojom.IsDoubleKind(field.kind)):
+    return False
+
+  if not hasattr(union, 'field_types'):
+    union.field_types = collections.defaultdict(lambda: -1)
+    for union_field in union.fields:
+      union.field_types[GetCppWrapperParamType(union_field.kind)] += 1
+  return not union.field_types[
+      GetCppWrapperParamType(field.kind)]
+
 def IsHashableKind(kind):
   """Check if the kind can be hashed.
 
@@ -638,6 +658,7 @@ class Generator(generator.Generator):
     "is_any_interface_kind": mojom.IsAnyInterfaceKind,
     "is_any_handle_or_interface_kind": mojom.IsAnyHandleOrInterfaceKind,
     "is_associated_kind": mojom.IsAssociatedKind,
+    "is_distinct_kind": IsDistinctUnionKind,
     "is_hashable": IsHashableKind,
     "is_map_kind": mojom.IsMapKind,
     "is_nullable_kind": mojom.IsNullableKind,