diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index 12af1b8..0e4baff 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -284,7 +284,7 @@
 === Union types ===
 
 Usage: { 'union': STRING, 'data': DICT }
-or:    { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME,
+or:    { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME-OR-DICT,
          'discriminator': ENUM-MEMBER-OF-BASE }
 
 Union types are used to let the user choose between several different
@@ -320,13 +320,16 @@
 the union can be named 'max', as this would collide with the implicit
 enum.  The value for each branch can be of any type.
 
-A flat union definition specifies a struct as its base, and
-avoids nesting on the wire.  All branches of the union must be
-complex types, and the top-level members of the union dictionary on
-the wire will be combination of members from both the base type and the
-appropriate branch type (when merging two dictionaries, there must be
-no keys in common).  The 'discriminator' member must be the name of a
-non-optional enum-typed member of the base struct.
+A flat union definition avoids nesting on the wire, and specifies a
+set of common members that occur in all variants of the union.  The
+'base' key must specifiy either a type name (the type must be a
+struct, not a union), or a dictionary representing an anonymous type.
+All branches of the union must be complex types, and the top-level
+members of the union dictionary on the wire will be combination of
+members from both the base type and the appropriate branch type (when
+merging two dictionaries, there must be no keys in common).  The
+'discriminator' member must be the name of a non-optional enum-typed
+member of the base struct.
 
 The following example enhances the above simple union example by
 adding an optional common member 'read-only', renaming the
@@ -334,10 +337,8 @@
 default of 'type', and reducing the number of {} required on the wire:
 
  { 'enum': 'BlockdevDriver', 'data': [ 'file', 'qcow2' ] }
- { 'struct': 'BlockdevOptionsBase',
-   'data': { 'driver': 'BlockdevDriver', '*read-only': 'bool' } }
  { 'union': 'BlockdevOptions',
-   'base': 'BlockdevOptionsBase',
+   'base': { 'driver': 'BlockdevDriver', '*read-only': 'bool' },
    'discriminator': 'driver',
    'data': { 'file': 'BlockdevOptionsFile',
              'qcow2': 'BlockdevOptionsQcow2' } }
@@ -366,10 +367,9 @@
 is identical on the wire to:
 
  { 'enum': 'Enum', 'data': ['one', 'two'] }
- { 'struct': 'Base', 'data': { 'type': 'Enum' } }
  { 'struct': 'Branch1', 'data': { 'data': 'str' } }
  { 'struct': 'Branch2', 'data': { 'data': 'int' } }
- { 'union': 'Flat', 'base': 'Base', 'discriminator': 'type',
+ { 'union': 'Flat': 'base': { 'type': 'Enum' }, 'discriminator': 'type',
    'data': { 'one': 'Branch1', 'two': 'Branch2' } }
 
 
diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
index 92ae619..e09c875 100644
--- a/scripts/qapi-types.py
+++ b/scripts/qapi-types.py
@@ -72,12 +72,14 @@
                  c_name=c_name(name))
 
     if base:
-        ret += mcgen('''
+        if not base.is_implicit():
+            ret += mcgen('''
     /* Members inherited from %(c_name)s: */
 ''',
-                     c_name=base.c_name())
+                         c_name=base.c_name())
         ret += gen_struct_members(base.members)
-        ret += mcgen('''
+        if not base.is_implicit():
+            ret += mcgen('''
     /* Own members: */
 ''')
     ret += gen_struct_members(members)
@@ -224,7 +226,7 @@
             return
         self._fwdecl += gen_fwd_object_or_array(name)
         self.decl += gen_object(name, base, members, variants)
-        if base:
+        if base and not base.is_implicit():
             self.decl += gen_upcast(name, base)
         # TODO Worth changing the visitor signature, so we could
         # directly use rather than repeat type.is_implicit()?
diff --git a/scripts/qapi.py b/scripts/qapi.py
index d91af94..a38ef52 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -327,6 +327,8 @@
 
 
 def find_base_members(base):
+    if isinstance(base, dict):
+        return base
     base_struct_define = find_struct(base)
     if not base_struct_define:
         return None
@@ -561,9 +563,10 @@
 
     # Else, it's a flat union.
     else:
-        # The object must have a string member 'base'.
+        # The object must have a string or dictionary 'base'.
         check_type(expr_info, "'base' for union '%s'" % name,
-                   base, allow_metas=['struct'])
+                   base, allow_dict=True, allow_optional=True,
+                   allow_metas=['struct'])
         if not base:
             raise QAPIExprError(expr_info,
                                 "Flat union '%s' must have a base"
@@ -1039,6 +1042,8 @@
             owner = owner[6:]
             if owner.endswith('-arg'):
                 return '(parameter of %s)' % owner[:-4]
+            elif owner.endswith('-base'):
+                return '(base of %s)' % owner[:-5]
             else:
                 assert owner.endswith('-wrapper')
                 # Unreachable and not implemented
@@ -1325,6 +1330,9 @@
         base = expr.get('base')
         tag_name = expr.get('discriminator')
         tag_member = None
+        if isinstance(base, dict):
+            base = (self._make_implicit_object_type(
+                    name, info, 'base', self._make_members(base, info)))
         if tag_name:
             variants = [self._make_variant(key, value)
                         for (key, value) in data.iteritems()]
diff --git a/tests/qapi-schema/flat-union-bad-base.err b/tests/qapi-schema/flat-union-bad-base.err
index 79b8a71..bee24a2 100644
--- a/tests/qapi-schema/flat-union-bad-base.err
+++ b/tests/qapi-schema/flat-union-bad-base.err
@@ -1 +1 @@
-tests/qapi-schema/flat-union-bad-base.json:9: 'base' for union 'TestUnion' should be a type name
+tests/qapi-schema/flat-union-bad-base.json:8: 'string' (member of TestTypeA) collides with 'string' (base of TestUnion)
diff --git a/tests/qapi-schema/flat-union-bad-base.json b/tests/qapi-schema/flat-union-bad-base.json
index e2e622b..74dd421 100644
--- a/tests/qapi-schema/flat-union-bad-base.json
+++ b/tests/qapi-schema/flat-union-bad-base.json
@@ -1,5 +1,4 @@
-# we require the base to be an existing struct
-# TODO: should we allow an anonymous inline base type?
+# we allow anonymous base, but enforce no duplicate keys
 { 'enum': 'TestEnum',
   'data': [ 'value1', 'value2' ] }
 { 'struct': 'TestTypeA',
@@ -7,7 +6,7 @@
 { 'struct': 'TestTypeB',
   'data': { 'integer': 'int' } }
 { 'union': 'TestUnion',
-  'base': { 'enum1': 'TestEnum', 'kind': 'str' },
+  'base': { 'enum1': 'TestEnum', 'string': 'str' },
   'discriminator': 'enum1',
   'data': { 'value1': 'TestTypeA',
             'value2': 'TestTypeB' } }
diff --git a/tests/qapi-schema/qapi-schema-test.json b/tests/qapi-schema/qapi-schema-test.json
index e722748..f571e1b 100644
--- a/tests/qapi-schema/qapi-schema-test.json
+++ b/tests/qapi-schema/qapi-schema-test.json
@@ -75,14 +75,10 @@
   'base': 'UserDefZero',
   'data': { 'string': 'str', 'enum1': 'EnumOne' } }
 
-{ 'struct': 'UserDefUnionBase2',
-  'base': 'UserDefZero',
-  'data': { 'string': 'str', 'enum1': 'QEnumTwo' } }
-
 # this variant of UserDefFlatUnion defaults to a union that uses members with
 # allocated types to test corner cases in the cleanup/dealloc visitor
 { 'union': 'UserDefFlatUnion2',
-  'base': 'UserDefUnionBase2',
+  'base': { '*integer': 'int', 'string': 'str', 'enum1': 'QEnumTwo' },
   'discriminator': 'enum1',
   'data': { 'value1' : 'UserDefC', # intentional forward reference
             'value2' : 'UserDefB' } }
diff --git a/tests/qapi-schema/qapi-schema-test.out b/tests/qapi-schema/qapi-schema-test.out
index d49fe1d..19cd214 100644
--- a/tests/qapi-schema/qapi-schema-test.out
+++ b/tests/qapi-schema/qapi-schema-test.out
@@ -66,7 +66,7 @@
     case value2: UserDefB
     case value3: UserDefB
 object UserDefFlatUnion2
-    base UserDefUnionBase2
+    base q_obj_UserDefFlatUnion2-base
     tag enum1
     case value1: UserDefC
     case value2: UserDefB
@@ -111,10 +111,6 @@
     base UserDefZero
     member string: str optional=False
     member enum1: EnumOne optional=False
-object UserDefUnionBase2
-    base UserDefZero
-    member string: str optional=False
-    member enum1: QEnumTwo optional=False
 object UserDefZero
     member integer: int optional=False
 object WrapAlternate
@@ -156,6 +152,10 @@
     member b: str optional=False
     member c: str optional=True
     member enum3: EnumOne optional=True
+object q_obj_UserDefFlatUnion2-base
+    member integer: int optional=True
+    member string: str optional=False
+    member enum1: QEnumTwo optional=False
 object q_obj___org.qemu_x-command-arg
     member a: __org.qemu_x-EnumList optional=False
     member b: __org.qemu_x-StructList optional=False