Making a discriminator conditonal doesn't make much sense. Instead,
the union could be made conditional.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 scripts/qapi/common.py                          | 11 +++++++++--
 tests/Makefile.include                          |  1 +
 .../flat-union-invalid-if-discriminator.err     |  1 +
 .../flat-union-invalid-if-discriminator.exit    |  1 +
 .../flat-union-invalid-if-discriminator.json    | 17 +++++++++++++++++
 .../flat-union-invalid-if-discriminator.out     |  0
 6 files changed, 29 insertions(+), 2 deletions(-)
 create mode 100644 tests/qapi-schema/flat-union-invalid-if-discriminator.err
 create mode 100644 tests/qapi-schema/flat-union-invalid-if-discriminator.exit
 create mode 100644 tests/qapi-schema/flat-union-invalid-if-discriminator.json
 create mode 100644 tests/qapi-schema/flat-union-invalid-if-discriminator.out

diff --git a/scripts/qapi/common.py b/scripts/qapi/common.py
index bbcb417a47..d75dfbf97a 100644
--- a/scripts/qapi/common.py
+++ b/scripts/qapi/common.py
@@ -576,7 +576,8 @@ def find_alternate_member_qtype(qapi_type):
 
 # Return the discriminator enum define if discriminator is specified as an
 # enum type, otherwise return None.
-def discriminator_find_enum_define(expr):
+def discriminator_find_enum_define(expr, info):
+    name = expr['union']
     base = expr.get('base')
     discriminator = expr.get('discriminator')
 
@@ -591,6 +592,11 @@ def discriminator_find_enum_define(expr):
     if not discriminator_member:
         return None
 
+    if discriminator_member.get('if'):
+        raise QAPISemError(info, 'The discriminator %s.%s for union %s '
+                           'must not be conditional' %
+                           (base, discriminator, name))
+
     return enum_types.get(discriminator_member['type'])
 
 
@@ -1024,7 +1030,8 @@ def check_exprs(exprs):
 
         if 'include' in expr:
             continue
-        if 'union' in expr and not discriminator_find_enum_define(expr):
+        info = expr_elem['info']
+        if 'union' in expr and not discriminator_find_enum_define(expr, info):
             name = '%sKind' % expr['union']
         elif 'alternate' in expr:
             name = '%sKind' % expr['alternate']
diff --git a/tests/Makefile.include b/tests/Makefile.include
index 7a46c1620d..e7429de9df 100644
--- a/tests/Makefile.include
+++ b/tests/Makefile.include
@@ -505,6 +505,7 @@ qapi-schema += flat-union-inline.json
 qapi-schema += flat-union-int-branch.json
 qapi-schema += flat-union-invalid-branch-key.json
 qapi-schema += flat-union-invalid-discriminator.json
+qapi-schema += flat-union-invalid-if-discriminator.json
 qapi-schema += flat-union-no-base.json
 qapi-schema += flat-union-optional-discriminator.json
 qapi-schema += flat-union-string-discriminator.json
diff --git a/tests/qapi-schema/flat-union-invalid-if-discriminator.err b/tests/qapi-schema/flat-union-invalid-if-discriminator.err
new file mode 100644
index 0000000000..0c94c9860d
--- /dev/null
+++ b/tests/qapi-schema/flat-union-invalid-if-discriminator.err
@@ -0,0 +1 @@
+tests/qapi-schema/flat-union-invalid-if-discriminator.json:13: The discriminator TestBase.enum1 for union TestUnion must not be conditional
diff --git a/tests/qapi-schema/flat-union-invalid-if-discriminator.exit b/tests/qapi-schema/flat-union-invalid-if-discriminator.exit
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/tests/qapi-schema/flat-union-invalid-if-discriminator.exit
@@ -0,0 +1 @@
+1
diff --git a/tests/qapi-schema/flat-union-invalid-if-discriminator.json b/tests/qapi-schema/flat-union-invalid-if-discriminator.json
new file mode 100644
index 0000000000..618ec36396
--- /dev/null
+++ b/tests/qapi-schema/flat-union-invalid-if-discriminator.json
@@ -0,0 +1,17 @@
+{ 'enum': 'TestEnum',
+  'data': [ 'value1', 'value2' ] }
+
+{ 'struct': 'TestBase',
+  'data': { 'enum1': { 'type': 'TestEnum', 'if': 'FOO' } } }
+
+{ 'struct': 'TestTypeA',
+  'data': { 'string': 'str' } }
+
+{ 'struct': 'TestTypeB',
+  'data': { 'integer': 'int' } }
+
+{ 'union': 'TestUnion',
+  'base': 'TestBase',
+  'discriminator': 'enum1',
+  'data': { 'value1': 'TestTypeA',
+            'value2': 'TestTypeB' } }
diff --git a/tests/qapi-schema/flat-union-invalid-if-discriminator.out b/tests/qapi-schema/flat-union-invalid-if-discriminator.out
new file mode 100644
index 0000000000..e69de29bb2
-- 
2.16.2.521.g9aa15f885a