Fix parsing conditional expression in set comprehension

Fixes #584.

Author: JukkaL

mypy/parse.py

@@ -1207,7 +1207,7 @@ def parse_dict_or_set_expr(self) -> Node:
         items = List[Tuple[Node, Node]]()
         self.expect('{')
         while self.current_str() != '}' and not self.eol():
-            key = self.parse_expression(precedence['<if>'])
+            key = self.parse_expression(precedence['<for>'])
             if self.current_str() in [',', '}'] and items == []:
                 return self.parse_set_expr(key)
             elif self.current_str() == 'for' and items == []:

mypy/test/data/parse.test

@@ -3086,3 +3086,34 @@ MypyFile:1(
         x
         IntExpr(1))
       VarArg)))
+
+[case testConditionalExpressionInSetComprehension]
+{ 1 if x else 2 for x in y }
+[out]
+MypyFile:1(
+  ExpressionStmt:1(
+    SetComprehension:1(
+      GeneratorExpr:1(
+        ConditionalExpr:1(
+          Condition(
+            NameExpr(x))
+          IntExpr(1)
+          IntExpr(2))
+        NameExpr(x)
+        NameExpr(y)))))
+
+[case testConditionalExpressionInListComprehension]
+a = [ 1 if x else 2 for x in y ]
+[out]
+MypyFile:1(
+  AssignmentStmt:1(
+    NameExpr(a)
+    ListComprehension:1(
+      GeneratorExpr:1(
+        ConditionalExpr:1(
+          Condition(
+            NameExpr(x))
+          IntExpr(1)
+          IntExpr(2))
+        NameExpr(x)
+        NameExpr(y)))))