tests: budget: add tests for unassignable accounts.

tests/print/auto.test

@@ -51,7 +51,7 @@ $ hledger balance -f- --auto
                 $-38
 >=
 
-# Balance assertions see the generated postings.
+# Balance assertions see postings generated by transaction modifier rules.
 <
 = trigger
   (target)   10
@@ -136,3 +136,31 @@ $ hledger -f- print --auto -x
 
 >=
 
+# Balance assignments may not be used with accounts also affected by transaction modifier rules.
+<
+= ^expenses:foo
+    budget:available   *-1
+    assets:checking     *1
+
+2018/10/17 * INITIAL
+    budget:available   $100
+    equity:opening
+
+2018/10/17 * SOME EXPENSE
+    expenses:foo                                 $50
+    assets:checking
+
+2018/10/17 * ASSERT
+    budget:other
+    budget:available                              =$0
+
+$ hledger print -f- --auto
+>2
+hledger: cannot assign amount to account 
+
+    budget:available
+
+because it is also included in transaction modifiers.
+
+>=1
+