fixes #712

compiler/sem.nim

@@ -120,7 +120,8 @@ proc commonType*(x, y: PType): PType =
     if a.kind == tyObject and b.kind == tyObject:
       result = commonSuperclass(a, b)
       # this will trigger an error later:
-      if result.isNil: return x
+      if result.isNil or result == a: return x
+      if result == b: return y
       if k != tyNone:
         let r = result
         result = newType(k, r.owner)

tests/exprs/tifexpr_typeinference.nim

@@ -0,0 +1,20 @@
+#bug #712
+
+import tables
+
+proc test(): TTable[string, string] =
+  discard
+
+proc test2(): TTable[string, string] =
+  discard
+
+var x = 5
+let blah =
+  case x
+  of 5:
+    test2()
+  of 2:
+    test()
+  else: test()
+
+echo blah.len