如何复现
(t/cf (t/fn [m :- (t/HMap :optional {:a String} :complete? true)] (if-let [b (get m :a)] true)))
观察结果
[[(:t/HMap :optional {:a String} :complete? true) -> true :filters {:then tt, :else ff}] {:then tt, :else ff}]
预期结果（伪代码）
[[(:t/HMap :optional {:a String} :complete? true) -> (:t/U nil true)]]
这是如果您将上面的if-let替换为if所得到的结果。
返回类型应为(t/Option true)，因为if-let将在map上没有设置:a键时返回nil。
细节
broz@macmicro:~$ grep try ~/.lein/profiles.clj
[lein-try "0.4.3"]
broz@macmicro:~$ lein try org.clojure/core.typed
nREPL server started on port 52406 on host 127.0.0.1 - nrepl://127.0.0.1:52406
REPL-y 0.3.5, nREPL 0.2.6
Clojure 1.6.0
Java HotSpot(TM) 64-Bit Server VM 1.8.0_40-b26
Doc: (doc function-name-here)
(find-doc "part-of-name-here")
源： (source function-name-here)
Javadoc: (javadoc java-object-or-class-here)
退出：Control+D 或 (exit) 或 (quit)
结果：存储在 vars *1, *2, *3，异常在 *e 中
user=> (require ['clojure.core.typed :as 't])
nil
user=> (t/cf (t/fn [m :- (t/HMap :optional {:a String} :complete? true)] (if-let [b (get m :a)] true)))
Initializing core.typed ...
Building core.typed base environments ...
Finished building base environments
"Elapsed time: 14055.732654 msecs"
core.typed initialized.
[[(:t/HMap :optional {:a String} :complete? true) -> true :filters {:then tt, :else ff}] {:then tt, :else ff}]
