use magic type for do

ast.ml

@@ -1,4 +1,5 @@
 type type_t =
+  | Magic_t
   | Void_t
   | Int_t
   | Bool_t

semantics.ml

@@ -24,7 +24,7 @@ let rec analyze_expr env ua t = function
   | Syntax.Call c ->
     (match Env.find c.func env with
     | Func_t (ret_t, tl) ->
-      if ret_t != t then errt ret_t t c.pos;
+      if ret_t != t && t != Magic_t then errt ret_t t c.pos;
       if List.length tl != List.length c.args
       then
         raise
@@ -63,7 +63,7 @@ let analyze_instr env ua ret_t = function
     let ae, et = analyze_expr env ua (Env.find a.var env) a.expr in
     Assign (a.var, ae), env, List.filter (fun x -> x <> a.var) ua
   | Syntax.Do d ->
-    let ae, _ = analyze_expr env ua Int_t d.expr in
+    let ae, _ = analyze_expr env ua Magic_t d.expr in
     Do ae, env, []
   | Syntax.Return r ->
     let ae, _ = analyze_expr env ua ret_t r.expr in

tests/17_err-losing-variable.test

@@ -0,0 +1,7 @@
+int foo(int a) {
+    return a;
+}
+
+void main() {
+    void b = foo(1);
+}