fix(library/print): typo in is_used_name

closes #408

src/library/print.cpp

@@ -29,7 +29,7 @@ bool is_used_name(expr const & t, name const & n) {
                 return false; // do not search their types
             return true; // continue search
         });
-    return false;
+    return found;
 }
 
 name pick_unused_name(expr const & t, name const & s) {

tests/lean/place_eqn.lean.expected.out

@@ -8,5 +8,5 @@ a : ℕ
 place_eqn.lean:3:0: error: failed to add declaration 'foo' to environment, value has metavariables
   λ (a : ℕ),
     nat.brec_on a
-      (λ (a : ℕ) (b : nat.below a),
-         nat.cases_on a (λ (b : nat.below 0), ?M_1) (λ (a : ℕ) (b : nat.below (succ a)), ?M_2) b)
+      (λ (a_1 : ℕ) (b : nat.below a_1),
+         nat.cases_on a_1 (λ (b_1 : nat.below 0), ?M_1) (λ (a_2 : ℕ) (b_1 : nat.below (succ a_2)), ?M_2) b)

tests/lean/ppbug.lean

@@ -0,0 +1 @@
+check char.rec_on

tests/lean/ppbug.lean.expected.out

@@ -0,0 +1,3 @@
+char.rec_on :
+  Π (n : char),
+    (Π (a a_1 a_2 a_3 a_4 a_5 a_6 a_7 : bool), ?C (char.mk a a_1 a_2 a_3 a_4 a_5 a_6 a_7)) → ?C n