fix(frontends/lean): process choice-exprs at check_constant_next

src/frontends/lean/parser.cpp

@@ -1111,6 +1111,9 @@ name parser::check_constant_next(char const * msg) {
             e = get_explicit_arg(e);
     }
 
+    while (is_choice(e))
+        e = get_choice(e, 0);
+
     if (is_constant(e)) {
         return const_name(e);
     } else {

tests/lean/run/const_choice.lean

@@ -0,0 +1,12 @@
+import data.nat.basic
+open nat
+
+definition pred (a : nat) : nat :=
+nat.cases_on a
+  zero
+  (fun a₁, a₁)
+
+example : pred 1 = 0 :=
+rfl
+
+print definition pred