feat(frontends/lean): do not allow coercion definition in sections

src/frontends/lean/builtin_cmds.cpp

@@ -377,6 +377,8 @@ environment open_cmd(parser & p) { return open_export_cmd(p, true); }
 environment export_cmd(parser & p) { return open_export_cmd(p, false); }
 
 environment coercion_cmd(parser & p) {
+    if (in_context(p.env()))
+        throw parser_error("invalid 'coercion' command, coercions cannot be defined in contexts", p.pos());
     bool persistent = false;
     parse_persistent(p, persistent);
     name f   = p.check_constant_next("invalid 'coercion' command, constant expected");

src/frontends/lean/decl_cmds.cpp

@@ -279,8 +279,11 @@ struct decl_modifiers {
                 m_is_instance = true;
                 p.next();
             } else if (p.curr_is_token(get_coercion_tk())) {
-                m_is_coercion = true;
+                auto pos = p.pos();
                 p.next();
+                if (in_context(p.env()))
+                    throw parser_error("invalid '[coercion]' modifier, coercions cannot be defined in contexts", pos);
+                m_is_coercion = true;
             } else if (p.curr_is_token(get_reducible_tk())) {
                 m_is_reducible = true;
                 p.next();

tests/lean/bad_coercions.lean

@@ -0,0 +1,20 @@
+import data.nat.basic
+open nat
+
+constant list.{l} : Type.{l} → Type.{l}
+constant vector.{l} : Type.{l} → nat → Type.{l}
+constant nil (A : Type) : list A
+
+set_option pp.coercions true
+context
+  parameter A : Type
+  parameter n : nat
+
+  definition foo2 [coercion] (v : vector A n) : list A :=
+  nil A
+
+  definition foo (v : vector A n) : list A :=
+  nil A
+
+  coercion foo
+end

tests/lean/bad_coercions.lean.expected.out

@@ -0,0 +1,2 @@
+bad_coercions.lean:13:18: error: invalid '[coercion]' modifier, coercions cannot be defined in contexts
+bad_coercions.lean:19:11: error: invalid 'coercion' command, coercions cannot be defined in contexts