feat(frontends/lean): improve error handling inside match-with expressions

src/frontends/lean/builtin_exprs.cpp

@@ -242,17 +242,8 @@ static expr parse_begin_end_core(parser & p, pos_info const & pos, name const &
             }
         }
     } catch (exception & ex) {
-        if (end_token == get_end_tk()) {
-            // When the end_token is 'end', the default parser
-            // sync_command does not work well because it will
-            // interpret 'end' as a synchronization point because 'end' is also a command.
-            while (!p.curr_is_token(get_end_tk())) {
-                if (p.curr() == scanner::token_kind::Eof)
-                    ex.rethrow();
-                p.next();
-            }
-            p.next(); // consume 'end'
-        }
+        if (end_token == get_end_tk())
+            consume_until_end(p);
         ex.rethrow();
     }
     auto end_pos = p.pos();

src/frontends/lean/decl_cmds.cpp

@@ -698,8 +698,10 @@ expr parse_local_equations(parser & p, expr const & fn) {
 
 /** \brief Use equations compiler infrastructure to implement match-with */
 expr parse_match(parser & p, unsigned, expr const *, pos_info const & pos) {
-    expr t  = p.parse_expr();
     buffer<expr> eqns;
+    expr t;
+    try {
+        t  = p.parse_expr();
         p.check_token_next(get_with_tk(), "invalid 'match' expression, 'with' expected");
         expr fn = mk_local(p.mk_fresh_name(), "match", mk_expr_placeholder(), binder_info());
         if (p.curr_is_token(get_end_tk())) {
@@ -740,6 +742,10 @@ expr parse_match(parser & p, unsigned, expr const *, pos_info const & pos) {
                 break;
             p.next();
         }
+    } catch (exception & ex) {
+        consume_until_end(p);
+        ex.rethrow();
+    }
     p.check_token_next(get_end_tk(), "invalid 'match' expression, 'end' expected");
     expr f = p.save_pos(mk_equations(1, eqns.size(), eqns.data()), pos);
     return p.mk_app(f, t, pos);

src/frontends/lean/util.cpp

@@ -22,6 +22,15 @@ Author: Leonardo de Moura
 #include "frontends/lean/tokens.h"
 
 namespace lean {
+void consume_until_end(parser & p) {
+    while (!p.curr_is_token(get_end_tk())) {
+        if (p.curr() == scanner::token_kind::Eof)
+            return;
+        p.next();
+    }
+    p.next();
+}
+
 bool parse_persistent(parser & p, bool & persistent) {
     if (p.curr_is_token_or_id(get_persistent_tk())) {
         p.next();

src/frontends/lean/util.h

@@ -14,6 +14,9 @@ Author: Leonardo de Moura
 
 namespace lean {
 class parser;
+/** \brief Consume tokes until 'end' token is consumed */
+void consume_until_end(parser & p);
+
 /** \brief Parse optional '[persistent]' modifier.
     return true if it is was found, and paremeter \c persistent to true.
 */

tests/lean/errors2.lean

@@ -0,0 +1,12 @@
+open nat
+
+namespace foo
+
+definition tst1 (a b : nat) : nat :=
+match a with
+| 0     := 1
+| (n+1) := foo
+end
+
+
+end foo

tests/lean/errors2.lean.expected.out

@@ -0,0 +1 @@
+errors2.lean:8:11: error: unknown identifier 'foo'