feat(frontends/lean/decl_cmds): allow 'empty' set of pattern matching equations

src/emacs/lean-syntax.el

@@ -119,7 +119,7 @@
      ;; modifiers
      (,(rx (or "\[persistent\]" "\[notation\]" "\[visible\]" "\[instance\]" "\[class\]" "\[parsing-only\]"
                "\[coercion\]" "\[reducible\]" "\[irreducible\]" "\[semireducible\]" "\[quasireducible\]" "\[wf\]"
-               "\[whnf\]" "\[multiple-instances\]"
+               "\[whnf\]" "\[multiple-instances\]" "\[none\]"
                "\[decls\]" "\[declarations\]" "\[all-transparent\]" "\[coercions\]" "\[classes\]"
                "\[notations\]" "\[abbreviations\]" "\[begin-end-hints\]" "\[tactic-hints\]" "\[reduce-hints\]"))
       . 'font-lock-doc-face)

src/frontends/lean/decl_cmds.cpp

@@ -604,6 +604,11 @@ expr parse_equations(parser & p, name const & n, expr const & type, buffer<name>
             }
         }
         check_eqn_prefix(p);
+        if (p.curr_is_token(get_none_tk())) {
+            // no equations have been provided
+            p.next();
+            eqns.push_back(Fun(fns, mk_no_equation(), p));
+        } else {
             for (expr const & fn : fns)
                 p.add_local(fn);
             while (true) {
@@ -658,6 +663,7 @@ expr parse_equations(parser & p, name const & n, expr const & type, buffer<name>
                 p.next();
             }
         }
+    }
     if (p.curr_is_token(get_wf_tk())) {
         auto pos = p.pos();
         p.next();

tests/lean/run/empty_eq.lean

@@ -0,0 +1,9 @@
+import data.fin
+open nat
+open fin
+
+definition case0 {C : fin zero → Type} : Π (f : fin zero), C f
+| [none]
+
+
+print definition case0