fix(frontends/lean/structure_cmd): fixes #1681

@kha I'm not sure if this is the right fix. I just avoided the loop that adds
`mk_expr_placeholder` if the function is not a projection.
I didn't spend time investigating why we need `mk_proj_app`.
I know the library doesn't compile if we don't use it, and just use
```
    return mk_app(copy_tag(ref, mk_constant(S_name + fname)), e);
```
:)

src/frontends/lean/structure_cmd.cpp

@@ -179,14 +179,16 @@ void get_structure_fields_flattened(environment const & env, name const & struct
 
 expr mk_proj_app(environment const & env, name const & S_name, name const & fname, expr const & e, expr const & ref) {
     if (is_structure_like(env, S_name)) {
-        auto nparams = std::get<1>(get_structure_info(env, S_name));
-        auto proj = mk_explicit(copy_tag(ref, mk_constant(S_name + fname)));
-        for (unsigned i = 0; i < nparams; i++)
-            proj = mk_app(proj, mk_expr_placeholder());
-        return mk_app(proj, e);
-    } else {
-        return mk_app(copy_tag(ref, mk_constant(S_name + fname)), e);
+        name proj_name = S_name + fname;
+        if (get_projection_info(env, proj_name)) {
+            auto proj    = mk_explicit(copy_tag(ref, mk_constant(proj_name)));
+            auto nparams = std::get<1>(get_structure_info(env, S_name));
+            for (unsigned i = 0; i < nparams; i++)
+                proj = mk_app(proj, mk_expr_placeholder());
+            return mk_app(proj, e);
+        }
     }
+    return mk_app(copy_tag(ref, mk_constant(S_name + fname)), e);
 }
 
 optional<expr> mk_base_projections(environment const & env, name const & S_name, name const & base_S_name, expr const & e) {

tests/lean/run/1681.lean

@@ -0,0 +1,19 @@
+def prod.foo (s : bool × bool) (fst : bool) : bool × bool :=
+match s with
+(a, b) := (fst, b)
+end
+
+variable s : bool × bool
+
+#check prod.foo s
+#check prod.foo s tt
+#check s.foo
+#check s.foo tt
+
+def prod.foo2 {α β} (s : α × β) (fst : α) : α × β :=
+match s with
+(a, b) := (fst, b)
+end
+
+#check s.foo2
+#check s.foo2 tt