fix(frontends/lean/structure_cmd): even less error recovery

src/frontends/lean/structure_cmd.cpp

@@ -507,6 +507,8 @@ struct structure_cmd_fn {
 
     /** \brief elaborate parameters and "parent" types */
     void elaborate_header() {
+        auto _ = m_p.no_error_recovery_scope(); // we require that m_p.elaborate_type(mk_let()) is a let, etc.
+
         buffer<expr> include_vars;
         m_p.get_include_variables(include_vars);
         buffer<expr> tmp_locals;

tests/lean/structure_elab_segfault.lean

@@ -1 +1,2 @@
+structure foo {A} (R) (x : list A) : Prop := (bar : R x)
 structure test : Type := (f : ∀ x y, true)

tests/lean/structure_elab_segfault.lean.expected.out

@@ -1,2 +1,4 @@
 structure_elab_segfault.lean:1:0: error: infer type failed, sort expected
   delayed[?m_1]
+structure_elab_segfault.lean:2:0: error: infer type failed, sort expected
+  delayed[?m_1]