fix(frontends/lean/structure_cmd): 'structure' command must set unfold-c attribute for auxiliary recursors

fixes #582

library/data/rat/basic.lean

@@ -109,7 +109,7 @@ theorem num_eq_zero_of_equiv_zero {a : prerat} : a ≡ zero → num a = 0 :=
 by rewrite [↑equiv, ↑zero, ↑of_int, mul_one, zero_mul]; intro H; exact H
 
 theorem inv_zero {d : int} (dp : d > 0) : inv (mk nat.zero d dp) = zero :=
-begin rewrite [↑inv, ↑int.cases_on, ↑cases_on, ▸*] end
+begin rewrite [↑inv, ▸*] end
 
 theorem inv_zero' : inv zero = zero := inv_zero (of_nat_succ_pos nat.zero)
 

src/frontends/lean/structure_cmd.cpp

@@ -18,6 +18,7 @@ Author: Leonardo de Moura
 #include "kernel/error_msgs.h"
 #include "kernel/inductive/inductive.h"
 #include "library/scoped_ext.h"
+#include "library/normalize.h"
 #include "library/placeholder.h"
 #include "library/locals.h"
 #include "library/reducible.h"
@@ -725,6 +726,8 @@ struct structure_cmd_fn {
                                              opaque);
         m_env = module::add(m_env, check(m_env, new_decl));
         m_env = set_reducible(m_env, n, reducible_status::Reducible);
+        if (optional<unsigned> idx = has_unfold_c_hint(m_env, rec_on_name))
+            m_env = add_unfold_c_hint(m_env, n, *idx);
         save_def_info(n);
         add_alias(n);
     }

tests/lean/582.hlean

@@ -0,0 +1,9 @@
+open prod
+variables {A B C : Type} (f : A → B → C) (a : A) (b b' : B)
+
+example (p : b = b') : prod.cases_on (a, b) f = f a b' :=
+begin
+  esimp,
+  state,
+  rewrite p
+end

tests/lean/582.hlean.expected.out

@@ -0,0 +1,9 @@
+582.hlean:7:2: proof state
+A : Type,
+B : Type,
+C : Type,
+f : A → B → C,
+a : A,
+b b' : B,
+p : b = b'
+⊢ f a b = f a b'