better error for iModIntro on mask-changing update

iris/proofmode/class_instances_updates.v

@@ -129,6 +129,12 @@ Proof. by rewrite /FromModal /= -bupd_intro. Qed.
 Global Instance from_modal_fupd E P `{!BiFUpd PROP} :
   FromModal True modality_id (|={E}=> P) (|={E}=> P) P.
 Proof. by rewrite /FromModal /= -fupd_intro. Qed.
+Global Instance from_modal_fupd_wrong_mask E1 E2 P `{!BiFUpd PROP} :
+  FromModal
+        (pm_error "Only non-mask-changing update modalities can be introduced directly.
+Use [iApply fupd_mask_intro] to introduce mask-changing update modalities")
+    modality_id (|={E1,E2}=> P) (|={E1,E2}=> P) P | 100.
+Proof. by intros []. Qed.
 
 Global Instance elim_modal_bupd `{!BiBUpd PROP} p P Q :
   ElimModal True p false (|==> P) P (|==> Q) (|==> Q).

tests/proofmode.ref

@@ -717,6 +717,16 @@ The command has indeed failed with message:
 Tactic failure: 
 "Goal and eliminated modality must have the same mask.
 Use [iMod (fupd_mask_subseteq E2)] to adjust the mask of your goal to [E2]".
+"iModIntro_mask_mismatch"
+     : string
+The command has indeed failed with message:
+Tactic failure: 
+"Only non-mask-changing update modalities can be introduced directly.
+Use [iApply fupd_mask_intro] to introduce mask-changing update modalities".
+The command has indeed failed with message:
+Tactic failure: 
+"Only non-mask-changing update modalities can be introduced directly.
+Use [iApply fupd_mask_intro] to introduce mask-changing update modalities".
 "test_pure_name"
      : string
 1 goal

tests/proofmode.v

@@ -1401,6 +1401,15 @@ Proof.
   Fail iIntros ">HP".
   iIntros "HP". Fail iMod "HP".
 Abort.
+
+Check "iModIntro_mask_mismatch".
+Lemma iMod_mask_mismatch `{!BiFUpd PROP} E1 E2 (P : PROP) :
+  ⊢ |={E1,E2}=> P.
+Proof.
+  Fail iModIntro.
+  Fail iIntros "!>".
+Abort.
+
 End error_tests.
 
 Section pure_name_tests.