From 1477fbae1961a615ebbfa0e60f741f5514edd438 Mon Sep 17 00:00:00 2001
From: Vassil Mladenov <vmladenov@fb.com>
Date: Wed, 23 Dec 2020 14:21:18 -0800
Subject: [PATCH] Pretty print capabilities in generic subtyping errors

Summary: Previously, the subtyping would proceed with standard decomposition of types and report the error, so the error would be something like `Expected \HH\Capabilities\WriteProp but got mixed`. This interrupts the error and reports the full coeffect list on both sides. Using a different code because we don't want this to be fixmeable unlike 4110.

Reviewed By: DavidSnider

Differential Revision: D25695994

fbshipit-source-id: 89c1e2609d20a6bdd36bc18a0100b88cba81a43f
---
 hphp/hack/src/errors/error_codes.ml                     |  1 +
 hphp/hack/src/errors/errors.ml                          | 10 ++++++++++
 hphp/hack/src/errors/errors.mli                         |  3 +++
 hphp/hack/src/oxidized/gen/error_codes.rs               |  3 ++-
 hphp/hack/src/typing/typing_subtype.ml                  | 17 +++++++++++++++++
 hphp/hack/test/typecheck/coeffects/pretty_subtyping.php |  7 +++++++
 .../test/typecheck/coeffects/pretty_subtyping.php.exp   |  6 ++++++
 7 files changed, 46 insertions(+), 1 deletion(-)
 create mode 100644 hphp/hack/test/typecheck/coeffects/pretty_subtyping.php
 create mode 100644 hphp/hack/test/typecheck/coeffects/pretty_subtyping.php.exp

diff --git a/hphp/hack/src/errors/error_codes.ml b/hphp/hack/src/errors/error_codes.ml
index 4ad11b5d9ad..aa2760f97fd 100644
--- a/hphp/hack/src/errors/error_codes.ml
+++ b/hphp/hack/src/errors/error_codes.ml
@@ -673,6 +673,7 @@ module Typing = struct
     | IFCPolicyMismatch [@value 4400]
     | OpCoeffects [@value 4401]
     | ImplementsDynamic [@value 4402] (* EXTEND HERE WITH NEW VALUES IF NEEDED *)
+    | SubtypeCoeffects [@value 4403]
   [@@deriving enum, show { with_path = false }]
 
   let err_code = to_enum
diff --git a/hphp/hack/src/errors/errors.ml b/hphp/hack/src/errors/errors.ml
index bb56430392c..3ea95bc4ab4 100644
--- a/hphp/hack/src/errors/errors.ml
+++ b/hphp/hack/src/errors/errors.ml
@@ -5270,6 +5270,16 @@ let enum_inclusion_not_enum
       ^ " which is not an enum" )
     []
 
+(* This is meant for subtyping functions with incompatible coeffects, i.e.
+ * (function ()[io]: void) </: (function ()[]: void) *)
+let coeffect_subtyping_error
+    pos_expected cap_expected pos_got cap_got (on_error : typing_error_callback)
+    =
+  on_error
+    ~code:(Typing.err_code Typing.SubtypeCoeffects)
+    (pos_expected, "Expected a function that requires " ^ cap_expected)
+    [(pos_got, "But got a function that requires " ^ cap_got)]
+
 let call_coeffect_error
     ~available_incl_unsafe ~available_pos ~required ~required_pos call_pos =
   add_list
diff --git a/hphp/hack/src/errors/errors.mli b/hphp/hack/src/errors/errors.mli
index 67c75dae945..5ce187742ff 100644
--- a/hphp/hack/src/errors/errors.mli
+++ b/hphp/hack/src/errors/errors.mli
@@ -1395,6 +1395,9 @@ val call_coeffect_error :
   Pos.t ->
   unit
 
+val coeffect_subtyping_error :
+  Pos.t -> string -> Pos.t -> string -> typing_error_callback -> unit
+
 val op_coeffect_error :
   locally_available:string ->
   available_pos:Pos.t ->
diff --git a/hphp/hack/src/oxidized/gen/error_codes.rs b/hphp/hack/src/oxidized/gen/error_codes.rs
index db28378d37c..2439043fe3e 100644
--- a/hphp/hack/src/oxidized/gen/error_codes.rs
+++ b/hphp/hack/src/oxidized/gen/error_codes.rs
@@ -3,7 +3,7 @@
 // This source code is licensed under the MIT license found in the
 // LICENSE file in the "hack" directory of this source tree.
 //
-// @generated SignedSource<<1dfdf4ca7240e17d0d8af4af23995191>>
+// @generated SignedSource<<47420593d224e913c0dd1f5666f320d0>>
 //
 // To regenerate this file, run:
 //   hphp/hack/src/oxidized/regen.sh
@@ -565,6 +565,7 @@ pub enum Typing {
     IFCPolicyMismatch = 4400,
     OpCoeffects = 4401,
     ImplementsDynamic = 4402,
+    SubtypeCoeffects = 4403,
 }
 impl TrivialDrop for Typing {}
 
diff --git a/hphp/hack/src/typing/typing_subtype.ml b/hphp/hack/src/typing/typing_subtype.ml
index 925bd3c4ed8..69122a91bd1 100644
--- a/hphp/hack/src/typing/typing_subtype.ml
+++ b/hphp/hack/src/typing/typing_subtype.ml
@@ -2200,6 +2200,23 @@ and simplify_subtype_params_with_variadic
 and simplify_subtype_implicit_params
     ~subtype_env { capability = sub_cap } { capability = super_cap } env =
   if TypecheckerOptions.any_coeffects (Env.get_tcopt env) then
+    let subtype_env =
+      {
+        subtype_env with
+        on_error =
+          begin
+            fun ?code:_ _ _ ->
+            let expected = Typing_coeffects.get_type sub_cap in
+            let got = Typing_coeffects.get_type super_cap in
+            Errors.coeffect_subtyping_error
+              (get_pos expected)
+              (Typing_print.coeffects env expected)
+              (get_pos got)
+              (Typing_print.coeffects env got)
+              subtype_env.on_error
+          end;
+      }
+    in
     match (sub_cap, super_cap) with
     | (CapTy sub, CapTy super) -> simplify_subtype ~subtype_env sub super env
     | (CapTy sub, CapDefaults _p) ->
diff --git a/hphp/hack/test/typecheck/coeffects/pretty_subtyping.php b/hphp/hack/test/typecheck/coeffects/pretty_subtyping.php
new file mode 100644
index 00000000000..8b22fcc75a6
--- /dev/null
+++ b/hphp/hack/test/typecheck/coeffects/pretty_subtyping.php
@@ -0,0 +1,7 @@
+<?hh
+
+function purification_fail(
+  (function ()[local]: void) $f
+): (function ()[]: void) {
+  return $f;
+}
diff --git a/hphp/hack/test/typecheck/coeffects/pretty_subtyping.php.exp b/hphp/hack/test/typecheck/coeffects/pretty_subtyping.php.exp
new file mode 100644
index 00000000000..eb857dd8a57
--- /dev/null
+++ b/hphp/hack/test/typecheck/coeffects/pretty_subtyping.php.exp
@@ -0,0 +1,6 @@
+File "pretty_subtyping.php", line 6, characters 10-11:
+Invalid return type (Typing[4403])
+  File "pretty_subtyping.php", line 5, characters 16-17:
+  Expected a function that requires the capability set {}
+  File "pretty_subtyping.php", line 4, characters 16-20:
+  But got a function that requires the capability WriteProperty
-- 
2.11.4.GIT