Reverse top-level Flows in supertype

Summary:
Although we accumulate provenance on the subtype during subtyping, we actually record typing time provenance on both types so the supertype may be a `Flow`.  In this case we actually want the inner most element of the flow rather than the outer most (which is what we want for the subtype)

This diff adds a function to reverse top-level flows and applies it to the supertype when rendering a subtyping error.

Reviewed By: madgen

Differential Revision: D63900892

fbshipit-source-id: 2c2034476c2ae0b5e68c01622f18bb9de8ecf090

Author: Michael Thomas

hphp/hack/src/typing/typing_error_utils.ml

@@ -5821,7 +5821,8 @@ end = struct
            (ty_super_descr, ty_sub_descr)
        in
        let left =
-         Typing_reason.to_string ("Expected " ^ ty_super_descr) r_super
+         Typing_reason.to_string ("Expected " ^ ty_super_descr)
+         @@ Typing_reason.reverse_flow r_super
        in
        let right = Typing_reason.to_string ("But got " ^ ty_sub_descr) r_sub in
        let reasons = left @ right in

hphp/hack/src/typing/typing_reason.ml

@@ -2063,6 +2063,14 @@ let rec flow_contains_tyvar = function
   | Flow { into; _ } -> flow_contains_tyvar into
   | _ -> false
 
+let reverse_flow t =
+  let rec aux ~k = function
+    | Flow { from; into; kind } ->
+      aux into ~k:(fun into -> k @@ Flow { from = into; into = from; kind })
+    | t -> k t
+  in
+  aux ~k:(fun r -> r) t
+
 (* Translate a reason to a (pos, string) list, suitable for error_l. This
  * previously returned a string, however the need to return multiple lines with
  * multiple locations meant that it needed to more than convert to a string *)

hphp/hack/src/typing/typing_reason.mli

@@ -80,6 +80,8 @@ type decl_t = decl_phase t_
 
 val localize : decl_t -> t
 
+val reverse_flow : t -> t
+
 (** Translate a reason to a (pos, string) list, suitable for error_l. This
     previously returned a string, however the need to return multiple lines with
     multiple locations meant that it needed to more than convert to a string *)

hphp/hack/test/extended_reasons/bad_inout_lvalue9.php.debug.exp

@@ -135,14 +135,14 @@ File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
  6 |   f(inout »tuple($x, $y)«);
  7 | }
 
-Expected `(int, int)` because the type of this inout parameter is implicitly a like-type
+Expected `(int, int)`
 
-File "bad_inout_lvalue9.php", line 3, character 30 - line 3, character 31:
+File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
 
- 3 | function f(inout varray<int> »$a«): void {}
+ 3 | function f(inout varray<int> $a): void {}
  4 | 
  5 | function test(int $x, int $y): void {
- 6 |   f(inout tuple($x, $y));
+ 6 |   f(inout »tuple($x, $y)«);
  7 | }
 
 But got `vec<int>`
@@ -285,14 +285,14 @@ File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
  6 |   f(inout »tuple($x, $y)«);
  7 | }
 
-Expected `(int, int)` because the type of this inout parameter is implicitly a like-type
+Expected `(int, int)`
 
-File "bad_inout_lvalue9.php", line 3, character 30 - line 3, character 31:
+File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
 
- 3 | function f(inout varray<int> »$a«): void {}
+ 3 | function f(inout varray<int> $a): void {}
  4 | 
  5 | function test(int $x, int $y): void {
- 6 |   f(inout tuple($x, $y));
+ 6 |   f(inout »tuple($x, $y)«);
  7 | }
 
 But got `dynamic` because the type of this inout parameter is implicitly a like-type

hphp/hack/test/extended_reasons/bad_inout_lvalue9.php.diff.exp

@@ -1,36 +0,0 @@
---- control
-
-+++ test
-
-@@ -20,20 +20,20 @@
-
-       7 | }
- 
- error: Typing[4110] Invalid argument to an inout parameter [1]
---> Expected (int, int) [1]
---> But got vec<int> [2]
-+-> Expected (int, int) because the type of this inout parameter is implicitly a like-type [2]
-+-> But got vec<int> [3]
- 
- hphp/hack/test/extended_reasons/bad_inout_lvalue9.php:6:11
--    1 | <?hh
--    2 | 
--[2] 3 | function f(inout varray<int> $a): void {}
--    4 | 
--    5 | function test(int $x, int $y): void {
--[1] 6 |   f(inout tuple($x, $y));
--    7 | }
-+      1 | <?hh
-+      2 | 
-+[2,3] 3 | function f(inout varray<int> $a): void {}
-+      4 | 
-+      5 | function test(int $x, int $y): void {
-+[1]   6 |   f(inout tuple($x, $y));
-+      7 | }
- 
- error: Typing[4110] Invalid argument to an inout parameter [1]
---> Expected (int, int) [1]
-+-> Expected (int, int) because the type of this inout parameter is implicitly a like-type [2]
- -> But got dynamic because the type of this inout parameter is implicitly a like-type [2]
- 
- hphp/hack/test/extended_reasons/bad_inout_lvalue9.php:6:11

hphp/hack/test/extended_reasons/bad_inout_lvalue9.php.exp

@@ -106,14 +106,14 @@ File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
  6 |   f(inout »tuple($x, $y)«);
  7 | }
 
-Expected `(int, int)` because the type of this inout parameter is implicitly a like-type
+Expected `(int, int)`
 
-File "bad_inout_lvalue9.php", line 3, character 30 - line 3, character 31:
+File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
 
- 3 | function f(inout varray<int> »$a«): void {}
+ 3 | function f(inout varray<int> $a): void {}
  4 | 
  5 | function test(int $x, int $y): void {
- 6 |   f(inout tuple($x, $y));
+ 6 |   f(inout »tuple($x, $y)«);
  7 | }
 
 But got `vec<int>`
@@ -205,14 +205,14 @@ File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
  6 |   f(inout »tuple($x, $y)«);
  7 | }
 
-Expected `(int, int)` because the type of this inout parameter is implicitly a like-type
+Expected `(int, int)`
 
-File "bad_inout_lvalue9.php", line 3, character 30 - line 3, character 31:
+File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
 
- 3 | function f(inout varray<int> »$a«): void {}
+ 3 | function f(inout varray<int> $a): void {}
  4 | 
  5 | function test(int $x, int $y): void {
- 6 |   f(inout tuple($x, $y));
+ 6 |   f(inout »tuple($x, $y)«);
  7 | }
 
 But got `dynamic` because the type of this inout parameter is implicitly a like-type

hphp/hack/test/extended_reasons/bad_inout_lvalue9.php.terse.exp

@@ -97,14 +97,14 @@ File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
  6 |   f(inout »tuple($x, $y)«);
  7 | }
 
-Expected `(int, int)` because the type of this inout parameter is implicitly a like-type
+Expected `(int, int)`
 
-File "bad_inout_lvalue9.php", line 3, character 30 - line 3, character 31:
+File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
 
- 3 | function f(inout varray<int> »$a«): void {}
+ 3 | function f(inout varray<int> $a): void {}
  4 | 
  5 | function test(int $x, int $y): void {
- 6 |   f(inout tuple($x, $y));
+ 6 |   f(inout »tuple($x, $y)«);
  7 | }
 
 But got `vec<int>`
@@ -187,14 +187,14 @@ File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
  6 |   f(inout »tuple($x, $y)«);
  7 | }
 
-Expected `(int, int)` because the type of this inout parameter is implicitly a like-type
+Expected `(int, int)`
 
-File "bad_inout_lvalue9.php", line 3, character 30 - line 3, character 31:
+File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
 
- 3 | function f(inout varray<int> »$a«): void {}
+ 3 | function f(inout varray<int> $a): void {}
  4 | 
  5 | function test(int $x, int $y): void {
- 6 |   f(inout tuple($x, $y));
+ 6 |   f(inout »tuple($x, $y)«);
  7 | }
 
 But got `dynamic` because the type of this inout parameter is implicitly a like-type

hphp/hack/test/extended_reasons/bad_inout_lvalue9.php.verbose.exp

@@ -106,14 +106,14 @@ File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
  6 |   f(inout »tuple($x, $y)«);
  7 | }
 
-Expected `(int, int)` because the type of this inout parameter is implicitly a like-type
+Expected `(int, int)`
 
-File "bad_inout_lvalue9.php", line 3, character 30 - line 3, character 31:
+File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
 
- 3 | function f(inout varray<int> »$a«): void {}
+ 3 | function f(inout varray<int> $a): void {}
  4 | 
  5 | function test(int $x, int $y): void {
- 6 |   f(inout tuple($x, $y));
+ 6 |   f(inout »tuple($x, $y)«);
  7 | }
 
 But got `vec<int>`
@@ -205,14 +205,14 @@ File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
  6 |   f(inout »tuple($x, $y)«);
  7 | }
 
-Expected `(int, int)` because the type of this inout parameter is implicitly a like-type
+Expected `(int, int)`
 
-File "bad_inout_lvalue9.php", line 3, character 30 - line 3, character 31:
+File "bad_inout_lvalue9.php", line 6, character 11 - line 6, character 23:
 
- 3 | function f(inout varray<int> »$a«): void {}
+ 3 | function f(inout varray<int> $a): void {}
  4 | 
  5 | function test(int $x, int $y): void {
- 6 |   f(inout tuple($x, $y));
+ 6 |   f(inout »tuple($x, $y)«);
  7 | }
 
 But got `dynamic` because the type of this inout parameter is implicitly a like-type