fix(declarations): suppress errors after merge, not during

Fixes facebook#6631, facebook#6717, facebook#7803.

There are two major flaws in the original approach:

1. Silencing errors at the merge site does not work for builtins,
as those errors are computed later, and
2. Putting that suppression in an if/else where the else block
parsed comment error suppressions can cause [declarations] to
actually create *more* errors, which was misleading a lot of people
and masked the source of bugs.

We now suppress entire files in error collation, at the very end
of the error reporting process.

src/typing/errors/error_suppressions.ml

@@ -167,6 +167,11 @@ let in_node_modules ~root ~file_options loc =
   | None -> false
   | Some (file, options) -> Files.is_within_node_modules ~root ~options (File_key.to_string file)
 
+let in_declarations ~file_options loc =
+  match Option.both (Loc.source loc) file_options with
+  | None -> false
+  | Some (file, options) -> Files.is_declaration options (File_key.to_string file)
+
 let check ~root ~file_options (err : Loc.t Errors.printable_error) (suppressions : t) (unused : t)
     =
   let locs =
@@ -176,11 +181,11 @@ let check ~root ~file_options (err : Loc.t Errors.printable_error) (suppressions
      * without a source. *)
     |> List.filter (fun loc -> Option.is_some (Loc.source loc))
   in
-  (* Ignore lint errors from node modules. *)
+  (* Ignore lint errors from node modules, and all errors from declarations directories. *)
   let ignore =
     match Errors.kind_of_printable_error err with
     | Errors.LintError _ -> in_node_modules ~root ~file_options (Errors.loc_of_printable_error err)
-    | _ -> false
+    | _ -> in_declarations ~file_options (Errors.loc_of_printable_error err)
   in
   if ignore then
     None

tests/config_declarations_react/.flowconfig

@@ -0,0 +1,14 @@
+[ignore]
+
+[include]
+
+[libs]
+
+[declarations]
+<PROJECT_ROOT>/node_modules/react-native/.*
+
+[options]
+suppress_comment=.*\\$FlowFixMe
+suppress_comment=.*\\$FlowIssue
+suppress_comment=\\(.\\|\n\\)*\\$FlowNewLine
+include_warnings=true

tests/config_declarations_react/A.js

@@ -0,0 +1,14 @@
+// Due to [declarations], this won't error
+const str2str = require('react-native/str2str')
+
+// But this identical one will
+const str2str_loud = require('react-d3/str2str')
+
+// Calling str2str with a string is correct
+const foo: string = str2str(' bar ');
+
+// But not with a number
+const fooNum: number = str2str(39); // Would not error if using [untyped]!
+
+// trim2 should still error as well
+str2str(39);

tests/config_declarations_react/B.js

@@ -0,0 +1,5 @@
+const React = require('React');
+
+// Will error. Ensuring this still errors properly despite the react-native decl
+const AsyncMode: boolean = React.unstable_AsyncMode;
+

tests/config_declarations_react/config_declarations_react.exp

@@ -0,0 +1,63 @@
+Error ------------------------------------------------------------------------------------------------------- A.js:11:24
+
+Cannot assign `str2str(...)` to `fooNum` because string [1] is incompatible with number [2].
+
+   A.js:11:24
+   11| const fooNum: number = str2str(39); // Would not error if using [untyped]!
+                              ^^^^^^^^^^^
+
+References:
+   node_modules/react-native/str2str.js:7:40
+    7| module.exports = function str2str(foo: string) {
+                                              ^^^^^^ [1]
+   A.js:11:15
+   11| const fooNum: number = str2str(39); // Would not error if using [untyped]!
+                     ^^^^^^ [2]
+
+
+Error ------------------------------------------------------------------------------------------------------- A.js:11:32
+
+Cannot call `str2str` with `39` bound to `foo` because number [1] is incompatible with string [2].
+
+   A.js:11:32
+   11| const fooNum: number = str2str(39); // Would not error if using [untyped]!
+                                      ^^ [1]
+
+References:
+   node_modules/react-native/str2str.js:7:40
+    7| module.exports = function str2str(foo: string) {
+                                              ^^^^^^ [2]
+
+
+Error -------------------------------------------------------------------------------------------------------- A.js:14:9
+
+Cannot call `str2str` with `39` bound to `foo` because number [1] is incompatible with string [2].
+
+   A.js:14:9
+   14| str2str(39);
+               ^^ [1]
+
+References:
+   node_modules/react-native/str2str.js:7:40
+    7| module.exports = function str2str(foo: string) {
+                                              ^^^^^^ [2]
+
+
+Error -------------------------------------------------------------------------------------------------------- B.js:4:28
+
+Cannot get `React.unstable_AsyncMode` because property `unstable_AsyncMode` is missing in exports [1].
+
+   4| const AsyncMode: boolean = React.unstable_AsyncMode;
+                                 ^^^^^^^^^^^^^^^^^^^^^^^^
+
+
+Error ---------------------------------------------------------------------------- node_modules/react-d3/str2str.js:4:19
+
+Cannot get `React.unstable_AsyncMode` because property `unstable_AsyncMode` is missing in exports [1].
+
+   4| const AsyncMode = React.unstable_AsyncMode;
+                        ^^^^^^^^^^^^^^^^^^^^^^^^
+
+
+
+Found 5 errors