[JsLIGO] Allow _ as an expression

Motivation and Context

Since we allow _ as a normal binder, we have to allow it as an expression.

See #1692 (closed)

Description

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

Underscore is now accepted everywhere a variable is accepted.

Checklist:

• If a new syntax has been introduced, put a message on slack ligo-lsp
• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).

src/bin/expect_tests/contract_tests.ml

@@ -7,6 +7,22 @@ let bad_contract = bad_test
 (* avoid pretty printing *)
 let () = Ligo_unix.putenv ~key:"TERM" ~data:"dumb"
 
+let%expect_test _ =
+  run_ligo_good
+    [ "compile"
+    ; "expression"
+    ; "jsligo"
+    ; "h2"
+    ; "--init-file"
+    ; contract "wildcard_in_type.jsligo"
+    ];
+  [%expect {|
+    { LAMBDA (pair int int) int { UNPAIR ; ADD } ;
+      DUP 2 ;
+      APPLY ;
+      SWAP ;
+      DROP } |}]
+
 let%expect_test _ =
   run_ligo_good
     [ "compile"