Fix parenthesization for binary operators

shared/src/main/scala/mlscript/JSBackend.scala

@@ -277,6 +277,12 @@ class JSBackend(allowUnresolvedSymbols: Boolean) {
           name -> translateTerm(value)
         }) :: Nil
       )
+    // Only parenthesize binary operators
+    // Custom operators do not need special handling since they are desugared to plain methods
+    case Bra(false, trm) => trm match { 
+      case App(Var(op), _) if JSBinary.operators.contains(op) => JSParenthesis(translateTerm(trm)) 
+      case trm => translateTerm(trm)
+    }
     case Bra(_, trm) => translateTerm(trm)
     case Tup(terms) =>
       JSArray(terms map { case (_, Fld(_, term)) => translateTerm(term) })

shared/src/main/scala/mlscript/codegen/Codegen.scala

@@ -924,6 +924,11 @@ final case class JSComment(text: Str) extends JSStmt {
   def toSourceCode: SourceCode = SourceCode(s"// $text")
 }
 
+final case class JSParenthesis(exp: JSExpr) extends JSExpr {
+  implicit def precedence: Int = 0
+  def toSourceCode: SourceCode = exp.embed
+}
+
 object JSCodeHelpers {
   def id(name: Str): JSIdent = JSIdent(name)
   def lit(value: Int): JSLit = JSLit(value.toString())

shared/src/test/diff/codegen/NuParentheses.mls

@@ -0,0 +1,61 @@
+:NewDefs
+
+
+:js
+16 / (2 / 2)
+//│ Num
+//│ // Prelude
+//│ let res;
+//│ class TypingUnit {}
+//│ const typing_unit = new TypingUnit;
+//│ // Query 1
+//│ res = 16 / (2 / 2);
+//│ // End of generated code
+//│ res
+//│     = 16
+
+:js
+1 - (3 - 5)
+//│ Int
+//│ // Prelude
+//│ class TypingUnit1 {}
+//│ const typing_unit1 = new TypingUnit1;
+//│ // Query 1
+//│ res = 1 - (3 - 5);
+//│ // End of generated code
+//│ res
+//│     = 3
+
+
+fun (--) minusminus(a, b) = a - b
+//│ fun (--) minusminus: (Int, Int) -> Int
+
+:js
+1 -- (3 -- 5)
+//│ Int
+//│ // Prelude
+//│ class TypingUnit3 {}
+//│ const typing_unit3 = new TypingUnit3;
+//│ // Query 1
+//│ res = minusminus(1, minusminus(3, 5));
+//│ // End of generated code
+//│ res
+//│     = 3
+
+
+fun (-+-) complex(a, b) = a - 2*b
+//│ fun (-+-) complex: (Int, Int) -> Int
+
+:js
+1 -+- (3 -+- 5)
+//│ Int
+//│ // Prelude
+//│ class TypingUnit5 {}
+//│ const typing_unit5 = new TypingUnit5;
+//│ // Query 1
+//│ res = complex(1, complex(3, 5));
+//│ // End of generated code
+//│ res
+//│     = 15
+
+

shared/src/test/diff/nu/UnaryMinus.mls

@@ -28,7 +28,7 @@
 1 - (3 - 5)
 //│ Int
 //│ res
-//│     = -7
+//│     = 3
 
 3 - 1
 //│ Int
@@ -53,7 +53,7 @@
 1 - (1 - 1)
 //│ Int
 //│ res
-//│     = -1
+//│     = 1
 
 1 - 1
 //│ Int