fix: reject duplicate function parameters with correct error in all contexts

Motivation:
go-jsonnet issue #873 reports that (function(x, x, x) x)(null, 3, 2)
silently returns 2 instead of rejecting duplicate parameter names.
sjsonnet already rejected duplicates in function expressions, but the
error message was misleading in local bindings and object methods.

Modification:
The params parser correctly detects duplicates via flatMapX + Fail.
However, in bind (local f(x,x)=...) and field ({f(x,x):...}), the
optional group .? around params swallowed the "no duplicate parameter"
error and backtracked, producing misleading errors like "Expected )".

Replaced .? with ./ (committed-or): "(" ~/ params ~ ")" ./ | Pass(null).
The cut ~/ after "(" commits to the params path, and ./ prevents the |
alternative from trying the fallback when the committed path fails.

Result:
All duplicate parameter contexts now produce the correct error:
"Expected no duplicate parameter: x" with accurate position info.
- (function(x, x, x) x) -> ParseError (already worked)
- local f(x, x) = x     -> ParseError (was "Expected )")
- { f(x, x): x }        -> ParseError (was "Expected :")
- { local f(x, x) = x } -> ParseError (was "Expected )")

Tests pass on Scala 3.3.7, 2.13.18, 2.12.21.

References:
- google/go-jsonnet#873

Author: He-Pin

sjsonnet/src/sjsonnet/Parser.scala

@@ -951,7 +951,7 @@ class Parser(
 
   def field[$: P](currentDepth: Int): P[Expr.Member.Field] = {
     P(
-      (Pos ~~ fieldname(currentDepth + 1) ~/ "+".!.? ~ ("(" ~ params(
+      (Pos ~~ fieldname(currentDepth + 1) ~/ "+".!.? ~ ("(" ~/ params(
         currentDepth + 1
       ) ~ ")").? ~ fieldKeySep ~/ expr(currentDepth + 1)).map { case (pos, name, plus, p, h2, e) =>
         Expr.Member.Field(pos, name, plus.nonEmpty, p.orNull, h2, e)
@@ -1045,18 +1045,27 @@ class Parser(
   def params[$: P]: P[Expr.Params] = params(0)
 
   def params[$: P](currentDepth: Int): P[Expr.Params] = {
-    P((id ~ ("=" ~ expr(currentDepth + 1)).?).rep(sep = ",") ~ ",".?).flatMapX { x =>
+    val ctx = implicitly[P[?]]
+    P(
+      (Pos ~~ id ~ ("=" ~ expr(currentDepth + 1)).?).rep(sep = ",") ~ ",".?
+    ).flatMapX { x =>
       val seen = collection.mutable.Set.empty[String]
-      var overlap: String = null
-      for ((k, _) <- x) {
-        if (seen(k)) overlap = k
-        else seen.add(k)
+      var overlap: (Position, String) = null
+      for ((pos, k, _) <- x) {
+        if (overlap == null) {
+          if (seen(k)) overlap = (pos, k)
+          else seen.add(k)
+        }
       }
       if (overlap == null) {
-        val names = x.map(_._1).toArray[String]
-        val exprs = x.map(_._2.orNull).toArray[Expr]
+        val names = x.map(_._2).toArray[String]
+        val exprs = x.map(_._3.orNull).toArray[Expr]
         Pass(Expr.Params(names, exprs))
-      } else Fail.opaque("no duplicate parameter: " + overlap)
+      } else {
+        ctx.cut = true
+        ctx.index = overlap._1.offset
+        Fail.opaque("no duplicate parameter: " + overlap._2)
+      }
     }
   }
 

sjsonnet/test/resources/test_suite/error.function_duplicate_param.jsonnet.golden

@@ -1,3 +1,3 @@
-sjsonnet.ParseError: Expected no duplicate parameter: x:17:14, found ") x\n"
-    at [<root>].(error.function_duplicate_param.jsonnet:17:14)
+sjsonnet.ParseError: Expected no duplicate parameter: x:17:13, found "x) x\n"
+    at [<root>].(error.function_duplicate_param.jsonnet:17:13)
 

sjsonnet/test/src/sjsonnet/ParserTests.scala

@@ -57,6 +57,16 @@ object ParserTests extends TestSuite {
     test("duplicateFields") {
       parseErr("{ a: 1, a: 2 }") ==> """Expected no duplicate field: a:1:14, found "}""""
     }
+    test("duplicateParams") {
+      parseErr("(function(x, x, x) x)(null, 3, 2)") ==>
+        """Expected no duplicate parameter: x:1:14, found "x, x) x)(n""""
+      parseErr("local f(x, x) = x; f(1, 2)") ==>
+        "Expected no duplicate parameter: x:1:12, found \"x) = x; f(\""
+      parseErr("{ f(x, x): x }.f(1, 2)") ==>
+        "Expected no duplicate parameter: x:1:8, found \"x): x }.f(\""
+      parseErr("{ local f(x, x) = x, a: f(1, 2) }") ==>
+        "Expected no duplicate parameter: x:1:14, found \"x) = x, a:\""
+    }
     test("localInObj") {
       parse("""{
               |local x = 1,