Print parens for single method argument only if a direct tuple type (#21510)

A type alias of a tuple type should be printed without parenthesis.

Closes scalameta/metals#6613

Author: tgodzik

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1806,18 +1806,24 @@ class Definitions {
     || (sym eq Any_typeTest)
     || (sym eq Any_typeCast)
 
-  /** Is this type a `TupleN` type?
+  /** Is `tp` a `TupleN` type?
+    *
+    * @return true if the type of `tp` is `TupleN[T1, T2, ..., Tn]`
+    */
+  def isDirectTupleNType(tp: Type)(using Context): Boolean =
+    val arity = tp.argInfos.length
+    arity <= MaxTupleArity && {
+      val tupletp = TupleType(arity)
+      tupletp != null && tp.isRef(tupletp.symbol)
+    }
+
+  /** Is `tp` (an alias of) a `TupleN` type?
    *
    * @return true if the dealiased type of `tp` is `TupleN[T1, T2, ..., Tn]`
    */
-  def isTupleNType(tp: Type)(using Context): Boolean = {
+  def isTupleNType(tp: Type)(using Context): Boolean =
     val tp1 = tp.dealias
-    val arity = tp1.argInfos.length
-    arity <= MaxTupleArity && {
-      val tupletp = TupleType(arity)
-      tupletp != null && tp1.isRef(tupletp.symbol)
-    }
-  }
+    isDirectTupleNType(tp1)
 
   def tupleType(elems: List[Type]): Type = {
     val arity = elems.length

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -168,7 +168,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
     changePrec(GlobalPrec) {
       val argStr: Text =
         if args.length == 2
-            && !defn.isTupleNType(args.head)
+            && !defn.isDirectTupleNType(args.head)
             && !isGiven
         then
           atPrec(InfixPrec) { argText(args.head) }

presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionSuite.scala

@@ -147,6 +147,50 @@ class CompletionSuite extends BaseCompletionSuite:
       "XtensionMethod(a: Int): XtensionMethod"
     )
 
+  @Test def tupleDirect =
+    check(
+      """
+        |trait Foo {
+        |  def setup: List[(String, String)]
+        |}
+        |object Foo {
+        |  val foo: Foo = ???
+        |  foo.setup.exist@@
+        |}""".stripMargin,
+      """|exists(p: ((String, String)) => Boolean): Boolean
+          |""".stripMargin
+    )
+
+  @Test def tupleAlias =
+    check(
+      """
+        |trait Foo {
+        |  def setup: List[Foo.TupleAliasResult]
+        |}
+        |object Foo {
+        |  type TupleAliasResult = (String, String)
+        |  val foo: Foo = ???
+        |  foo.setup.exist@@
+        |}""".stripMargin,
+      """|exists(p: TupleAliasResult => Boolean): Boolean
+         |""".stripMargin
+    )
+
+  @Test def listAlias =
+    check(
+      """
+        |trait Foo {
+        |  def setup: List[Foo.ListAliasResult]
+        |}
+        |object Foo {
+        |  type ListAliasResult = List[String]
+        |  val foo: Foo = ???
+        |  foo.setup.exist@@
+        |}""".stripMargin,
+      """|exists(p: ListAliasResult => Boolean): Boolean
+         |""".stripMargin
+    )
+
   @Ignore("This test should be handled by compiler fuzzy search")
   @Test def fuzzy =
     check(