Add pattern completion for unapply contexts (#20274)

Fixes #19972.

Add pattern completion for `Unapply` tree contexts.

A typical example would be
```scala
optionList match
    case List(S@@)
```
which should be prompted `Some(value)`, due to `List.unapplySeq`
expecting `Option[T]` patterns as arguments.

Author: natsukagami

Diff for: presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala

@@ -70,6 +70,8 @@ class Completions(
         false
       case (_: (Import | Export)) :: _ => false
       case _ :: (_: (Import | Export)) :: _ => false
+      // UnApply has patterns included in MatchCaseCompletions
+      case _ :: (_: UnApply) :: _ => false
       case _ => true
 
   private lazy val isNew: Boolean = Completion.isInNewContext(adjustedPath)
@@ -405,6 +407,36 @@ class Completions(
           true,
         )
 
+      // unapply pattern
+      case Ident(name) :: (unapp : UnApply) :: _ =>
+        (
+          CaseKeywordCompletion.contribute(
+            EmptyTree, // no selector
+            completionPos,
+            indexedContext,
+            config,
+            search,
+            parent = unapp,
+            autoImports,
+            patternOnly = Some(name.decoded)
+          ),
+          false,
+        )
+      case Select(_, name) :: (unapp : UnApply) :: _ =>
+        (
+          CaseKeywordCompletion.contribute(
+            EmptyTree, // no selector
+            completionPos,
+            indexedContext,
+            config,
+            search,
+            parent = unapp,
+            autoImports,
+            patternOnly = Some(name.decoded)
+          ),
+          false,
+        )
+
       // class FooImpl extends Foo:
       //   def x|
       case OverrideExtractor(td, completing, start, exhaustive, fallbackName) =>

Diff for: presentation-compiler/src/main/dotty/tools/pc/completions/MatchCaseCompletions.scala

@@ -27,6 +27,8 @@ import dotty.tools.dotc.core.Types.NoType
 import dotty.tools.dotc.core.Types.OrType
 import dotty.tools.dotc.core.Types.Type
 import dotty.tools.dotc.core.Types.TypeRef
+import dotty.tools.dotc.core.Types.AppliedType
+import dotty.tools.dotc.typer.Applications.UnapplyArgs
 import dotty.tools.dotc.util.SourcePosition
 import dotty.tools.pc.AutoImports.AutoImportsGenerator
 import dotty.tools.pc.AutoImports.SymbolImport
@@ -75,10 +77,24 @@ object CaseKeywordCompletion:
       patternOnly,
       hasBind
     )
+
     val printer = ShortenedTypePrinter(search, IncludeDefaultParam.Never)(using indexedContext)
     val selTpe = selector match
       case EmptyTree =>
         parent match
+          /* Parent is an unapply pattern */
+          case UnApply(fn, implicits, patterns) if !fn.tpe.isErroneous =>
+            patternOnly match
+              case None => None
+              case Some(value) =>
+                val argPts = UnapplyArgs(fn.tpe.widen.finalResultType, fn, patterns, parent.srcPos).argTypes
+                patterns.zipWithIndex
+                  .find:
+                    case (Ident(v), tpe) => v.decoded == value
+                    case (Select(_, v), tpe) => v.decoded == value
+                    case t => false
+                  .map((_, id) => argPts(id).widen.deepDealias)
+          /* Parent is a function expecting a case match expression */
           case TreeApply(fun, _) if !fun.tpe.isErroneous =>
             fun.tpe.paramInfoss match
               case (head :: Nil) :: _
@@ -105,7 +121,8 @@ object CaseKeywordCompletion:
           if patternOnly.isEmpty then
             val selectorTpe = selTpe.show
             val tpeLabel =
-              if !selectorTpe.contains("x$1") then selectorTpe
+              if !selectorTpe.contains("x$1") /* selector of a function type? */ then
+                selectorTpe
               else selector.symbol.info.show
             val label = s"case ${tpeLabel} =>"
             List(

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

@@ -634,14 +634,58 @@ class CompletionSuite extends BaseCompletionSuite:
          |""".stripMargin
     )
 
+  @Test def patRecursive =
+    check(
+      s"""|object Main {
+          |  Option(List(Option(1))) match {
+          |    case Some(List(None, Som@@))
+          |}
+          |""".stripMargin,
+      """|Some(value) scala
+         |Some scala
+         |""".stripMargin
+    )
+    check(
+      s"""|object Main {
+          |  (null: Option[Option[Option[Option[Int]]]]) match
+          |    case Some(Some(Some(Som@@))))
+          |}
+          |""".stripMargin,
+      """|Some(value) scala
+         |Some scala
+         |""".stripMargin
+    )
+    check(
+      s"""|object Main {
+          |  Option(Option(1)) match {
+          |    case Some(Som@@)
+          |}
+          |""".stripMargin,
+      """|Some(value) scala
+         |Some scala
+         |""".stripMargin
+    )
+    check(
+      s"""|object Test:
+          |  case class NestedClass(x: Int)
+          |object TestRun:
+          |  Option(Test.NestedClass(5)) match
+          |    case Some(Test.Neste@@)
+          |""".stripMargin,
+      """|NestedClass(x) test.Test
+         |NestedClass test.Test
+         |""".stripMargin
+    )
+
   @Test def pat1 =
     check(
       s"""|object Main {
           |  Option(1) match {
           |    case List(Som@@)
           |}
           |""".stripMargin,
-      """|Some[A](value: A): Some[A]
+      """|Some(value) scala
+         |Some scala
          |Some scala
          |""".stripMargin
     )