Add support for completions for extension definition

Author: rochala

compiler/src/dotty/tools/dotc/interactive/Completion.scala

@@ -3,6 +3,7 @@ package dotty.tools.dotc.interactive
 import scala.language.unsafeNulls
 
 import dotty.tools.dotc.ast.untpd
+import dotty.tools.dotc.ast.NavigateAST
 import dotty.tools.dotc.config.Printers.interactiv
 import dotty.tools.dotc.core.Contexts._
 import dotty.tools.dotc.core.Decorators._
@@ -129,16 +130,29 @@ object Completion {
       case _ => 0
     }
 
+  /**
+   * Inspect `path` to deterimine whether enclosing tree is a result of tree extension.
+   * If so, completion should use untyped path containing tree before extension to get proper results.
+   */
+  def pathBeforeDesugaring(path: List[Tree], pos: SourcePosition)(using Context): List[Tree] =
+    val hasUntypedTree = path.headOption.forall(NavigateAST.untypedPath(_, exactMatch = true).nonEmpty)
+    if hasUntypedTree then
+      path
+    else
+      NavigateAST.untypedPath(pos.span).collect:
+        case tree: untpd.Tree => tree
+
   private def computeCompletions(pos: SourcePosition, path: List[Tree])(using Context): (Int, List[Completion]) = {
-    val mode = completionMode(path, pos)
-    val rawPrefix = completionPrefix(path, pos)
+    val path0 = pathBeforeDesugaring(path, pos)
+    val mode = completionMode(path0, pos)
+    val rawPrefix = completionPrefix(path0, pos)
 
     val hasBackTick = rawPrefix.headOption.contains('`')
     val prefix = if hasBackTick then rawPrefix.drop(1) else rawPrefix
 
     val completer = new Completer(mode, prefix, pos)
 
-    val completions = path match {
+    val completions = path0 match {
         // Ignore synthetic select from `This` because in code it was `Ident`
         // See example in dotty.tools.languageserver.CompletionTest.syntheticThis
         case Select(qual @ This(_), _) :: _ if qual.span.isSynthetic  => completer.scopeCompletions
@@ -153,7 +167,7 @@ object Completion {
     val backtickedCompletions =
       describedCompletions.map(completion => backtickCompletions(completion, hasBackTick))
 
-    val offset = completionOffset(path)
+    val offset = completionOffset(path0)
 
     interactiv.println(i"""completion with pos     = $pos,
                           |                prefix  = ${completer.prefix},

language-server/test/dotty/tools/languageserver/CompletionTest.scala

@@ -1528,4 +1528,13 @@ class CompletionTest {
         )
       )
   }
+
+  @Test def desugaredErrorStatement: Unit =
+    code"""|trait Foo
+           |object T:
+           |  extension (x: Fo$m1)
+           |"""
+      .completion(m1, Set(
+          ("Foo",Class,"Foo")
+      ))
 }

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

@@ -125,7 +125,7 @@ class Completions(
   end CursorPos
 
   private lazy val cursorPos =
-    calculateTypeInstanceAndNewPositions(path)
+    calculateTypeInstanceAndNewPositions(Completion.pathBeforeDesugaring(path, pos))
 
   private def calculateTypeInstanceAndNewPositions(
       path: List[Tree]

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

@@ -1326,6 +1326,26 @@ class CompletionSuite extends BaseCompletionSuite:
       """|AClass[A <: Int] test.O
          |AClass test.O
          |AbstractTypeClassManifest - scala.reflect.ClassManifestFactory
+         """.stripMargin
+    )
+
+  @Test def `extension-definition-scope` =
+    check(
+      """|object T:
+         |  extension (x: ListBuffe@@)
+         |""".stripMargin,
+      """|ListBuffer[T] - scala.collection.mutable
+         |ListBuffer - scala.collection.mutable
+         |""".stripMargin,
+    )
+
+  @Test def `extension-definition-symbol-search` =
+    check(
+      """|trait Foo
+         |object T:
+         |  extension (x: Fo@@)
+         |""".stripMargin,
+      """|Foo test
          |""".stripMargin
     )