Create extension method when infer method is not possible

Author: zielinsky

presentation-compiler/src/main/dotty/tools/pc/InferredMethodProvider.scala

@@ -227,7 +227,25 @@ final class InferredMethodProvider(
                 s"\n$indent$signature",
               )
             ) ::: imports
-        case None => Nil
+        case None => 
+          extensionMethodEdits(signature, container)
+
+    def extensionMethodEdits(signature: String, container: Tree): List[TextEdit] =
+      val containerTypeStr = printType(container.tpe.widenDealias)
+      
+      val pos = insertPosition()
+      val indent = indentation(params.text(), pos.start - 1)
+      val extensionSignature = s"extension (x: $containerTypeStr)\n  $indent$signature"
+      
+      val lspPos = pos.toLsp
+      lspPos.setEnd(lspPos.getStart())
+
+      List(
+        TextEdit(
+          lspPos,
+          s"$extensionSignature\n$indent",
+        )
+      ) ::: imports
 
     path match
       /**

presentation-compiler/test/dotty/tools/pc/tests/edit/InsertInferredMethodSuite.scala

@@ -495,6 +495,48 @@ class InsertInferredMethodSuite extends BaseCodeActionSuite:
          |""".stripMargin,
     )
 
+  @Test def `extension-method` =
+    checkEdit(
+      """|object Main:
+         |  val x = 1
+         |  x.<<incr>>
+         |""".stripMargin,
+      """|object Main:
+         |  val x = 1
+         |  extension (x: Int)
+         |    def incr = ???
+         |  x.incr
+         |""".stripMargin,
+    )
+
+  @Test def `extension-method-1` =
+    checkEdit(
+      """|object Main:
+         |  val x = 1
+         |  x.<<add>>(2)
+         |""".stripMargin,
+      """|object Main:
+         |  val x = 1
+         |  extension (x: Int)
+         |    def add(arg0: Int) = ???
+         |  x.add(2)
+         |""".stripMargin,
+    )
+
+  @Test def `extension-method-2` =
+    checkEdit(
+      """|object Main:
+         |  val s = "hello"
+         |  s.<<shout>>
+         |""".stripMargin,
+      """|object Main:
+         |  val s = "hello"
+         |  extension (x: String)
+         |    def shout = ???
+         |  s.shout
+         |""".stripMargin,
+    )
+
   def checkEdit(
       original: String,
       expected: String