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
-caseNone=>Nil
+caseNone=>
+ extensionMethodEdits(signature, container)
+
+defextensionMethodEdits(signature: String, container: Tree):List[TextEdit] =
+valcontainerTypeStr= printType(container.tpe.widenDealias)
+
+valpos= insertPosition()
+valindent= indentation(params.text(), pos.start -1)
+valextensionSignature=s"extension (x: $containerTypeStr)\n$indent$signature"
+
+vallspPos= pos.toLsp
+ lspPos.setEnd(lspPos.getStart())
+
+List(
+TextEdit(
+ lspPos,
+s"$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,
  )
 
+@Testdef`extension-method`=
+ checkEdit(
+"""|object Main:
+ | val x = 1
+ | x.<<incr>>
+ |""".stripMargin,
+"""|object Main:
+ | val x = 1
+ | extension (x: Int)
+ | def incr = ???
+ | x.incr
+ |""".stripMargin,
+ )
+
+@Testdef`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,
+ )
+
+@Testdef`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,
+ )
+
 defcheckEdit(
 original: String,
 expected: String