Add test for multiline class def

Author: ghostbuster91

compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -22,6 +22,7 @@ import config.Printers.refcheck
 import reporting.*
 import Constants.Constant
 import cc.{stripCapturing, isUpdateMethod, CCState}
+import dotty.tools.dotc.util.Chars.{isLineBreakChar, isWhitespace}
 
 object RefChecks {
   import tpd.*
@@ -1278,7 +1279,7 @@ object RefChecks {
     val classHasBraces = classText.contains("{") && classText.contains("}")
 
     // Indentation for inserted methods
-    val lineStart = content.lastIndexWhere(c => c == '\n', end = span.end - 1) + 1
+    val lineStart = content.lastIndexWhere(isLineBreakChar, end = span.end - 1) + 1
     val baseIndent = new String(content.slice(lineStart, span.end).takeWhile(c => c == ' ' || c == '\t'))
     val indent = baseIndent + "  "
 
@@ -1295,7 +1296,7 @@ object RefChecks {
       val braceEnd   = classText.lastIndexOf('}')
       val bodyBetweenBraces = classText.slice(braceStart + 1, braceEnd)
       val bodyIsEmpty = bodyBetweenBraces.forall(_.isWhitespace)
-      val bodyContainsNewLine = bodyBetweenBraces.contains(System.lineSeparator())
+      val bodyContainsNewLine = bodyBetweenBraces.exists(isLineBreakChar)
 
       val prefix = if (bodyContainsNewLine) "" else System.lineSeparator()
       val patchText =

compiler/test/dotty/tools/dotc/reporting/CodeActionTest.scala

@@ -291,6 +291,36 @@ class CodeActionTest extends DottyTest:
           afterPhase= "erasure"
       )
 
+  @Test def insertMissingMethodsIntoMultilineClassDefinition =
+    checkCodeAction(
+      code =
+        """trait Animal {
+          |  def name: String
+          |}
+          |trait Car {
+          |  def wheels: Int
+          |}
+          |class Dog extends Animal 
+          |               with Car {
+          |}
+          |""".stripMargin,
+         title = "Add missing methods",
+      expected =
+        """trait Animal {
+          |  def name: String
+          |}
+          |trait Car {
+          |  def wheels: Int
+          |}
+          |class Dog extends Animal 
+          |               with Car {
+          |  def name: String = ???
+          |  def wheels: Int = ???
+          |}
+          |""".stripMargin,
+          afterPhase= "erasure"
+      )
+
   // Make sure we're not using the default reporter, which is the ConsoleReporter,
   // meaning they will get reported in the test run and that's it.
   private def newContext =