Fix #5191: Don't lift the receiver of a call to a method with varargs

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

@@ -228,9 +228,23 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
 
     /** Whether `liftFun` is needed? It is the case if default arguments are used.
      */
-    protected def needLiftFun: Boolean =
+    protected def needLiftFun: Boolean = {
+      def requiredArgNum(tp: Type): Int = tp.widen match {
+        case funType: MethodType =>
+          val paramInfos = funType.paramInfos
+          val argsNum = paramInfos.size
+          if (argsNum >= 1 && paramInfos.last.isRepeatedParam)
+            // Repeated arguments do not count as required arguments
+            argsNum - 1
+          else
+            argsNum
+        case funType: PolyType => requiredArgNum(funType.resultType)
+        case tp => args.size
+      }
+
       !isJavaAnnotConstr(methRef.symbol) &&
-      args.size < reqiredArgNum(funType)
+      args.size < requiredArgNum(funType)
+    }
 
     /** A flag signalling that the typechecking the application was so far successful */
     private[this] var _ok = true
@@ -250,12 +264,6 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
       case tp => tp //was: funType
     }
 
-    def reqiredArgNum(tp: Type): Int = tp.widen match {
-      case funType: MethodType => funType.paramInfos.size
-      case funType: PolyType => reqiredArgNum(funType.resultType)
-      case tp => args.size
-    }
-
     lazy val liftedFunType =
       if (needLiftFun) {
         liftFun()

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -475,4 +475,29 @@ class TestBCode extends DottyBytecodeTest {
         diffInstructions(testInstructions, refInstructions))
     }
   }
+
+  /** Test that the receiver of a call to a method with varargs is not unnecessarily lifted */
+  @Test def i5191 = {
+    val source =
+      """class Test {
+        |  def foo(args: String*): String = ""
+        |  def self = this
+        |
+        |  def test = self.foo()
+        |}
+      """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn   = dir.lookupName("Test.class", directory = false).input
+      val clsNode = loadClassNode(clsIn)
+      val method  = getMethod(clsNode, "test")
+
+      val liftReceiver = instructionsFromMethod(method).exists {
+        case VarOp(Opcodes.ASTORE, _) => true // receiver lifted in local val
+        case _ => false
+      }
+      assertFalse("Receiver of a call to a method with varargs is unnecessarily lifted",
+        liftReceiver)
+    }
+  }
 }