Make explicit arguments for context bounds an error from 3.5

Author: odersky

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

@@ -189,4 +189,5 @@ class ReTyper(nestingLevel: Int = 0) extends Typer(nestingLevel) with ReChecking
   override protected def checkEqualityEvidence(tree: tpd.Tree, pt: Type)(using Context): Unit = ()
   override protected def matchingApply(methType: MethodOrPoly, pt: FunProto)(using Context): Boolean = true
   override protected def typedScala2MacroBody(call: untpd.Tree)(using Context): Tree = promote(call)
+  override protected def migrate[T](migration: => T, disabled: => T = ()): T = disabled
 }

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

@@ -157,6 +157,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   // Overridden in derived typers
   def newLikeThis(nestingLevel: Int): Typer = new Typer(nestingLevel)
 
+  // Overridden to do nothing in derived typers
+  protected def migrate[T](migration: => T, disabled: => T = ()): T = migration
+
   /** Find the type of an identifier with given `name` in given context `ctx`.
    *   @param name       the name of the identifier
    *   @param pt         the expected type

tests/warn/context-bounds-migration.scala

@@ -0,0 +1,9 @@
+
+class C[T]
+def foo[X: C] = ()
+
+given [T]: C[T] = C[T]()
+
+def Test =
+  foo(C[Int]())  // warning
+  foo(using C[Int]()) // ok