Fix #10259: Don't complain about old style conversion for extension method

Author: prolativ

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

@@ -792,8 +792,8 @@ trait Implicits:
         val inferred = inferImplicit(adjust(to), from, from.span)
 
         inferred match {
-          case SearchSuccess(_, ref, _) =>
-            if isOldStyleFunctionConversion(ref.underlying) then
+          case SearchSuccess(tree, ref, _)
+            if isOldStyleFunctionConversion(ref.underlying) && !tree.isInstanceOf[Applications.ExtMethodApply] =>
               report.migrationWarning(
                 i"The conversion ${ref} will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.",
                 from

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -39,6 +39,7 @@ class CompilationTests {
       compileFilesInDir("tests/new", defaultOptions),
       compileFilesInDir("tests/pos-scala2", scala2CompatMode),
       compileFilesInDir("tests/pos-custom-args/erased", defaultOptions.and("-Yerased-terms")),
+      compileFilesInDir("tests/pos-custom-args/fatal-warnings", defaultOptions.and("-Xfatal-warnings")),
       compileFilesInDir("tests/pos", defaultOptions),
       compileFilesInDir("tests/pos-deep-subtype", allowDeepSubtypes),
       compileFile(

tests/pos-custom-args/fatal-warnings/i10259.scala

@@ -0,0 +1,8 @@
+trait S[T] extends (T => T):
+  def apply(x: T) = ???
+  extension (x: T) def show: String
+
+given S[Int] with
+  extension (x: Int) def show = x.toString
+
+val x = 10.show