Fix scala#8311: Propagate more information about extension method arguments

Author: odersky

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

@@ -3251,12 +3251,21 @@ class Typer extends Namer
         case IgnoredProto(_: FunOrPolyProto) => false
         case _ => true
       }
+      // If the expected type is a selection of an extension method, deepen it
+      // to also propagate the argument type (which is the receiver we have
+      // typechecked already). This is needed for i8311.scala. Doing so
+      // for all expected types does not work since it would block the case
+      // where we have an argument that must be converted with another
+      // implicit conversion to the receiver type.
+      def sharpenedPt = pt match
+        case pt: SelectionProto if pt.name.isExtensionName => pt.deepenProto
+        case _ => pt
       var resMatch: Boolean = false
       wtp match {
         case wtp: ExprType =>
           readaptSimplified(tree.withType(wtp.resultType))
         case wtp: MethodType if wtp.isImplicitMethod &&
-          ({ resMatch = constrainResult(tree.symbol, wtp, pt); resMatch } || !functionExpected) =>
+          ({ resMatch = constrainResult(tree.symbol, wtp, sharpenedPt); resMatch } || !functionExpected) =>
           if (resMatch || ctx.mode.is(Mode.ImplicitsEnabled))
             adaptNoArgsImplicitMethod(wtp)
           else

tests/pos/i8311.scala

@@ -0,0 +1,19 @@
+
+trait Show[O]:
+  extension (o: O)
+    def show: String
+
+class Box[A]
+class Foo
+
+object test:
+
+  given box[A](using Show[A]) as Show[Box[A]] = _.toString
+  given foo as Show[Foo] = _.toString
+
+  def run(s: Box[Box[Foo]]): Unit =
+    val x = summon[Show[Box[Box[Foo]]]]
+    x.extension_show(s)
+    val r: String = box.extension_show(s)
+    println(s"step: ${box[Box[Foo]].extension_show(s)}")
+    println(s"step: ${s.show}")

tests/pos/reference/extension-methods.scala

@@ -104,7 +104,7 @@ object ExtMethods:
     summon[Ord[Lst[Lst[Int]]]]
 
     assert(Lst.ord[Lst[Int]].extension_less(xss)(Lst(Lst(3))))
-    // fails type inference: assert(xss `less` Lst(Lst(3)))
+    assert(xss `less` Lst(Lst(3)))
     assert(xss.flatten `less` Lst(3))
 
   extension (s: String)