Ignore selection prototypes when typing type applications

Ignore selection prototypes at first when typing type applications.
If we need them later for overloading disambiguation, reveal the ignored
type.

The reason for doing this is that a selection might come from an extension method,
and in this case we should not require the selected name as a member of the result.

This change breaks one test (overloading-specifity-2.scala) that explicitly
tested that we don't consult implicit arguments for disambiguation since the
expected type was a selection that already determined the outcome. This is logic
no longer holds. We have to see whether this change breaks any code in practice.

Fixes scala#23773

[Cherry-picked 30aa805][modified]

Author: tgodzik

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1859,6 +1859,11 @@ object Types extends TypeUtils {
     /** If this is a proto type, WildcardType, otherwise the type itself */
     def dropIfProto: Type = this
 
+    /** If this is a (possibly applied) selection proto type, ignore the
+     *  selection part
+     */
+    def ignoreSelectionProto(using Context): Type = this
+
     /** If this is an AndType, the number of factors, 1 for all other types */
     def andFactorCount: Int = 1
 

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -306,9 +306,13 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         ~ toText(resultType)
         ~ "]"
       case IgnoredProto(ignored) =>
-        "?" ~ (("(ignored: " ~ toText(ignored) ~ ")") provided printDebug)
-      case tp @ PolyProto(targs, resType) =>
-        "[applied to [" ~ toTextGlobal(targs, ", ") ~ "] returning " ~ toText(resType)
+        "?" ~ ("(ignored: " ~ toText(ignored) ~ ")").provided(printDebug)
+      case tp @ PolyProto(targs, resultType) =>
+        "[applied to ["
+        ~ toTextGlobal(targs, ", ")
+        ~ "] returning "
+        ~ toText(resultType)
+        ~ "]"
       case _ =>
         super.toText(tp)
     }

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

@@ -1248,7 +1248,7 @@ trait Applications extends Compatibility {
     val isNamed = hasNamedArg(tree.args)
     val typedArgs = if (isNamed) typedNamedArgs(tree.args) else tree.args.mapconserve(typedType(_))
     record("typedTypeApply")
-    typedExpr(tree.fun, PolyProto(typedArgs, pt)) match {
+    typedExpr(tree.fun, PolyProto(typedArgs, pt.ignoreSelectionProto)) match {
       case fun: TypeApply if !ctx.isAfterTyper =>
         val function = fun.fun
         val args = (fun.args ++ tree.args).map(_.show).mkString(", ")

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

@@ -252,6 +252,9 @@ object ProtoTypes {
     override def deepenProtoTrans(using Context): SelectionProto =
       derivedSelectionProto(name, memberProto.deepenProtoTrans, compat, nameSpan)
 
+    override def ignoreSelectionProto(using Context): IgnoredProto =
+      IgnoredProto(this)
+
     override def computeHash(bs: Hashable.Binders): Int = {
       val delta = (if (compat eq NoViewsAllowed) 1 else 0) | (if (privateOK) 2 else 0)
       addDelta(doHash(bs, name, memberProto), delta)
@@ -584,6 +587,9 @@ object ProtoTypes {
     override def deepenProtoTrans(using Context): FunProto =
       derivedFunProto(args, resultType.deepenProtoTrans, constrainResultDeep = true)
 
+    override def ignoreSelectionProto(using Context): FunProto =
+      derivedFunProto(args, resultType.ignoreSelectionProto)
+
     override def withContext(newCtx: Context): ProtoType =
       if newCtx `eq` protoCtx then this
       else new FunProto(args, resType)(typer, applyKind, state)(using newCtx)
@@ -694,6 +700,9 @@ object ProtoTypes {
 
     override def deepenProtoTrans(using Context): PolyProto =
       derivedPolyProto(targs, resultType.deepenProtoTrans)
+
+    override def ignoreSelectionProto(using Context): PolyProto =
+      derivedPolyProto(targs, resultType.ignoreSelectionProto)
   }
 
   /** A prototype for expressions [] that are known to be functions:

tests/run/overloading-specifity-2.scala tests/neg/overloading-specifity-2.scalatests/run/overloading-specifity-2.scala renamed to tests/neg/overloading-specifity-2.scala

@@ -23,5 +23,5 @@ object Test extends App {
     def foo[T]: Show[T] = new Show[T](2)
   }
 
-  assert(a.foo[Int].i == 1) // error: no implicit argument of type Test.Context was found for parameter ctx
+  assert(a.foo[Int].i == 1) // error: no implicit argument of type Test.Context was found for parameter ctx, was OK
 }

tests/pos/i23773.scala

@@ -0,0 +1,10 @@
+trait Foo[T]
+
+def foo[A]: Int = ???
+def foo[A: Foo]: Int = ???
+
+extension (x: Int)
+  def succ: Int = x + 1
+
+val a = foo[Int]
+val b = foo[Int].succ // error