Make overload pruning based on result types less aggressive (#21744)

`adaptByResult` was introduced in 2015 in
54835b6 as a last step in overloading
resolution:

> Take expected result type into account more often for overloading
resolution
>
> Previously, the expected result type of a FunProto type was ignored
and taken into
> account only in case of ambiguities. arrayclone-new.scala shows that
this is not enough.
> In a case like
>
>     val x: Array[Byte] = Array(1, 2)
>
> we typed 1, 2 to be Int, so overloading resulution would give the
Array.apply of
> type (Int, Int*)Array[Int]. But that's a dead end, since Array[Int] is
not a subtype
> of Array[Byte].
>
> This commit proposes the following modified rule for overloading
resulution:
>
> A method alternative is applicable if ... (as before), and if its
result type
>   is copmpatible with the expected type of the method application.
>
> The commit does not pre-select alternatives based on comparing with
the expected
> result type. I tried that but it slowed down typechecking by a factor
of at least 4.
> Instead, we proceed as usual, ignoring the result type except in case
of
> ambiguities, but check whether the result of overloading resolution
has a
> compatible result type. If that's not the case, we filter all
alternatives
> for result type compatibility and try again.

In i21410.scala this means we end up checking:

    F[?U] <:< Int
(where ?U is unconstrained, because the check is done without looking at
the
    argument types)

The problem is that the subtype check returning false does not mean that
there is no instantiation of `?U` that would make this check return
true, just that type inference was not able to come up with one. This
could happen for any number of reason but commonly will happen with
match types since inference cannot do much with them.

We cannot avoid this by taking the argument types into account, because
this logic was added precisely to handle cases where the argument types
mislead you because adaptation isn't taken into account. Instead, we can
approximate type variables in the result type to trade false negatives
for false positives which should be less problematic here.

Fixes #21410.

Author: smarter

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

@@ -2118,16 +2118,27 @@ trait Applications extends Compatibility {
   def resolveOverloaded(alts: List[TermRef], pt: Type)(using Context): List[TermRef] =
     record("resolveOverloaded")
 
-    /** Is `alt` a method or polytype whose result type after the first value parameter
+    /** Is `alt` a method or polytype whose approximated result type after the first value parameter
      *  section conforms to the expected type `resultType`? If `resultType`
      *  is a `IgnoredProto`, pick the underlying type instead.
+     *
+     *  Using an approximated result type is necessary to avoid false negatives
+     *  due to incomplete type inference such as in tests/pos/i21410.scala and tests/pos/i21410b.scala.
      */
     def resultConforms(altSym: Symbol, altType: Type, resultType: Type)(using Context): Boolean =
       resultType.revealIgnored match {
         case resultType: ValueType =>
           altType.widen match {
-            case tp: PolyType => resultConforms(altSym, instantiateWithTypeVars(tp), resultType)
-            case tp: MethodType => constrainResult(altSym, tp.resultType, resultType)
+            case tp: PolyType => resultConforms(altSym, tp.resultType, resultType)
+            case tp: MethodType =>
+              val wildRes = wildApprox(tp.resultType)
+
+              class ResultApprox extends AvoidWildcardsMap:
+                // Avoid false negatives by approximating to a lower bound
+                variance = -1
+
+              val approx = ResultApprox()(wildRes)
+              constrainResult(altSym, approx, resultType)
             case _ => true
           }
         case _ => true
@@ -2499,6 +2510,7 @@ trait Applications extends Compatibility {
       if t.exists && alt.symbol.exists then
         val (trimmed, skipped) = trimParamss(t.stripPoly, alt.symbol.rawParamss)
         val mappedSym = alt.symbol.asTerm.copy(info = t)
+        mappedSym.annotations = alt.symbol.annotations
         mappedSym.rawParamss = trimmed
         val (pre, totalSkipped) = mappedAltInfo(alt.symbol) match
           case Some((pre, prevSkipped)) =>

tests/pos/i21410.scala

@@ -0,0 +1,12 @@
+class A
+object Test:
+  type F[X] <: Any = X match
+    case A => Int
+
+  def foo[T](x: String): T = ???
+  def foo[U](x: U): F[U] = ???
+
+  val x1 = foo(A())
+  val y: Int = x1
+
+  val x2: Int = foo(A()) // error

tests/pos/i21410b.scala

@@ -0,0 +1,10 @@
+object Test:
+  def foo[T](x: Option[T]): T = ???
+  def foo[T <: Tuple](x: T): Tuple.Map[T, List] = ???
+
+  val tup: (Int, String) = (1, "")
+
+  val x = foo(tup)
+  val y: (List[Int], List[String]) = x
+
+  val x2: (List[Int], List[String]) = foo(tup) // error

tests/pos/i21410c.scala

@@ -0,0 +1,11 @@
+class AppliedPIso[A, B]()
+case class User(age: Int)
+
+object Test:
+  extension [From, To](from: From)
+    def focus(): AppliedPIso[From, From] = ???
+    transparent inline def focus(inline lambda: (From => To)): Any = ???
+
+
+  val u = User(1)
+  val ap: AppliedPIso[User, User] = u.focus(_.age) // error