Tune usage of resultConforms to prefer the chosen alternative

`resultConforms` should only make us change our mind when we're sure that the
chosen alternative does not fit the result type but another alternative would.
The problem is that due to approximations and adaptation, we can guess wrong on
both counts.

In i22713 in particular, we guessed that the chosen alternative does not
work (but it does because of Unit insertion) and that another alternative would,
but it doesn't because `resultConforms` prefers false positives to false
negatives (the result type is under-approximated to `Nothing` which is
indeed a subtype of `Unit`).

We only really need to prefer false positives to false negatives when
considering whether to discard the currently chosen alternative (because there's
probably a good reason we chose it in the first place), so this commit
introduces a parameter to `resultConforms` to control this.

Fixes #22713.

Author: smarter

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

@@ -2125,7 +2125,7 @@ trait Applications extends Compatibility {
      *  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 =
+    def resultConforms(altSym: Symbol, altType: Type, resultType: Type, avoidFalseNegatives: Boolean = false)(using Context): Boolean =
       resultType.revealIgnored match {
         case resultType: ValueType =>
           altType.widen match {
@@ -2134,8 +2134,9 @@ trait Applications extends Compatibility {
               val wildRes = wildApprox(tp.resultType)
 
               class ResultApprox extends AvoidWildcardsMap:
-                // Avoid false negatives by approximating to a lower bound
-                variance = -1
+                if avoidFalseNegatives then
+                  // Prefer false negatives to false positives by approximating to a lower bound
+                  variance = -1
 
               val approx = ResultApprox()(wildRes)
               constrainResult(altSym, approx, resultType)
@@ -2157,7 +2158,7 @@ trait Applications extends Compatibility {
      *  do they prune much, on average.
      */
     def adaptByResult(chosen: TermRef, alts: List[TermRef]) = pt match {
-      case pt: FunProto if !explore(resultConforms(chosen.symbol, chosen, pt.resultType)) =>
+      case pt: FunProto if !explore(resultConforms(chosen.symbol, chosen, pt.resultType, avoidFalseNegatives = true)) =>
         val conformingAlts = alts.filterConserve(alt =>
           (alt ne chosen) && explore(resultConforms(alt.symbol, alt, pt.resultType)))
         conformingAlts match {

tests/pos/i22713.scala

@@ -0,0 +1,14 @@
+trait Invariant[T]
+
+trait Foo
+trait Bar[T]
+
+trait Worker:
+  def schedule(command: Foo): Invariant[?]
+  def schedule[V](callable: Bar[V]): Invariant[V]
+
+object Test:
+  def test(worker: Worker, foo: Foo): Option[Unit] =
+    Some(worker.schedule(foo))
+    // error: Found: Foo
+    //        Required: Bar[V]