Fix VarianceChecker's relativeVariance

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

@@ -87,6 +87,10 @@ object Formatting {
         def show(x: H *: T) =
           CtxShow(toStr(x.head) *: toShown(x.tail).asInstanceOf[Tuple])
 
+      given Show[typer.VarianceChecker.VarianceError] with
+        def show(x: typer.VarianceChecker.VarianceError) =
+          CtxShow("VarianceError(" + toStr(x.tvar) + ", " + toStr(x.required) + ")")
+
       given Show[FlagSet] with
         def show(x: FlagSet) = x.flagsString
 

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

@@ -73,9 +73,9 @@ class VarianceChecker(using Context) {
 
     /** The variance of a symbol occurrence of `tvar` seen at the level of the definition of `base`.
      *  The search proceeds from `base` to the owner of `tvar`.
-     *  Initially the state is covariant, but it might change along the search.
+     *  Initially the state is the accumulator's variance, but it might change along the search.
      */
-    def relativeVariance(tvar: Symbol, base: Symbol, v: Variance = Covariant): Variance = /*trace(i"relative variance of $tvar wrt $base, so far: $v")*/
+    def relativeVariance(tvar: Symbol, base: Symbol, v: Variance = varianceFromInt(variance)): Variance = /*trace(i"relative variance of $tvar wrt $base, so far: $v")*/
       if base == tvar.owner then
         v
       else if base.is(Param) && base.owner.isTerm && !base.owner.isAllOf(PrivateLocal) then

tests/neg/i21625.alt4.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i21625.alt4.scala:2:11 -----------------------------------------------------------------------------
+2 |  def test[F[_ >: L] >: Unit]: Unit // error
+  |           ^^^^^^^^^^^^^^^^^
+  |           covariant type L occurs in contravariant position in type [_$1 >: L] >: Unit of type F

tests/neg/i21625.alt4.scala

@@ -0,0 +1,3 @@
+trait Foo[+L] {
+  def test[F[_ >: L] >: Unit]: Unit // error
+}

tests/pos/i21625.alt1.scala

@@ -0,0 +1,3 @@
+trait Foo[-L] {
+  def test[F[_ >: L]]: Unit
+}

tests/pos/i21625.alt2.scala

@@ -0,0 +1,3 @@
+trait Foo[-L] {
+  def test[F[_ >: L] >: Unit]: Unit
+}

tests/pos/i21625.alt3.scala

@@ -0,0 +1,3 @@
+trait Foo[-L] {
+  def test[F[_ >: L] >: Nothing]: Unit
+}

tests/pos/i21625.scala

@@ -0,0 +1,3 @@
+trait Foo[-L] {
+  def test[LF, F[_ >: L] >: LF]: Unit
+}