Interpolate only variables at negative polarity

Interpolating covariant variables risks restricting capture sets to early.
For instance, when a variable has the capture set of a called function in
its capture set. When we have indirectly recursive calls it could be that
the capture set of a called function is not yet fully formed.

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -294,8 +294,7 @@ object CaptureSet:
       }
 
     def solve(variance: Int)(using Context): Unit =
-      if variance > 0 then isSolved = true
-      else if variance < 0 then
+      if variance < 0 then
         val approx = upperApprox
         if approx.isConst then
           elems = approx.elems

tests/neg-custom-args/captures/boxmap.check

@@ -1,7 +1,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/boxmap.scala:14:2 ----------------------------------------
 14 |  () => b[Box[B]]((x: A) => box(f(x)))  // error
    |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |  Found:    {f} () => Box[B]
+   |  Found:    {f} () => ? Box[B]
    |  Required: () => Box[B]
 
 longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/capt1.check

@@ -1,14 +1,14 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/capt1.scala:3:2 ------------------------------------------
 3 |  () => if x == null then y else y  // error
   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-  |  Found:    {x} () => C
+  |  Found:    {x} () => ? C
   |  Required: () => C
 
 longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/capt1.scala:6:2 ------------------------------------------
 6 |  () => if x == null then y else y  // error
   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-  |  Found:    {x} () => C
+  |  Found:    {x} () => ? C
   |  Required: Matchable
 
 longer explanation available when compiling with `-explain`
@@ -40,7 +40,7 @@ longer explanation available when compiling with `-explain`
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/capt1.scala:31:24 ----------------------------------------
 31 |  val z2 = h[() => Cap](() => x)(() => C())  // error
    |                        ^^^^^^^
-   |                        Found:    {x} () => Cap
+   |                        Found:    {x} () => ? Cap
    |                        Required: () => Cap
 
 longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/try.check

@@ -1,7 +1,7 @@
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/try.scala:28:43 ------------------------------------------
 28 |  val b = handle[Exception, () => Nothing] {  // error
    |                                           ^
-   |                                           Found:    ? (x: CanThrow[Exception]) => {x} () => Nothing
+   |                                           Found:    ? (x: CanThrow[Exception]) => {x} () => ? Nothing
    |                                           Required: CanThrow[Exception] => () => Nothing
 29 |    (x: CanThrow[Exception]) => () => raise(new Exception)(using x)
 30 |  } {