Type infer issue

Compiler version

Tested with 3.0.2 and 3.1.0-RC2

Minimized code

https://github.com/vigoo/scala3-type-infer-bug

Contains a minimal example (the issue originally came up in a parser library under development) and two files from ZIO, Zippable and Unzippable as the issue seems to be related to these specific implicits.

Output

[error] 26 |    val _ = assertSameType(xy2)
[error]    |                           ^^^
[error]    |                      Found:    (Main.xy2 : Example[Int, (AnyVal, Int)])
[error]    |                      Required: Example[T, T]
[error]    |
[error]    |                      where:    T is a type variable

Expectation

The

 val x = pure(1)
 val y = pure(2)
 val xy1 = x zip y

and

val xy2 = pure(1) zip pure(2)

should both be inferred to the same result, Example[(Int, Int), (Int, Int)].

[smarter]

In fact, taking a closer look at https://github.com/lampepfl/dotty/issues/13646#issuecomment-933551826, we do have I qualifying to be instantiated in the zip example, but it doesn’t get instantiated because of another heuristic: https://github.com/lampepfl/dotty/blob/1ed25ce458d36e773f732267975fbddb0a0be26c/compiler/src/dotty/tools/dotc/typer/Inferencing.scala#L184 which relies on: https://github.com/lampepfl/dotty/blob/1ed25ce458d36e773f732267975fbddb0a0be26c/compiler/src/dotty/tools/dotc/core/Types.scala#L4625-L4630

But I does have a more precise upper-bound than Any in fact, it’s upper-bounded by a type variable A coming from the pure method. hasNonWildcardUpperBound returns false anyway because it relies on currentEntry which only returns the non-variable bounds, if we instead do something like:

diff --git compiler/src/dotty/tools/dotc/core/Types.scala compiler/src/dotty/tools/dotc/core/Types.scala
index 1d36efcb2a0..ce84ad92840 100644
--- compiler/src/dotty/tools/dotc/core/Types.scala
+++ compiler/src/dotty/tools/dotc/core/Types.scala
@@ -4626,7 +4626,7 @@ object Types {
      *  does it not contain wildcard types?
      */
     def hasNonWildcardUpperBound(using Context): Boolean =
-      val hi = currentEntry.hiBound
+      val hi = TypeComparer.fullUpperBound(origin).orElse(currentEntry.hiBound)
       !hi.isRef(defn.AnyClass) && !hi.containsWildcardTypes

     /** Unwrap to instance (if instantiated) or origin (if not), until result

… then the test case pass. So, probably something worth digging into more.

[SethTisue]

Could someone suggest a better title for the ticket, to make it easier to find in the future?