Minimise and fix upstream ZIO's problem

Author: dwijnand

community-build/community-projects/zio

@@ -0,0 +1,29 @@
+// Minimised from zio's ZLayer ++
+class Cov[+W]:
+  def add[X >: W, Y](y: Cov[Y]): Cov[X & Y] = ???
+  def pre[Y >: W, X](x: Cov[X]): Cov[X & Y] = ???
+
+class Test:
+  def a1[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & B & C]       = a.add(b).add(c)
+  def a2[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A with B with C] = a.add(b).add(c) // error
+
+  def b1[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = a.add(b).add(c) // error (a2)
+  def b2[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = a.add(b).add(c)
+  def b3[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = a.add(b.add(c))
+  def b4[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = a.add(b.add(c))
+
+
+  def c3[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & B & C]       = a.pre(b).pre(c)
+  def c4[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A with B with C] = a.pre(b).pre(c) // error
+
+  def d1[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = a.pre(b).pre(c) // error (c4)
+  def d2[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = a.pre(b).pre(c)
+  def d3[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = a.pre(b.pre(c))
+  def d4[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = a.pre(b.pre(c))
+
+
+  def add[X, Y](x: Cov[X], y: Cov[Y]): Cov[X & Y] = ???
+  def e1[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = add(add(a, b), c) // alt assoc: ok!
+  def e2[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = add(add(a, b), c) // reg assoc: ok
+  def e3[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[A & (B & C)] = add(a, add(b, c)) // reg assoc: ok
+  def e4[A, B, C](a: Cov[A], b: Cov[B], c: Cov[C]): Cov[(A & B) & C] = add(a, add(b, c)) // alt assoc: ok!

tests/neg/i18453.zio.scala

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+trait Box[T]
+
+trait Foo
+
+class Test:
+  def meth[A](func: A => A & Foo)(using boxA: Box[A]): Unit = ???
+  def test[B]                    (using boxB: Box[B]): Unit =
+    def nest(p: B): B & Foo = ???
+    meth(nest)

tests/pos/i18453.min.scala

@@ -0,0 +1,9 @@
+trait Foo[A]
+trait Bar[B] extends Foo[B]
+
+class Test[C]:
+  def put[X >: Bar[C]](fn: X => Unit, x1: X): Unit = ()
+  def id(foo: Foo[C]): Foo[C] = foo
+
+  def t1(foo2: Foo[C]): Unit =
+    put(id, foo2) // was: error: exp: Bar[C], got (foo2 : Foo[C])