Fix #5980: Distribute type parameters of covariant type constructors

When performing subtyping comparisons of the `F[A] & F[B] <:< C` kind,
if `F` is a type parameter (e.g. as in `trait Foo[F[+_]]`), the compiler
decides the relationship to be true if either `F[A] <: C` or `F[B] <: C`.
However, in case of covariant `F[+_]`, it is possible to distribute the
type arguments when performing the `&` operation: `F[A] & F[B] ~> F[A & B]`.
Hence there is one more case to be considered when checking `<:<`
relationship. This commit adds such a check to the `<:<` logic. Now,
`F[A] & F[B] <:< C` is true (for covariant `F`) if either of the following
is true:
- F[A] <: C
- F[B] <: C
- F[A & B] <: C

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -738,7 +738,9 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
             return recur(AndType(tp11, tp121), tp2) && recur(AndType(tp11, tp122), tp2)
           case _ =>
         }
-        either(recur(tp11, tp2), recur(tp12, tp2))
+        val tp1norm = simplifyAndTypeWithFallback(tp11, tp12, tp1)
+        if (tp1 ne tp1norm) recur(tp1norm, tp2)
+        else either(recur(tp11, tp2), recur(tp12, tp2))
       case tp1: MatchType =>
         def compareMatch = tp2 match {
           case tp2: MatchType =>
@@ -1641,6 +1643,18 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
         NoType
     }
 
+  private[this] def andTypeGen(tp1: Type, tp2: Type, op: (Type, Type) => Type,
+      original: (Type, Type) => Type = _ & _, isErased: Boolean = ctx.erasedTypes): Type = trace(s"glb(${tp1.show}, ${tp2.show})", subtyping, show = true) {
+    val t1 = distributeAnd(tp1, tp2)
+    if (t1.exists) t1
+    else {
+      val t2 = distributeAnd(tp2, tp1)
+      if (t2.exists) t2
+      else if (isErased) erasedGlb(tp1, tp2, isJava = false)
+      else liftIfHK(tp1, tp2, op, original)
+    }
+  }
+
   /** Form a normalized conjunction of two types.
    *  Note: For certain types, `&` is distributed inside the type. This holds for
    *  all types which are not value types (e.g. TypeBounds, ClassInfo,
@@ -1659,16 +1673,11 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
    *
    *  In these cases, a MergeError is thrown.
    */
-  final def andType(tp1: Type, tp2: Type, isErased: Boolean = ctx.erasedTypes): Type = trace(s"glb(${tp1.show}, ${tp2.show})", subtyping, show = true) {
-    val t1 = distributeAnd(tp1, tp2)
-    if (t1.exists) t1
-    else {
-      val t2 = distributeAnd(tp2, tp1)
-      if (t2.exists) t2
-      else if (isErased) erasedGlb(tp1, tp2, isJava = false)
-      else liftIfHK(tp1, tp2, AndType(_, _), _ & _)
-    }
-  }
+  final def andType(tp1: Type, tp2: Type, isErased: Boolean = ctx.erasedTypes): Type =
+    andTypeGen(tp1, tp2, AndType(_, _), isErased = isErased)
+
+  final def simplifyAndTypeWithFallback(tp1: Type, tp2: Type, fallback: Type): Type =
+    andTypeGen(tp1, tp2, (_, _) => fallback)
 
   /** Form a normalized conjunction of two types.
    *  Note: For certain types, `|` is distributed inside the type. This holds for

tests/pos/i5980.scala

@@ -0,0 +1,34 @@
+trait A
+trait B
+
+trait Covariant[F[+_]] {
+  trait G[+X]
+
+  def fx: F[A  &   B] = fy
+  def fy: F[A] & F[B] = fx
+
+  def gx: G[A  &   B] = gy
+  def gy: G[A] & G[B] = gx
+}
+
+trait Contravariant[F[-_]] {
+  trait G[-X]
+
+  def fx: F[A  |   B] = fy
+  def fy: F[A] & F[B] = fx
+
+  def gx: G[A  |   B] = gy
+  def gy: G[A] & G[B] = gx
+}
+
+trait LiskovViolation[F[+_]] {
+  trait A { def children: F[A] }
+  trait B { def children: F[B] }
+  trait C extends A with B { def children: F[A] & F[B] = ??? }
+
+  def fc1: C     = new C {}
+  def fc2: A & B = fc1
+
+  def fy1: F[A & B] = fc1.children
+  def fy2: F[A & B] = fc2.children
+}

tests/pos/infinite-loop-potential.scala

@@ -0,0 +1,10 @@
+object InfiniteSubtypingLoopPossibility {
+  trait A[X]
+  trait B extends A[B]
+  trait Min[+S <: B with A[S]]
+
+  def c: Any = ???
+  c match {
+    case pc: Min[_] =>
+  }
+}