Reuse methods for prefix and owner

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

@@ -48,9 +48,12 @@ object TypeApplications {
       || {
         val tparams = fn.typeParams
         val paramRefs = tparams.map(_.paramRef)
-        val prefix = fn match { case fn: TypeRef => fn.prefix       case _ => NoPrefix }
-        val owner  = fn match { case fn: TypeRef => fn.symbol.owner case _ => NoSymbol }
+        val prefix = fn.normalizedPrefix
+        val owner = fn.typeSymbol.maybeOwner
         tp.typeParams.corresponds(tparams) { (param1, param2) =>
+          // see tests/neg/variances-constr.scala
+          // its B parameter should have info <: Any, using class C as the owner
+          // rather than info <: A, using class Inner2 as the owner
           param2.paramInfo.asSeenFrom(prefix, owner) frozen_<:< param1.paramInfo.substParams(tp, paramRefs)
         }
       }
@@ -303,7 +306,7 @@ class TypeApplications(val self: Type) extends AnyVal {
   def etaExpand(using Context): Type =
     val tparams = self.typeParams
     val resType = self.appliedTo(tparams.map(_.paramRef))
-    self match
+    self.dealias match
       case self: TypeRef if tparams.nonEmpty && self.symbol.isClass =>
         val owner = self.symbol.owner
         // Calling asSeenFrom on the type parameter infos is important

tests/neg/i7820.check

@@ -0,0 +1,34 @@
+-- [E046] Cyclic Error: tests/neg/i7820.scala:1:23 ---------------------------------------------------------------------
+1 |trait A1 { type F[X <: F[_, _], Y] }  // error: cyclic reference involving type F
+  |                       ^
+  |                       Cyclic reference involving type F
+  |
+  |                        Run with -explain-cyclic for more details.
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E046] Cyclic Error: tests/neg/i7820.scala:2:23 ---------------------------------------------------------------------
+2 |trait A2 { type F[X <: F, Y] }        // error: cyclic reference involving type F
+  |                       ^
+  |                       Cyclic reference involving type F
+  |
+  |                        Run with -explain-cyclic for more details.
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E046] Cyclic Error: tests/neg/i7820.scala:3:23 ---------------------------------------------------------------------
+3 |trait A3 { type F[X >: F, Y] }        // error: cyclic reference involving type F
+  |                       ^
+  |                       Cyclic reference involving type F
+  |
+  |                        Run with -explain-cyclic for more details.
+  |
+  | longer explanation available when compiling with `-explain`
+-- Warning: tests/neg/i7820.scala:1:25 ---------------------------------------------------------------------------------
+1 |trait A1 { type F[X <: F[_, _], Y] }  // error: cyclic reference involving type F
+  |                         ^
+  |                         `_` is deprecated for wildcard arguments of types: use `?` instead
+  |                         This construct can be rewritten automatically under -rewrite -source 3.4-migration.
+-- Warning: tests/neg/i7820.scala:1:28 ---------------------------------------------------------------------------------
+1 |trait A1 { type F[X <: F[_, _], Y] }  // error: cyclic reference involving type F
+  |                            ^
+  |                            `_` is deprecated for wildcard arguments of types: use `?` instead
+  |                            This construct can be rewritten automatically under -rewrite -source 3.4-migration.

tests/neg/i7820.scala

@@ -1,3 +1,3 @@
 trait A1 { type F[X <: F[_, _], Y] }  // error: cyclic reference involving type F
-trait A2 { type F[X <: F, Y] }        // error: cyclic reference involving type F // error
-trait A3 { type F[X >: F, Y] }        // error: cyclic reference involving type F // error
+trait A2 { type F[X <: F, Y] }        // error: cyclic reference involving type F
+trait A3 { type F[X >: F, Y] }        // error: cyclic reference involving type F