Better comparisons for type projections (#17092)

We already implemented in essence the rule suggested in
lampepfl/dotty-feature-requests#14:

```
Γ ⊨ p : T
------------------------ (Sel-<:-Proj)
Γ ⊨ p.A <: T#A
```

This rule is implemented in `isSubPrefix`. But we did not get to check
that since we concluded prematurely that an alias type would never
match. The alias type in question in i17064.scala was

```scala
Outer#Inner
```
Since `Outer` is not a path, the asSeenFrom to recover the info of
`Outer#Inner` got approximated with a `Nothing` argument and therefore
the alias failed. It is important in this case that we could still
succeed with a `isSubPrefix` test, which comes later. So we should not
return `false` when the prefix is not a singleton.

Fixes #17064

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

@@ -283,17 +283,28 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           val ctx = comparerContext
           given Context = ctx // optimization for performance
           val info2 = tp2.info
+
+          /** Does `tp2` have a stable prefix?
+           *  If that's not the case, following an alias via asSeenFrom could be lossy
+           *  so we should not conclude `false` if comparing aliases fails.
+           *  See pos/i17064.scala for a test case
+           */
+          def hasStablePrefix(tp: NamedType) =
+            tp.prefix.isStable
+
           info2 match
             case info2: TypeAlias =>
               if recur(tp1, info2.alias) then return true
-              if tp2.asInstanceOf[TypeRef].canDropAlias then return false
+              if tp2.asInstanceOf[TypeRef].canDropAlias && hasStablePrefix(tp2) then
+                return false
             case _ =>
           tp1 match
             case tp1: NamedType =>
               tp1.info match {
                 case info1: TypeAlias =>
                   if recur(info1.alias, tp2) then return true
-                  if tp1.asInstanceOf[TypeRef].canDropAlias then return false
+                  if tp1.asInstanceOf[TypeRef].canDropAlias && hasStablePrefix(tp2) then
+                    return false
                 case _ =>
               }
               val sym2 = tp2.symbol
@@ -302,7 +313,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                 // For convenience we want X$ <:< X.type
                 // This is safe because X$ self-type is X.type
                 sym1 = sym1.companionModule
-              if ((sym1 ne NoSymbol) && (sym1 eq sym2))
+              if (sym1 ne NoSymbol) && (sym1 eq sym2) then
                 ctx.erasedTypes ||
                 sym1.isStaticOwner ||
                 isSubPrefix(tp1.prefix, tp2.prefix) ||

tests/neg/i15525.scala

@@ -37,7 +37,7 @@ def element22(
       transmittable20.Type / transmittable21.Type
   } = ???
 
-def test22 = // error
+def test22 =
   Resolution(
     element22(
       Resolution(element0), Resolution(element0), // error // error

tests/pos/i17064.scala

@@ -0,0 +1,10 @@
+class HiddenInner[+O<:Outer](val outer:O){
+}
+
+class Outer{
+  type Inner = HiddenInner[this.type]
+}
+
+val o : Outer       = new Outer
+def a : o.Inner     = new o.Inner(o)
+val b : Outer#Inner = a // DOES NOT COMPILE