Do not flag match types as Deferred

This already wasn't the case for unpickled match types,
which caused varying results for `ImplicitRunInfo#isAnchor`,
by not reaching the `isMatchAlias` condition.

Ensures both #20071 and #20136 each have the same result,
when compiled with a classpath dependency as when merged.
Note that they both still fail (20071 compiles but shouldn't),
but at least do so consistently.

Also update TreeUnpickler MATCHtpt doc to align with changes from #19871

Co-authored-by: Guillaume Martres <[email protected]>

Author: EugeneFlesselle

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1542,8 +1542,6 @@ class TreeUnpickler(reader: TastyReader,
               // as the reduction of the match type definition!
               //
               // We also override the type, as that's what Typer does.
-              // The difference here is that a match type that reduces to a non-match type
-              // makes the TypeRef for that definition will have a TypeAlias info instead of a MatchAlias.
               tpt.overwriteType(tpt.tpe.normalized)
               tpt
             case TYPEBOUNDStpt =>

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -268,8 +268,8 @@ class Namer { typer: Typer =>
             else if flags.isAllOf(EnumValue) && ctx.owner.isStaticOwner then flags |= JavaStatic
           case tree: TypeDef =>
             def analyzeRHS(rhs: Tree): Unit = rhs match
-              case _: TypeBoundsTree | _: MatchTypeTree =>
-                flags |= Deferred // Typedefs with Match rhs classify as abstract
+              case _: TypeBoundsTree =>
+                flags |= Deferred
               case LambdaTypeTree(_, body) =>
                 analyzeRHS(body)
               case _ =>