Move ExpandSAMs to miniphase group before HoistSuperArgs

There are interactions for treating partial functions that require
a split into different groups.

See pos/i9391.scala for a test case.

Also: maintain Extension flags for forwarders. This causes an error
in RefChecks for run/fragables-extension.scala otherwise. Previously
the error was not detected since methods created in the same group
as RefChecks are not checked.

Author: odersky

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -61,9 +61,9 @@ class Compiler {
          new CookComments,           // Cook the comments: expand variables, doc, etc.
          new CheckStatic,            // Check restrictions that apply to @static members
          new BetaReduce,             // Reduce closure applications
+         new ExpandSAMs,             // Expand single abstract method closures to anonymous classes
          new init.Checker) ::        // Check initialization of objects
     List(new ElimRepeated,           // Rewrite vararg parameters and arguments
-         new ExpandSAMs,             // Expand single abstract method closures to anonymous classes
          new ProtectedAccessors,     // Add accessors for protected members
          new ExtensionMethods,       // Expand methods of value classes with extension methods
          new UncacheGivenAliases,    // Avoid caching RHS of simple parameterless given aliases

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -349,7 +349,8 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
         coord = fns.map(_.span).reduceLeft(_ union _))
     val constr = newConstructor(cls, Synthetic, Nil, Nil).entered
     def forwarder(fn: TermSymbol, name: TermName) = {
-      val fwdMeth = fn.copy(cls, name, Synthetic | Method | Final).entered.asTerm
+      val maybeExtension = fn.flags & Extension
+      val fwdMeth = fn.copy(cls, name, Synthetic | Method | Final | maybeExtension).entered.asTerm
       if (fwdMeth.allOverriddenSymbols.exists(!_.is(Deferred))) fwdMeth.setFlag(Override)
       polyDefDef(fwdMeth, tprefs => prefss => ref(fn).appliedToTypes(tprefs).appliedToArgss(prefss))
     }

compiler/src/dotty/tools/dotc/transform/HoistSuperArgs.scala

@@ -45,6 +45,10 @@ class HoistSuperArgs extends MiniPhase with IdentityDenotTransformer { thisPhase
 
   def phaseName: String = HoistSuperArgs.name
 
+  override def runsAfterGroupsOf: Set[String] = Set(ExpandSAMs.name)
+    // ExpanSAMs applied to partial functions creates methods that need
+    // to be fully defined before hoisting. Test case is pos/i9391.scala.
+
   override def runsAfter: Set[String] = Set(ByNameClosures.name)
     // By name closures need to be introduced first in order to be hoisted out here.
     // There's an interaction with by name closures in that the <cbn-arg> marker

tests/pos/i9391.scala

@@ -0,0 +1,3 @@
+def g(x: => Any): Any = x
+
+val a: PartialFunction[Any => Any, Any] = (f => g(f(0)) match { case v => v })    // was an error, now OK