Backport "Fix retained flags in exports" to LTS (#20912)

Backports #19636 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

Diff for: compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -532,8 +532,15 @@ object Flags {
   /** Flags that can apply to a module class */
   val RetainedModuleClassFlags: FlagSet = RetainedModuleValAndClassFlags | Enum
 
-  /** Flags retained in export forwarders */
-  val RetainedExportFlags = Given | Implicit | Inline | Transparent
+  /** Flags retained in term export forwarders */
+  val RetainedExportTermFlags = Infix | Given | Implicit | Inline | Transparent | Erased | HasDefaultParams | NoDefaultParams | ExtensionMethod
+
+  val MandatoryExportTermFlags = Exported | Method | Final
+
+  /** Flags retained in type export forwarders */
+  val RetainedExportTypeFlags = Infix
+
+  val MandatoryExportTypeFlags = Exported | Final
 
   /** Flags that apply only to classes */
   val ClassOnlyFlags = Sealed | Open | Abstract.toTypeFlags

Diff for: compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1200,7 +1200,7 @@ class Namer { typer: Typer =>
                 target = target.EtaExpand(target.typeParams)
               newSymbol(
                 cls, forwarderName,
-                Exported | Final,
+                MandatoryExportTypeFlags | (sym.flags & RetainedExportTypeFlags),
                 TypeAlias(target),
                 coord = span)
               // Note: This will always create unparameterzied aliases. So even if the original type is
@@ -1246,11 +1246,8 @@ class Namer { typer: Typer =>
                     then addPathMethodParams(pathMethod.info, mbr.info.widenExpr)
                     else mbr.info.ensureMethodic
                   (EmptyFlags, mbrInfo)
-              var flagMask = RetainedExportFlags
-              if sym.isTerm then flagMask |= HasDefaultParams | NoDefaultParams
-              var mbrFlags = Exported | Method | Final | maybeStable | sym.flags & flagMask
-              if sym.is(ExtensionMethod) || pathMethod.exists then
-                mbrFlags |= ExtensionMethod
+              var mbrFlags = MandatoryExportTermFlags | maybeStable | (sym.flags & RetainedExportTermFlags)
+              if pathMethod.exists then mbrFlags |= ExtensionMethod
               val forwarderName = checkNoConflict(alias, isPrivate = false, span)
               newSymbol(cls, forwarderName, mbrFlags, mbrInfo, coord = span)
 

Diff for: tests/pos/i19301.scala

@@ -0,0 +1,12 @@
+//> using options -Xfatal-warnings
+
+object Extensions:
+  infix def foo(x: String): Unit = ()
+  extension (arg1: Int) infix def X (arg2: Int): Int = arg1 * arg2
+  infix type X[A, B]
+
+export Extensions.*
+
+val x = 1 X 2
+type Foo = Int X Int
+val u = Extensions foo ""