Allow infix enum

Fixes #18933

Author: nicolasstucki

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3928,7 +3928,9 @@ object Parsers {
     }
 
     private def checkAccessOnly(mods: Modifiers, where: String): Modifiers =
-      val mods1 = mods & (AccessFlags | Enum)
+      // We allow `infix to mark the `enum`s type as infix.
+      // Syntax rules disallow the soft infix modifier on `case`s.
+      val mods1 = mods & (AccessFlags | Enum | Infix)
       if mods1 ne mods then
         syntaxError(em"Only access modifiers are allowed on enum $where")
       mods1

tests/neg/i18933.check

@@ -0,0 +1,10 @@
+-- Error: tests/neg/i18933.scala:3:8 -----------------------------------------------------------------------------------
+3 |  infix case B(b: B) // error // error
+  |        ^^^^
+  |        end of statement expected but 'case' found
+-- [E006] Not Found Error: tests/neg/i18933.scala:3:2 ------------------------------------------------------------------
+3 |  infix case B(b: B) // error // error
+  |  ^^^^^
+  |  Not found: infix
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i18933.scala

@@ -0,0 +1,3 @@
+enum Extends[A, B]:
+  case A(a: A)
+  infix case B(b: B) // error // error

tests/pos/i18933.scala

@@ -0,0 +1,4 @@
+//> using options -Werror
+
+infix enum Extends[A, B]:
+  case Ev[B, A <: B]() extends (A Extends B)