diff -r 5f8f0bf8c72c -r b958e053d993 src/Pure/Thy/export_theory.scala
--- a/src/Pure/Thy/export_theory.scala	Sat Apr 09 11:56:48 2022 +0200
+++ b/src/Pure/Thy/export_theory.scala	Sat Apr 09 12:02:38 2022 +0200
@@ -264,12 +264,13 @@
 
   def decode_syntax: XML.Decode.T[Syntax] =
     XML.Decode.variant(List(
-      { case (Nil, Nil) => No_Syntax },
-      { case (List(delim), Nil) => Prefix(delim) },
+      { case (Nil, Nil) => No_Syntax case _ => ??? },
+      { case (List(delim), Nil) => Prefix(delim) case _ => ??? },
       { case (Nil, body) =>
           import XML.Decode._
           val (ass, delim, pri) = triple(int, string, int)(body)
-          Infix(Assoc(ass), delim, pri) }))
+          Infix(Assoc(ass), delim, pri)
+        case _ => ??? }))
 
 
   /* types */
@@ -684,11 +685,11 @@
   val decode_recursion: XML.Decode.T[Recursion] = {
     import XML.Decode._
     variant(List(
-      { case (Nil, a) => Primrec(list(string)(a)) },
-      { case (Nil, Nil) => Recdef },
-      { case (Nil, a) => Primcorec(list(string)(a)) },
-      { case (Nil, Nil) => Corec },
-      { case (Nil, Nil) => Unknown_Recursion }))
+      { case (Nil, a) => Primrec(list(string)(a)) case _ => ??? },
+      { case (Nil, Nil) => Recdef case _ => ??? },
+      { case (Nil, a) => Primcorec(list(string)(a)) case _ => ??? },
+      { case (Nil, Nil) => Corec case _ => ??? },
+      { case (Nil, Nil) => Unknown_Recursion case _ => ??? }))
   }
 
 
@@ -713,10 +714,10 @@
   val decode_rough_classification: XML.Decode.T[Rough_Classification] = {
     import XML.Decode._
     variant(List(
-      { case (Nil, a) => Equational(decode_recursion(a)) },
-      { case (Nil, Nil) => Inductive },
-      { case (Nil, Nil) => Co_Inductive },
-      { case (Nil, Nil) => Unknown }))
+      { case (Nil, a) => Equational(decode_recursion(a)) case _ => ??? },
+      { case (Nil, Nil) => Inductive case _ => ??? },
+      { case (Nil, Nil) => Co_Inductive case _ => ??? },
+      { case (Nil, Nil) => Unknown case _ => ??? }))
   }