3 files changed, 8 insertions, 3 deletions
diff --git a/src/dotty/tools/dotc/core/Contexts.scala b/src/dotty/tools/dotc/core/Contexts.scala
index 2477fd4a4..f048c27aa 100644
--- a/src/dotty/tools/dotc/core/Contexts.scala
+++ b/src/dotty/tools/dotc/core/Contexts.scala
@@ -601,7 +601,12 @@ object Contexts {
     implicit val ctx: Context = initctx
   }
 
-  class GADTMap(var bounds: SimpleMap[Symbol, TypeBounds])
+  class GADTMap(initBounds: SimpleMap[Symbol, TypeBounds]) {
+    private var myBounds = initBounds
+    def setBounds(sym: Symbol, b: TypeBounds): Unit =
+      myBounds = myBounds.updated(sym, b)
+    def bounds = myBounds
+  }
 
   /** Initial size of superId table */
   private final val InitialSuperIdsSize = 4096
diff --git a/src/dotty/tools/dotc/core/TypeComparer.scala b/src/dotty/tools/dotc/core/TypeComparer.scala
index 5cb384361..6ea67617c 100644
--- a/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -919,7 +919,7 @@ class TypeComparer(initctx: Context) extends DotClass {
 
   def narrowGADTBounds(tr: NamedType, bounds: TypeBounds): Boolean =
     isSubType(bounds.lo, bounds.hi) &&
-    { ctx.gadt.bounds = ctx.gadt.bounds.updated(tr.symbol, bounds); true }
+    { ctx.gadt.setBounds(tr.symbol, bounds); true }
 
   // Tests around `matches`
 
diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index 624a86c1c..f6f5a638e 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -675,7 +675,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         val c = ctx.fresh.setFreshGADTBounds
         for (sym <- gadtSyms)
           if (!c.gadt.bounds.contains(sym))
-            c.gadt.bounds = c.gadt.bounds.updated(sym, TypeBounds.empty)
+            c.gadt.setBounds(sym, TypeBounds.empty)
         c
       }
     val pat1 = typedPattern(tree.pat, selType)(gadtCtx)