Fix decoding error on extra members for nullary case classes and objects with MapBasedCodecs, closes #600

Author: sirthias

derivation/src/main/scala/io/bullet/borer/derivation/MapBasedCodecs.scala

@@ -246,7 +246,12 @@ object MapBasedCodecs extends DerivationApi {
         new Deriver[Decoder, T, quotes.type]:
 
           def deriveForCaseObject(moduleTermSymbol: Symbol): Expr[Decoder[T]] =
-            '{ Decoder[T](r => r.readMapClose(r.readMapOpen(0), ${ Ref(moduleTermSymbol).asExprOf[T] })) }
+            '{
+              Decoder[T] { r =>
+                if (r.readMapOpen(0)) while (!r.tryReadBreak()) r.skipElement()
+                ${ Ref(moduleTermSymbol).asExprOf[T] }
+              }
+            }
 
           def deriveForCaseClass(
               tpe: TypeRepr,
@@ -443,7 +448,7 @@ object MapBasedCodecs extends DerivationApi {
                     else fail("Case classes mit > 128 fields are not supported")
                   } else
                     '{
-                      if (count < 0) $r.readBreak()
+                      if (count < 0) while (! $r.tryReadBreak()) $r.skipElement()
                       ${ companionApply(companion, typeArgs(tpe), Nil).asExprOf[T] }
                     }
                 }

derivation/src/test/scala/io/bullet/borer/derivation/MiscSpec.scala

@@ -162,4 +162,17 @@ class MiscSpec extends AbstractBorerSpec {
       |    1| }
       |""".stripMargin
   }
+
+  test("Extra members") {
+    sealed trait Base
+    case class A(x: Int) extends Base
+    case class B()       extends Base
+    case object C        extends Base
+
+    implicit val baseCodecs: Decoder[Base] = MapBasedCodecs.deriveAllDecoders
+
+    verifyDecoding[Base]("""{"A":{"x":100, "extra": "should be ignored"}}""", A(100))
+    verifyDecoding[Base]("""{"B":{"extra": "should be ignored"}}""", B())
+    verifyDecoding[Base]("""{"C":{"extra": "should be ignored"}}""", C)
+  }
 }