Refrain from adding ctor to class from ct.sym

src/compiler/scala/tools/nsc/settings/ScalaSettings.scala

@@ -90,7 +90,7 @@ trait ScalaSettings extends StandardScalaSettings with Warnings { _: MutableSett
       // TODO validate release <= java.specification.version
     }
   } withAbbreviation "--release"
-  def releaseValue: Option[String] = Option(release.value).filter(_ != "")
+  def releaseValue: Option[String] = Option(release.value).filter(_ != "")  // release.valueSetByUser
 
   /**
    * -X "Advanced" settings

src/compiler/scala/tools/nsc/symtab/classfile/ClassfileParser.scala

@@ -549,7 +549,11 @@ abstract class ClassfileParser(reader: ReusableInstance[ReusableDataReader]) {
       val needsConstructor = (
            !sawPrivateConstructor
         && !(instanceScope containsName nme.CONSTRUCTOR)
-        && ((sflags & INTERFACE) == 0 || (sflags | JAVA_ANNOTATION) != 0)
+        && ((sflags & INTERFACE) == 0 || (sflags & JAVA_ANNOTATION) != 0)
+        && {
+          val isSig = file.name.endsWith(".sig")
+          !isSig
+        }
       )
       if (needsConstructor)
         instanceScope enter clazz.newClassConstructor(NoPosition)

test/files/neg/t12565.check

@@ -0,0 +1,4 @@
+t12565.scala:5: error: java.time.Instant does not have a constructor
+  def f = new java.time.Instant
+          ^
+1 error

test/files/neg/t12565.scala

@@ -0,0 +1,9 @@
+// scalac: -Werror --release 8
+// javaVersion: 9+
+
+class C {
+  def f = new java.time.Instant
+}
+object Test extends App {
+  println(new C().f)
+}