SI-10093 don't move member traits to constructor body in constructors

Fixes a regression introduced in c8e6050. Member traits with only
abstract definitions (`isInterface`) were moved into the primary
constructor by mistake. (Flatten moved the classes back.)

The member trait was duplicated into the constructor of specialized
subclasses, causing it to be generated multiple times.

Also removes some unnecessary `isMixinConstructor` checks: the mixin
constructor is always the primary constructor.

This commit also clarifies (and tests) what `isInterface` means: for
scala-defined traits, it means there are only abstract members. For
java-defined interfaces, it is always true.

3 files changed, 44 insertions, 2 deletions

diff --git a/test/files/pos/t10093.flags b/test/files/pos/t10093.flags
new file mode 100644
index 0000000000..85d8eb2ba2
--- /dev/null
+++ b/test/files/pos/t10093.flags
@@ -0,0 +1 @@
+-Xfatal-warnings
diff --git a/test/files/pos/t10093.scala b/test/files/pos/t10093.scala
new file mode 100644
index 0000000000..a894a54926
--- /dev/null
+++ b/test/files/pos/t10093.scala
@@ -0,0 +1,5 @@
+class A[@specialized(Int) T](val value: T) {
+  trait B
+  def useValue(x:T): Unit = ()
+  useValue(value)
+}
diff --git a/test/junit/scala/tools/nsc/symtab/FlagsTest.scala b/test/junit/scala/tools/nsc/symtab/FlagsTest.scala
index e88b3f9e96..4e78ca7f22 100644
--- a/test/junit/scala/tools/nsc/symtab/FlagsTest.scala
+++ b/test/junit/scala/tools/nsc/symtab/FlagsTest.scala
@@ -2,15 +2,17 @@ package scala.tools.nsc
 package symtab
 
 import org.junit.Assert._
-import scala.tools.testing.AssertUtil._
 import org.junit.Test
 import org.junit.runner.RunWith
 import org.junit.runners.JUnit4
 
+import scala.tools.testing.BytecodeTesting
+
 @RunWith(classOf[JUnit4])
-class FlagsTest {
+class FlagsTest extends BytecodeTesting {
   object symbolTable extends SymbolTableForUnitTesting
   import symbolTable._
+
   import Flags._
 
   def sym = NoSymbol.newTermSymbol(nme.EMPTY)
@@ -87,4 +89,38 @@ class FlagsTest {
     val dep = typeOf[java.lang.Deprecated].typeSymbol
     assertTrue(dep.isJavaAnnotation && dep.isJava)
   }
+
+  @Test
+  def interfaceFlag(): Unit = {
+    // scala traits are `isInterface` if they have only type defs and abstract methods / fields.
+    // java interfaces are always `isInterface`.
+    val scalaCode =
+      """package p
+        |trait T1 {
+        |  import scala.collection
+        |  def m: Int
+        |  val f: Int
+        |  type T <: AnyRef
+        |}
+        |trait T2 {
+        |  def m = 1
+        |}
+        |trait T3 {
+        |  val f = 1
+        |}
+        |trait T4 {
+        |  println()
+        |}
+      """.stripMargin
+    val javaI1 = "package p; interface I1 { int m(); }"
+    val javaI2 = "package p; interface I2 { default int m() { return 1; } }"
+    compiler.compileClasses(code = scalaCode, javaCode = (javaI1, "I1.java") :: (javaI2, "I2.java") :: Nil)
+    import compiler.global.rootMirror._
+    assert( getRequiredClass("p.T1").isInterface)
+    assert(!getRequiredClass("p.T2").isInterface)
+    assert(!getRequiredClass("p.T3").isInterface)
+    assert(!getRequiredClass("p.T4").isInterface)
+    assert( getRequiredClass("p.I1").isInterface)
+    assert( getRequiredClass("p.I2").isInterface)
+  }
 }