Ignore null in generic lambda tparams

Because of the way lambdas are synthesized by the JVM, at the call-site,
it is not possible to know which generic types do lambdas have via the
current reflection API.

Related but not strictly reasons why this happens can be found in JDK's
issue tracker: https://bugs.openjdk.java.net/browse/JDK-8178523?jql=text%20%7E%20%22lambda%20generic%20type%22

As a result, we ignore nulls that are returned by
`getGenericParameterTypes`.

Fixes sbt#389.

internal/zinc-apiinfo/src/main/scala/sbt/internal/inc/ClassToAPI.scala

@@ -244,12 +244,14 @@ object ClassToAPI {
     accumulate(t).filterNot(_ == null).distinct
   }
 
-  @deprecated("No longer used", "0.13.0")
-  def parents(c: Class[_]): Seq[api.Type] = types(allSuperTypes(c))
-  def types(ts: Seq[Type]): Array[api.Type] = (ts filter (_ ne null) map reference).toArray
+  def types(ts: Seq[Type]): Array[api.Type] =
+    ts.filter(_ ne null).map(reference).toArray
   def upperBounds(ts: Array[Type]): api.Type =
     api.Structure.of(lzy(types(ts)), lzyEmptyDefArray, lzyEmptyDefArray)
 
+  @deprecated("No longer used", "0.13.0")
+  def parents(c: Class[_]): Seq[api.Type] = types(allSuperTypes(c))
+
   @deprecated("Use fieldToDef[4] instead", "0.13.9")
   def fieldToDef(enclPkg: Option[String])(f: Field): api.FieldLike = {
     val c = f.getDeclaringClass
@@ -496,12 +498,18 @@ object ClassToAPI {
         api.Projection.of(api.Singleton.of(pathFromString(p)), cls)
     }
   }
+
+  // sbt/zinc#389: Ignore nulls coming from generic parameter types of lambdas
+  private[this] def ignoreNulls[T: scala.reflect.ClassTag](genericTypes: Array[T]): Array[T] =
+    genericTypes.filter(_ != null)
+
   def referenceP(t: ParameterizedType): api.Parameterized = {
-    val targs = t.getActualTypeArguments
+    val targs = ignoreNulls(t.getActualTypeArguments)
     val args = if (targs.isEmpty) emptyTypeArray else arrayMap(targs)(t => reference(t): api.Type)
     val base = reference(t.getRawType)
     api.Parameterized.of(base, args)
   }
+
   def reference(t: Type): api.Type =
     t match {
       case _: WildcardType       => reference("_")

zinc/src/sbt-test/source-dependencies/java-lambda-typeparams/Example.java

@@ -0,0 +1,26 @@
+package typeparameters;
+
+import java.util.function.Supplier;
+
+public class Example {
+
+  static <I, O> void call() {
+    Supplier<BaseBlah<I, O>> blah = () ->
+      new BaseBlah<I, O>() {
+        @Override
+        protected O getResponseInternal(I i) {
+          return null;
+        }
+      };
+  }
+
+  public static void main(String[] args) {
+    Example.<String, String>call();
+  }
+}
+
+abstract class BaseBlah<I, O> {
+  protected O getResponseInternal(I i) {
+    return null;
+  }
+}

zinc/src/sbt-test/source-dependencies/java-lambda-typeparams/test

@@ -0,0 +1 @@
+> compile