Fix sbt#389: Ignore null in generic lambdas parameters

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`.

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

@@ -548,13 +548,19 @@ object ClassToAPI {
   }.toMap
   def primitive(name: String): api.Type = PrimitiveRefs(name)
 
+  // sbt/zinc#389: Ignore nulls coming from generic parameter types of lambdas
+  private[this] def ignoreNulls(genericTypes: Array[Type]): Array[Type] =
+    genericTypes.filterNot(_ == null)
+
   private[this] def returnType(f: Field): Type = f.getGenericType
   private[this] def returnType(m: Method): Type = m.getGenericReturnType
   private[this] def exceptionTypes(c: Constructor[_]): Array[Type] = c.getGenericExceptionTypes
 
   private[this] def exceptionTypes(m: Method): Array[Type] = m.getGenericExceptionTypes
-  private[this] def parameterTypes(m: Method): Array[Type] = m.getGenericParameterTypes
-  private[this] def parameterTypes(c: Constructor[_]): Array[Type] = c.getGenericParameterTypes
+  private[this] def parameterTypes(m: Method): Array[Type] =
+    ignoreNulls(m.getGenericParameterTypes)
+  private[this] def parameterTypes(c: Constructor[_]): Array[Type] =
+    ignoreNulls(c.getGenericParameterTypes)
 
   private[this] def typeParameterTypes[T](m: Constructor[T]): Array[TypeVariable[Constructor[T]]] =
     m.getTypeParameters

project/Dependencies.scala

@@ -79,6 +79,7 @@ object Dependencies {
   val scalaCheck = "org.scalacheck" %% "scalacheck" % "1.13.4"
   val scalatest = "org.scalatest" %% "scalatest" % "3.0.1"
   val junit = "junit" % "junit" % "4.11"
+  val typetools = "net.jodah" % "typetools" % "0.5.0"
   val sjsonnew = Def.setting {
     "com.eed3si9n" %% "sjson-new-core" % contrabandSjsonNewVersion.value
   }

zinc/src/sbt-test/source-dependencies/java-typeparameters/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-typeparameters/test

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