Always cast return value to return type. Fixes #5204.

headius · headius · commit ff90e424e0d3 · 2018-06-11T13:27:50.000-05:00
This issue was discovered by implementing an interface that
returns a JRuby type, like RubyArray. Because the JRuby types
implement IRubyObject, the logic for casting was not used, when at
least the cast should be present in all cases.
diff --git a/core/src/main/java/org/jruby/java/codegen/RealClassGenerator.java b/core/src/main/java/org/jruby/java/codegen/RealClassGenerator.java
@@ -822,12 +822,13 @@ public static void coerceResultAndReturn(SkinnyMethodAdapter mv, Class returnTyp
                     }
                 }
             } else {
+                // if the return type is not an IRubyObject implementer, coerce to that type before casting
                 if (!IRubyObject.class.isAssignableFrom(returnType)) {
                     mv.ldc(Type.getType(returnType));
                     mv.invokeinterface(
                         p(IRubyObject.class), "toJava", sig(Object.class, Class.class));
-                    mv.checkcast(p(returnType));
                 }
+                mv.checkcast(p(returnType));
                 mv.areturn();
             }
         } else {

Original file line number	Diff line number	Diff line change
`@@ -822,12 +822,13 @@ public static void coerceResultAndReturn(SkinnyMethodAdapter mv, Class returnTyp`
`822`	`822`	`}`
`823`	`823`	`}`
`824`	`824`	`} else {`
	`825`	`+ // if the return type is not an IRubyObject implementer, coerce to that type before casting`
`825`	`826`	`if (!IRubyObject.class.isAssignableFrom(returnType)) {`
`826`	`827`	`mv.ldc(Type.getType(returnType));`
`827`	`828`	`mv.invokeinterface(`
`828`	`829`	`p(IRubyObject.class), "toJava", sig(Object.class, Class.class));`
`829`		`- mv.checkcast(p(returnType));`
`830`	`830`	`}`
	`831`	`+ mv.checkcast(p(returnType));`
`831`	`832`	`mv.areturn();`
`832`	`833`	`}`
`833`	`834`	`} else {`