diff --git a/dev/core/src/com/google/gwt/dev/javac/JSORestrictionsChecker.java b/dev/core/src/com/google/gwt/dev/javac/JSORestrictionsChecker.java
index f251fa8..e3eb085 100644
--- a/dev/core/src/com/google/gwt/dev/javac/JSORestrictionsChecker.java
+++ b/dev/core/src/com/google/gwt/dev/javac/JSORestrictionsChecker.java
@@ -75,8 +75,6 @@
       "Methods cannot be overridden in JavaScriptObject subclasses";
   public static final String ERR_JS_FUNCTION_ONLY_ALLOWED_ON_FUNCTIONAL_INTERFACE =
       "@JsFunction is only allowed on functional interface";
-  public static final String ERR_JS_FUNCTION_CANNOT_HAVE_DEFAULT_METHODS =
-      "JsFunction cannot have default methods";
 
   private enum ClassState {
     NORMAL, JSO
@@ -221,10 +219,6 @@
         errorOn(type, ERR_JS_FUNCTION_ONLY_ALLOWED_ON_FUNCTIONAL_INTERFACE);
         return;
       }
-      // If a functional interface has more than one method, it means it has default methods.
-      if (binding.methods().length > 1) {
-        errorOn(type, ERR_JS_FUNCTION_CANNOT_HAVE_DEFAULT_METHODS);
-      }
     }
 
     private ClassState checkType(TypeDeclaration type) {
diff --git a/dev/core/src/com/google/gwt/dev/jjs/ast/JMethod.java b/dev/core/src/com/google/gwt/dev/jjs/ast/JMethod.java
index f168d42..f31c96a 100644
--- a/dev/core/src/com/google/gwt/dev/jjs/ast/JMethod.java
+++ b/dev/core/src/com/google/gwt/dev/jjs/ast/JMethod.java
@@ -227,7 +227,7 @@
   }
 
   private boolean isJsFunctionMethod() {
-    return enclosingType.isJsFunction();
+    return enclosingType.isJsFunction() && isAbstract();
   }
 
   public boolean isOrOverridesJsFunctionMethod() {
@@ -248,7 +248,9 @@
   public boolean isJsOverlay() {
     return isJsOverlay
         || getEnclosingType().isJsoType()
-        || getEnclosingType().isJsNative() && JProgram.isClinit(this);
+        // Clinits are implicit overlays on native types and JsFunction interfaces.
+        || JProgram.isClinit(this)
+            && (getEnclosingType().isJsNative() || getEnclosingType().isJsFunction());
   }
 
   public void setSyntheticAccidentalOverride() {
diff --git a/dev/core/src/com/google/gwt/dev/jjs/impl/JsInteropRestrictionChecker.java b/dev/core/src/com/google/gwt/dev/jjs/impl/JsInteropRestrictionChecker.java
index 3f8dc71..2e070c2 100644
--- a/dev/core/src/com/google/gwt/dev/jjs/impl/JsInteropRestrictionChecker.java
+++ b/dev/core/src/com/google/gwt/dev/jjs/impl/JsInteropRestrictionChecker.java
@@ -298,8 +298,10 @@
 
     String memberDescription = JjsUtils.getReadableDescription(member);
 
-    if (!member.getEnclosingType().isJsNative()) {
-      logError(member, "JsOverlay '%s' can only be declared in a native type.", memberDescription);
+    if (!member.getEnclosingType().isJsNative() && !member.getEnclosingType().isJsFunction()) {
+      logError(member,
+          "JsOverlay '%s' can only be declared in a native type or a JsFunction interface.",
+          memberDescription);
     }
 
     if (member instanceof JConstructor) {
@@ -676,6 +678,26 @@
     return true;
   }
 
+  private void checkMemberOfJsFunction(JMember member) {
+    if (member.getJsMemberType() != JsMemberType.NONE) {
+      logError(member,
+          "JsFunction interface member '%s' cannot be JsMethod nor JsProperty.",
+          JjsUtils.getReadableDescription(member));
+    }
+
+    if (member.isJsOverlay() || member.isSynthetic()) {
+      return;
+    }
+
+    if (member instanceof JMethod && ((JMethod) member).isOrOverridesJsFunctionMethod()) {
+      return;
+    }
+
+    logError(member, "JsFunction interface '%s' cannot declare non-JsOverlay member '%s'.",
+        JjsUtils.getReadableDescription(member.getEnclosingType()),
+        JjsUtils.getReadableDescription(member));
+  }
+
   private void checkJsFunction(JDeclaredType type) {
     if (!isClinitEmpty(type, false)) {
       logError("JsFunction '%s' cannot have static initializer.", type);
@@ -687,7 +709,39 @@
 
     if (type.isJsType()) {
       logError("'%s' cannot be both a JsFunction and a JsType at the same time.", type);
+      return;
     }
+
+    // Functional interface restriction already enforced by JSORestrictionChecker. It is safe
+    // to assume here that there is a single abstract method.
+    for (JMember member : type.getMembers()) {
+      checkMemberOfJsFunction(member);
+    }
+  }
+
+  private void checkMemberOfJsFunctionImplementation(JMember member) {
+    if (member.getJsMemberType() != JsMemberType.NONE) {
+      logError(member,
+          "JsFunction implementation member '%s' cannot be JsMethod nor JsProperty.",
+          JjsUtils.getReadableDescription(member));
+    }
+
+    if (!(member instanceof JMethod)) {
+      return;
+    }
+
+    JMethod method = (JMethod) member;
+    if (method.isOrOverridesJsFunctionMethod()
+        || method.isSynthetic()
+        || method.getOverriddenMethods().isEmpty()) {
+      return;
+    }
+
+    // Methods that are not effectively static dispatch are disallowed. In this case these
+    // could only be overridable methods of java.lang.Object, i.e. toString, hashCode and equals.
+    logError(method, "JsFunction implementation '%s' cannot implement method '%s'.",
+        JjsUtils.getReadableDescription(member.getEnclosingType()),
+        JjsUtils.getReadableDescription(method));
   }
 
   private void checkJsFunctionImplementation(JDeclaredType type) {
@@ -695,18 +749,23 @@
       logError("JsFunction implementation '%s' must be final.",
           type);
     }
+
     if (type.getImplements().size() != 1) {
       logError("JsFunction implementation '%s' cannot implement more than one interface.",
           type);
     }
 
+    if (type.getSuperClass() != jprogram.getTypeJavaLangObject()) {
+      logError("JsFunction implementation '%s' cannot extend a class.", type);
+    }
+
     if (type.isJsType()) {
       logError("'%s' cannot be both a JsFunction implementation and a JsType at the same time.",
           type);
+      return;
     }
-
-    if (type.getSuperClass() != jprogram.getTypeJavaLangObject()) {
-      logError("JsFunction implementation '%s' cannot extend a class.", type);
+    for (JMember member : type.getMembers()) {
+      checkMemberOfJsFunctionImplementation(member);
     }
   }
 
diff --git a/dev/core/test/com/google/gwt/dev/javac/JSORestrictionsTest.java b/dev/core/test/com/google/gwt/dev/javac/JSORestrictionsTest.java
index 4c6e8ff..d5e1d6d 100644
--- a/dev/core/test/com/google/gwt/dev/javac/JSORestrictionsTest.java
+++ b/dev/core/test/com/google/gwt/dev/javac/JSORestrictionsTest.java
@@ -411,18 +411,6 @@
         "Line 2: " + JSORestrictionsChecker.ERR_JS_FUNCTION_ONLY_ALLOWED_ON_FUNCTIONAL_INTERFACE);
   }
 
-  public void testJsFunctionNotOnInterfaceWithDefaultMethod() {
-    StringBuilder buggyCode = new StringBuilder();
-    buggyCode.append("import jsinterop.annotations.JsFunction;\n");
-    buggyCode.append("@JsFunction public interface Buggy {\n");
-    buggyCode.append("int foo(int x);\n");
-    buggyCode.append("default void bar() { }\n");
-    buggyCode.append("}\n");
-
-    shouldGenerateError(SourceLevel.JAVA8, buggyCode,
-        "Line 2: " + JSORestrictionsChecker.ERR_JS_FUNCTION_CANNOT_HAVE_DEFAULT_METHODS);
-  }
-
   /**
    * Test that when compiling buggyCode, the TypeOracleUpdater emits
    * expectedError somewhere in its output. The code should define a class named
diff --git a/dev/core/test/com/google/gwt/dev/jjs/impl/JsInteropRestrictionCheckerTest.java b/dev/core/test/com/google/gwt/dev/jjs/impl/JsInteropRestrictionCheckerTest.java
index 44007b6..a110ab3 100644
--- a/dev/core/test/com/google/gwt/dev/jjs/impl/JsInteropRestrictionCheckerTest.java
+++ b/dev/core/test/com/google/gwt/dev/jjs/impl/JsInteropRestrictionCheckerTest.java
@@ -1147,6 +1147,10 @@
         "@JsFunction",
         "public interface Function {",
         "  int getFoo();",
+        "  @JsOverlay",
+        "  default void m() {}",
+        "  @JsOverlay",
+        "  static void n() {}",
         "}",
         "public static final class Buggy implements Function {",
         "  public int getFoo() { return 0; }",
@@ -1168,19 +1172,34 @@
 
   public void testJsFunctionFails() throws Exception {
     addSnippetImport("jsinterop.annotations.JsFunction");
+    addSnippetImport("jsinterop.annotations.JsMethod");
+    addSnippetImport("jsinterop.annotations.JsOverlay");
     addSnippetImport("jsinterop.annotations.JsProperty");
     addSnippetImport("jsinterop.annotations.JsType");
     addSnippetClassDecl(
         "@JsFunction",
-        "interface Function {",
+        "public interface Function {",
         "  int getFoo();",
         "}",
-        "static final class Buggy implements Function {",
+        "public static final class Buggy implements Function {",
+        "  @JsProperty",
         "  public int getFoo() { return 0; }",
+        "  @JsMethod",
+        "  private void bleh() {}",
+        "  @JsProperty",
+        "  public int prop = 0;",
+        "  public String toString() { return \"\"; }",
+        "  public boolean equals(Object o) { return false; }",
+        "  public int hashCode() { return 0; }",
         "}",
         "@JsFunction",
-        "interface InvalidFunction {",
+        "public interface InvalidFunction {",
+        "  @JsProperty",
         "  int getFoo();",
+        "  default void m() {}",
+        "  int f = 0;",
+        "  static void n() {}",
+        "  @JsOverlay",
         "  String s = null;",
         "}",
         "static class NonFinalJsFunction implements Function {",
@@ -1204,19 +1223,38 @@
         "}");
 
     assertBuggyFails(
-        "Line 14: JsFunction 'EntryPoint.InvalidFunction' cannot have static initializer.",
-        "Line 18: JsFunction implementation 'EntryPoint.NonFinalJsFunction' must be final.",
-        "Line 22: 'EntryPoint.JsFunctionMarkedAsJsType' cannot be both a JsFunction implementation "
+        "Line 14: JsFunction implementation member 'int EntryPoint.Buggy.getFoo()' "
+            + "cannot be JsMethod nor JsProperty.",
+        "Line 16: JsFunction implementation member 'void EntryPoint.Buggy.bleh()' cannot "
+            + "be JsMethod nor JsProperty.",
+        "Line 18: JsFunction implementation member 'int EntryPoint.Buggy.prop' cannot "
+            + "be JsMethod nor JsProperty.",
+        "Line 19: JsFunction implementation 'EntryPoint.Buggy' cannot implement method "
+            + "'String EntryPoint.Buggy.toString()'.",
+        "Line 20: JsFunction implementation 'EntryPoint.Buggy' cannot implement method "
+            + "'boolean EntryPoint.Buggy.equals(Object)'.",
+        "Line 21: JsFunction implementation 'EntryPoint.Buggy' cannot implement method "
+            + "'int EntryPoint.Buggy.hashCode()'.",
+        "Line 26: JsFunction interface member 'int EntryPoint.InvalidFunction.getFoo()' cannot "
+            + "be JsMethod nor JsProperty.",
+        "Line 27: JsFunction interface 'EntryPoint.InvalidFunction' cannot declare non-JsOverlay "
+            + "member 'void EntryPoint.InvalidFunction.m()'.",
+        "Line 28: JsFunction interface 'EntryPoint.InvalidFunction' cannot declare non-JsOverlay "
+            + "member 'int EntryPoint.InvalidFunction.f'.",
+        "Line 29: JsFunction interface 'EntryPoint.InvalidFunction' cannot declare non-JsOverlay "
+            + "member 'void EntryPoint.InvalidFunction.n()'.",
+        "Line 31: JsFunction implementation 'EntryPoint.NonFinalJsFunction' must be final.",
+        "Line 35: 'EntryPoint.JsFunctionMarkedAsJsType' cannot be both a JsFunction implementation "
             + "and a JsType at the same time.",
-        "Line 26: JsFunction 'EntryPoint.JsFunctionExtendsInterface' cannot extend other"
+        "Line 39: JsFunction 'EntryPoint.JsFunctionExtendsInterface' cannot extend other"
             + " interfaces.",
-        "Line 29: 'EntryPoint.InterfaceExtendsJsFunction' cannot extend "
+        "Line 42: 'EntryPoint.InterfaceExtendsJsFunction' cannot extend "
             + "JsFunction 'EntryPoint.Function'.",
-        "Line 30: Cannot do instanceof against JsFunction implementation "
+        "Line 43: Cannot do instanceof against JsFunction implementation "
             + "'EntryPoint.Buggy'.",
-        "Line 31: JsFunction implementation 'EntryPoint.JsFunctionExtendingBaseClass' cannot "
+        "Line 44: JsFunction implementation 'EntryPoint.JsFunctionExtendingBaseClass' cannot "
             + "extend a class.",
-        "Line 34: JsFunction implementation 'EntryPoint.JsFunctionMultipleInterfaces' cannot "
+        "Line 47: JsFunction implementation 'EntryPoint.JsFunctionMultipleInterfaces' cannot "
             + "implement more than one interface.");
   }
 
@@ -1753,8 +1791,10 @@
         "}");
 
     assertBuggyFails(
-        "Line 6: JsOverlay 'int EntryPoint.Buggy.f' can only be declared in a native type.",
-        "Line 7: JsOverlay 'void EntryPoint.Buggy.m()' can only be declared in a native type.");
+        "Line 6: JsOverlay 'int EntryPoint.Buggy.f' can only be declared in a native type "
+            + "or a JsFunction interface.",
+        "Line 7: JsOverlay 'void EntryPoint.Buggy.m()' can only be declared in a native type "
+            + "or a JsFunction interface.");
   }
 
   public void testJsTypeExtendsNativeJsTypeSucceeds() throws Exception {
diff --git a/user/test-super/com/google/gwt/dev/jjs/super/com/google/gwt/dev/jjs/test/Java8Test.java b/user/test-super/com/google/gwt/dev/jjs/super/com/google/gwt/dev/jjs/test/Java8Test.java
index d38bf76..a040c85 100644
--- a/user/test-super/com/google/gwt/dev/jjs/super/com/google/gwt/dev/jjs/test/Java8Test.java
+++ b/user/test-super/com/google/gwt/dev/jjs/super/com/google/gwt/dev/jjs/test/Java8Test.java
@@ -1600,4 +1600,27 @@
           }
         }.fAsPredicate().apply());
   }
+
+  @JsFunction
+  interface JsFunctionInterface {
+    Double m();
+    @JsOverlay
+    default Double callM() {
+      return this.m();
+    }
+  }
+
+  private static native JsFunctionInterface createNative() /*-{
+    return function () { return 5; };
+  }-*/;
+  public void testJsFunction_withOverlay() {
+    JsFunctionInterface f = new JsFunctionInterface() {
+      @Override
+      public Double m() {
+        return new Double(2.0);
+      }
+    };
+    assertEquals(2, f.callM().intValue());
+    assertEquals(5, createNative().callM().intValue());
+  }
 }
diff --git a/user/test/com/google/gwt/dev/jjs/test/Java8Test.java b/user/test/com/google/gwt/dev/jjs/test/Java8Test.java
index e435da0..e77792f 100644
--- a/user/test/com/google/gwt/dev/jjs/test/Java8Test.java
+++ b/user/test/com/google/gwt/dev/jjs/test/Java8Test.java
@@ -296,6 +296,10 @@
     assertFalse(isGwtSourceLevel8());
   }
 
+  public void testJsFunction_withOverlay() {
+    assertFalse(isGwtSourceLevel8());
+  }
+
   private boolean isGwtSourceLevel8() {
     return JUnitShell.getCompilerOptions().getSourceLevel().compareTo(SourceLevel.JAVA8) >= 0;
   }