diff --git a/src/main/java/org/codehaus/groovy/classgen/InnerClassCompletionVisitor.java b/src/main/java/org/codehaus/groovy/classgen/InnerClassCompletionVisitor.java
index 4c7ade06d96..032c796ed12 100644
--- a/src/main/java/org/codehaus/groovy/classgen/InnerClassCompletionVisitor.java
+++ b/src/main/java/org/codehaus/groovy/classgen/InnerClassCompletionVisitor.java
@@ -108,7 +108,9 @@ public void visitClass(final ClassNode node) {
         classNode = node;
         thisField = null;
 
-        if (node.isEnum() || node.isInterface() || isTrait(node.getOuterClass())) return;
+        // GROOVY-8587: trait helpers must not get the standard MOP methods, but
+        // GROOVY-12226: classes declared within a trait are ordinary inner classes
+        if (node.isEnum() || node.isInterface() || isTraitHelper(node)) return;
 
         // if the class has an inner class, add methods to support private member access
         if (node.getInnerClasses().hasNext()) {
@@ -149,6 +151,17 @@ public void visitConstructor(final ConstructorNode node) {
         }
     }
 
+    /**
+     * Tests for one of the classes generated for a trait, i.e. the helper, the
+     * field helper or the static field helper. Classes declared within a trait
+     * are not included.
+     */
+    private static boolean isTraitHelper(final ClassNode node) {
+        ClassNode outerClass = node.getOuterClass();
+        return isTrait(outerClass) && (node.getModifiers() & ACC_SYNTHETIC) != 0
+                && node.getName().startsWith(outerClass.getName() + "$Trait$");
+    }
+
     private static void makeBridgeConstructor(final ClassNode c, final Parameter[] p) {
         Parameter[] newP = new Parameter[p.length + 1];
         for (int i = 0; i < p.length; i += 1) {
diff --git a/src/test/groovy/org/codehaus/groovy/transform/traitx/TraitASTTransformationTest.groovy b/src/test/groovy/org/codehaus/groovy/transform/traitx/TraitASTTransformationTest.groovy
index 72c17cb0137..730ded1ebb0 100644
--- a/src/test/groovy/org/codehaus/groovy/transform/traitx/TraitASTTransformationTest.groovy
+++ b/src/test/groovy/org/codehaus/groovy/transform/traitx/TraitASTTransformationTest.groovy
@@ -1048,6 +1048,54 @@ final class TraitASTTransformationTest {
         '''
     }
 
+    // GROOVY-12226
+    @Test
+    void testStaticInnerClassInTrait2() {
+        assertScript shell, '''
+            trait T {
+                static class Outer {
+                    def outerMethod() { 'foo' }
+                    def outerProperty = 'bar'
+                    class Inner {
+                        def callOuter() { outerMethod() }
+                        def readOuter() { outerProperty }
+                    }
+                    def viaInner() {
+                        def inner = new Inner()
+                        inner.callOuter() + inner.readOuter()
+                    }
+                }
+            }
+            class Foo implements T {
+            }
+            assert new T.Outer().viaInner() == 'foobar'
+        '''
+    }
+
+    // GROOVY-12226: class name collides with the trait helper prefix
+    @Test
+    void testStaticInnerClassInTrait3() {
+        assertScript shell, '''
+            trait T {
+                static class Trait$Outer {
+                    def outerMethod() { 'foo' }
+                    def outerProperty = 'bar'
+                    class Inner {
+                        def callOuter() { outerMethod() }
+                        def readOuter() { outerProperty }
+                    }
+                    def viaInner() {
+                        def inner = new Inner()
+                        inner.callOuter() + inner.readOuter()
+                    }
+                }
+            }
+            class Foo implements T {
+            }
+            assert new T.Trait$Outer().viaInner() == 'foobar'
+        '''
+    }
+
     @Test
     void testNonStaticInnerClassInTrait() {
         shouldFail shell, '''