Issue #212 reproduced.

commons-compiler-tests/src/test/java/org/codehaus/commons/compiler/tests/ReportedBugsTest.java

@@ -1938,4 +1938,25 @@ interface Nukable {
         );
         this.assertCompilationUnitMainReturnsTrue(body, "MyClass");
     }
+
+    @Test public void
+    testIssue212AssignmentConversionNotPossibleFromTypeInterfaceClassToTypeAbstractClass() throws Exception {
+
+        String body = (
+            ""
+            + "import org.codehaus.commons.compiler.tests.issue212.base.IBase;\n"
+            + "import org.codehaus.commons.compiler.tests.issue212.base.BaseClass;\n"
+            + "import org.codehaus.commons.compiler.tests.issue212.base.DerivedClass;\n"
+            + "import org.codehaus.commons.compiler.tests.issue212.base.MapperClass;\n"
+            + "\n"
+            + "public class MyClass {\n"
+            + "    public static String main() {\n"
+            + "        IBase     o1 = new MapperClass().mapper().path();\n"
+            + "        BaseClass o2 = new MapperClass().mapper().path();\n"
+            + "        return o2.toString();\n"
+            + "    }\n"
+            + "}\n"
+        );
+        this.assertCompilationUnitMainEquals("BaseClass [baseId=0]DerivedClass [name=default]", body, "MyClass");
+    }
 }

commons-compiler-tests/src/test/java/org/codehaus/commons/compiler/tests/issue212/base/BaseClass.java

@@ -0,0 +1,24 @@
+
+package org.codehaus.commons.compiler.tests.issue212.base;
+
+public abstract class BaseClass implements IBase {
+
+    private String baseId;
+
+    public
+    BaseClass() {}
+
+    public
+    BaseClass(String baseId) {
+        super();
+        this.baseId = baseId;
+    }
+
+    @Override public abstract BaseClass
+    path();
+
+    @Override public String
+    toString() {
+        return "BaseClass [baseId=" + this.baseId + "]";
+    }
+}

commons-compiler-tests/src/test/java/org/codehaus/commons/compiler/tests/issue212/base/DerivedClass.java

@@ -0,0 +1,29 @@
+
+package org.codehaus.commons.compiler.tests.issue212.base;
+
+public
+class DerivedClass extends BaseClass {
+
+    private String name;
+
+    public
+    DerivedClass() {
+        super();
+    }
+
+    public
+    DerivedClass(String baseId, String name) {
+        super(baseId);
+        this.name = name;
+    }
+
+    @Override public BaseClass
+    path() {
+        return new DerivedClass("0", "default");
+    }
+
+    @Override public String
+    toString() {
+        return super.toString() + "DerivedClass [name=" + this.name + "]";
+    }
+}

commons-compiler-tests/src/test/java/org/codehaus/commons/compiler/tests/issue212/base/IBase.java

@@ -0,0 +1,7 @@
+
+package org.codehaus.commons.compiler.tests.issue212.base;
+
+public
+interface IBase {
+    IBase path();
+}

commons-compiler-tests/src/test/java/org/codehaus/commons/compiler/tests/issue212/base/MapperClass.java

@@ -0,0 +1,11 @@
+
+package org.codehaus.commons.compiler.tests.issue212.base;
+
+public
+class MapperClass {
+
+    public BaseClass
+    mapper() {
+        return new DerivedClass();
+    }
+}

commons-compiler-tests/src/test/java/util/CommonsCompilerTestSuite.java

@@ -461,14 +461,23 @@ class ClassBodyTest extends CompileAndExecuteTest {
 
     /**
      * Asserts that the given <var>compilationUnit</var> can be cooked by the {@link ISimpleCompiler} and its {@code
-     * public static boolean }<em>className</em>{@code .main()} method returns TRUE, <em>or</em> issues an error that matches
+     * public static boolean }<var>className</var>{@code .main()} method returns TRUE, <em>or</em> issues an error that matches
      * the <var>messageRegex</var>.
      */
     protected void
     assertCompilationUnitMainReturnsTrue(String compilationUnit, String className, String messageRegex) throws Exception {
         new SimpleCompilerTest(compilationUnit, className).assertResultTrue(messageRegex);
     }
 
+    /**
+     * Asserts that the given <var>compilationUnit</var> can be cooked by the {@link ISimpleCompiler}, and its {@code
+     * public static any-type }<var>className</var>{@code .main()} method returns a value that equals <var>expected</var>.
+     */
+    protected void
+    assertCompilationUnitMainEquals(Object expected, String compilationUnit, String className) throws Exception {
+        new SimpleCompilerTest(compilationUnit, className).assertResultEquals(expected);
+    }
+
     public
     class SimpleCompilerTest extends CompileAndExecuteTest {
 
@@ -732,6 +741,16 @@ class CompileAndExecuteTest {
             Assert.assertTrue("Test result is FALSE", (Boolean) result);
         }
 
+        /**
+         * Asserts that cooking completes normally and executing returns a value that equals <var>expected</var>.
+         */
+        public void
+        assertResultEquals(@Nullable Object expected) throws Exception {
+            this.cook();
+            Object result = this.execute();
+            Assert.assertEquals(expected, result);
+        }
+
         /**
          * Asserts that cooking completes normally and executing returns {@code null}.
          */