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diff --git a/src/main/java/com/google/devtools/common/options/GenericTypeHelper.java b/src/main/java/com/google/devtools/common/options/GenericTypeHelper.java
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+// Copyright 2014 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.common.options;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.primitives.Primitives;
+import com.google.common.reflect.TypeToken;
+
+import java.lang.reflect.Method;
+import java.lang.reflect.ParameterizedType;
+import java.lang.reflect.Type;
+import java.lang.reflect.TypeVariable;
+
+/**
+ * A helper class for {@link OptionsParserImpl} to help checking the return type
+ * of a {@link Converter} against the type of a field or the element type of a
+ * list.
+ *
+ * <p>This class has to go through considerable contortion to get the correct result
+ * from the Java reflection system, unfortunately. If the generic reflection part
+ * had been better designed, some of this would not be necessary.
+ */
+class GenericTypeHelper {
+
+  /**
+   * Returns the raw type of t, if t is either a raw or parameterized type.
+   * Otherwise, this method throws an {@link AssertionError}.
+   */
+  @VisibleForTesting
+  static Class<?> getRawType(Type t) {
+    if (t instanceof Class<?>) {
+      return (Class<?>) t;
+    } else if (t instanceof ParameterizedType) {
+      return (Class<?>) ((ParameterizedType) t).getRawType();
+    } else {
+      throw new AssertionError("A known concrete type is not concrete");
+    }
+  }
+
+  /**
+   * If type is a parameterized type, searches the given type variable in the list
+   * of declared type variables, and then returns the corresponding actual type.
+   * Returns null if the type variable is not defined by type.
+   */
+  private static Type matchTypeVariable(Type type, TypeVariable<?> variable) {
+    if (type instanceof ParameterizedType) {
+      Class<?> rawInterfaceType = getRawType(type);
+      TypeVariable<?>[] typeParameters = rawInterfaceType.getTypeParameters();
+      for (int i = 0; i < typeParameters.length; i++) {
+        if (variable.equals(typeParameters[i])) {
+          return ((ParameterizedType) type).getActualTypeArguments()[i];
+        }
+      }
+    }
+    return null;
+  }
+
+  /**
+   * Resolves the return type of a method, in particular if the generic return
+   * type ({@link Method#getGenericReturnType()}) is a type variable
+   * ({@link TypeVariable}), by checking all super-classes and directly
+   * implemented interfaces.
+   *
+   * <p>The method m must be defined by the given type or by its raw class type.
+   *
+   * @throws AssertionError if the generic return type could not be resolved
+   */
+  // TODO(bazel-team): also check enclosing classes and indirectly implemented
+  // interfaces, which can also contribute type variables. This doesn't happen
+  // in the existing use cases.
+  public static Type getActualReturnType(Type type, Method method) {
+    Type returnType = method.getGenericReturnType();
+    if (returnType instanceof Class<?>) {
+      return returnType;
+    } else if (returnType instanceof ParameterizedType) {
+      return returnType;
+    } else if (returnType instanceof TypeVariable<?>) {
+      TypeVariable<?> variable = (TypeVariable<?>) returnType;
+      while (type != null) {
+        Type candidate = matchTypeVariable(type, variable);
+        if (candidate != null) {
+          return candidate;
+        }
+
+        Class<?> rawType = getRawType(type);
+        for (Type interfaceType : rawType.getGenericInterfaces()) {
+          candidate = matchTypeVariable(interfaceType, variable);
+          if (candidate != null) {
+            return candidate;
+          }
+        }
+
+        type = rawType.getGenericSuperclass();
+      }
+    }
+    throw new AssertionError("The type " + returnType
+        + " is not a Class, ParameterizedType, or TypeVariable");
+  }
+
+  /**
+   * Determines if a value of a particular type (from) is assignable to a field of
+   * a particular type (to). Also allows assigning wrapper types to primitive
+   * types.
+   *
+   * <p>The checks done here should be identical to the checks done by
+   * {@link java.lang.reflect.Field#set}. I.e., if this method returns true, a
+   * subsequent call to {@link java.lang.reflect.Field#set} should succeed.
+   */
+  public static boolean isAssignableFrom(Type to, Type from) {
+    if (to instanceof Class<?>) {
+      Class<?> toClass = (Class<?>) to;
+      if (toClass.isPrimitive()) {
+        return Primitives.wrap(toClass).equals(from);
+      }
+    }
+    return TypeToken.of(to).isAssignableFrom(from);
+  }
+
+  private GenericTypeHelper() {
+    // Prevents Java from creating a public constructor.
+  }
+}