Merge pull request #654 from google/sync-master-2018/07/23

Moe Sync

Author: eamonnmcmanus

Diff for: common/src/main/java/com/google/auto/common/MoreElements.java

@@ -37,6 +37,7 @@
 import javax.lang.model.element.Modifier;
 import javax.lang.model.element.PackageElement;
 import javax.lang.model.element.TypeElement;
+import javax.lang.model.element.TypeParameterElement;
 import javax.lang.model.element.VariableElement;
 import javax.lang.model.type.TypeKind;
 import javax.lang.model.type.TypeMirror;
@@ -129,6 +130,33 @@ public static TypeElement asType(Element element) {
     return element.accept(TypeElementVisitor.INSTANCE, null);
   }
 
+  /**
+   * Returns the given {@link Element} instance as {@link TypeParameterElement}.
+   *
+   * <p>This method is functionally equivalent to an {@code instanceof} check and a cast, but should
+   * always be used over that idiom as instructed in the documentation for {@link Element}.
+   *
+   * @throws NullPointerException if {@code element} is {@code null}
+   * @throws IllegalArgumentException if {@code element} isn't a {@link TypeParameterElement}.
+   */
+  public static TypeParameterElement asTypeParameter(Element element) {
+    return element.accept(TypeParameterElementVisitor.INSTANCE, null);
+  }
+
+  private static final class TypeParameterElementVisitor
+      extends CastingElementVisitor<TypeParameterElement> {
+    private static final TypeParameterElementVisitor INSTANCE = new TypeParameterElementVisitor();
+
+    TypeParameterElementVisitor() {
+      super("type parameter element");
+    }
+
+    @Override
+    public TypeParameterElement visitTypeParameter(TypeParameterElement e, Void ignore) {
+      return e;
+    }
+  }
+
   private static final class VariableElementVisitor extends CastingElementVisitor<VariableElement> {
     private static final VariableElementVisitor INSTANCE = new VariableElementVisitor();
 

Diff for: common/src/test/java/com/google/auto/common/MoreElementsTest.java

@@ -15,6 +15,7 @@
  */
 package com.google.auto.common;
 
+import static com.google.common.collect.Iterables.getOnlyElement;
 import static com.google.common.truth.Truth.assertThat;
 import static com.google.common.truth.Truth.assertWithMessage;
 import static org.junit.Assert.assertFalse;
@@ -32,6 +33,7 @@
 import java.util.AbstractList;
 import java.util.Arrays;
 import java.util.HashSet;
+import java.util.List;
 import java.util.NoSuchElementException;
 import java.util.Set;
 import javax.lang.model.element.AnnotationMirror;
@@ -110,6 +112,26 @@ public void asPackage_illegalArgument() {
     }
   }
 
+  @Test
+  public void asTypeParameterElement() {
+    Element typeParameterElement =
+        getOnlyElement(
+            compilation
+                .getElements()
+                .getTypeElement(List.class.getCanonicalName())
+                .getTypeParameters());
+    assertThat(MoreElements.asTypeParameter(typeParameterElement)).isEqualTo(typeParameterElement);
+  }
+
+  @Test
+  public void asTypeParameterElement_illegalArgument() {
+    try {
+      MoreElements.asTypeParameter(javaLangPackageElement);
+      fail();
+    } catch (IllegalArgumentException expected) {
+    }
+  }
+
   @Test
   public void asType() {
     assertThat(MoreElements.asType(stringElement)).isEqualTo(stringElement);