Support non-generic type for `TypeToken.getParameterized` for legacy reasons (#2447)

This partially restores the behavior before a589ef2008,
except that back then for a non-generic type a bogus `TypeToken(ParameterizedType)`
was created, whereas now a `TypeToken(Class)` is created instead.

gson/src/main/java/com/google/gson/reflect/TypeToken.java

@@ -337,9 +337,12 @@ public class TypeToken<T> {
    * As seen here the result is a {@code TypeToken<?>}; this method cannot provide any type safety,
    * and care must be taken to pass in the correct number of type arguments.
    *
+   * <p>If {@code rawType} is a non-generic class and no type arguments are provided, this method
+   * simply delegates to {@link #get(Class)} and creates a {@code TypeToken(Class)}.
+   *
    * @throws IllegalArgumentException
-   *   If {@code rawType} is not of type {@code Class}, if it is not a generic type, or if the
-   *   type arguments are invalid for the raw type
+   *   If {@code rawType} is not of type {@code Class}, or if the type arguments are invalid for
+   *   the raw type
    */
   public static TypeToken<?> getParameterized(Type rawType, Type... typeArguments) {
     Objects.requireNonNull(rawType);
@@ -354,10 +357,16 @@ public class TypeToken<T> {
     Class<?> rawClass = (Class<?>) rawType;
     TypeVariable<?>[] typeVariables = rawClass.getTypeParameters();
 
-    // Note: Does not check if owner type of rawType is generic because this factory method
-    // does not support specifying owner type
-    if (typeVariables.length == 0) {
-      throw new IllegalArgumentException(rawClass.getName() + " is not a generic type");
+    int expectedArgsCount = typeVariables.length;
+    int actualArgsCount = typeArguments.length;
+    if (actualArgsCount != expectedArgsCount) {
+      throw new IllegalArgumentException(rawClass.getName() + " requires " + expectedArgsCount +
+          " type arguments, but got " + actualArgsCount);
+    }
+
+    // For legacy reasons create a TypeToken(Class) if the type is not generic
+    if (typeArguments.length == 0) {
+      return get(rawClass);
     }
 
     // Check for this here to avoid misleading exception thrown by ParameterizedTypeImpl
@@ -366,13 +375,6 @@ public class TypeToken<T> {
           + " it requires specifying an owner type");
     }
 
-    int expectedArgsCount = typeVariables.length;
-    int actualArgsCount = typeArguments.length;
-    if (actualArgsCount != expectedArgsCount) {
-      throw new IllegalArgumentException(rawClass.getName() + " requires " + expectedArgsCount +
-          " type arguments, but got " + actualArgsCount);
-    }
-
     for (int i = 0; i < expectedArgsCount; i++) {
       Type typeArgument = Objects.requireNonNull(typeArguments[i], "Type argument must not be null");
       Class<?> rawTypeArgument = $Gson$Types.getRawType(typeArgument);

gson/src/test/java/com/google/gson/reflect/TypeTokenTest.java

@@ -146,6 +146,9 @@ public final class TypeTokenTest {
     class LocalGenericClass<T> {}
     TypeToken<?> expectedLocalType = new TypeToken<LocalGenericClass<Integer>>() {};
     assertThat(TypeToken.getParameterized(LocalGenericClass.class, Integer.class)).isEqualTo(expectedLocalType);
+
+    // For legacy reasons, if requesting parameterized type for non-generic class, create a `TypeToken(Class)`
+    assertThat(TypeToken.getParameterized(String.class)).isEqualTo(TypeToken.get(String.class));
   }
 
   @Test
@@ -158,7 +161,7 @@ public final class TypeTokenTest {
     assertThat(e).hasMessageThat().isEqualTo("rawType must be of type Class, but was java.lang.String[]");
 
     e = assertThrows(IllegalArgumentException.class, () -> TypeToken.getParameterized(String.class, Number.class));
-    assertThat(e).hasMessageThat().isEqualTo("java.lang.String is not a generic type");
+    assertThat(e).hasMessageThat().isEqualTo("java.lang.String requires 0 type arguments, but got 1");
 
     e = assertThrows(IllegalArgumentException.class, () -> TypeToken.getParameterized(List.class, new Type[0]));
     assertThat(e).hasMessageThat().isEqualTo("java.util.List requires 1 type arguments, but got 0");