Produce a more helpful error for isInstanceOf(int.class) and simila…

…r, and make analogous `isNotInstanceOf` calls fail.

Users should write `isInstanceOf(Integer.class)` instead.

As always, the biggest danger is in `isNotInstanceOf` assertions, which currently silently pass for any input if given a primitive type.

If we wanted, we could automatically treat `int.class` like `Integer.class`. But I think we can get away with being stricter, and I think it's clearest to have users write the boxed type: It's not as if Truth can tell whether its input was "originally" an `int` or not.

(As a followup, I was considering also rejecting `Void.class`. But such a change seems as likely to break someone's clever "make this always pass/fail" hack than to detect real problems. Still, it may be worth a thought someday.)

Before:
```
expected instance of: int
but was instance of : java.lang.Integer
with value          : 1
```

After:
```
java.lang.IllegalArgumentException: Cannot check instanceof for primitive type int. Pass the wrapper class instead.
```
RELNOTES=n/a
PiperOrigin-RevId: 478924507

core/src/main/java/com/google/common/truth/Subject.java

@@ -19,6 +19,7 @@
 import static com.google.common.base.CaseFormat.UPPER_CAMEL;
 import static com.google.common.base.CharMatcher.whitespace;
 import static com.google.common.base.MoreObjects.firstNonNull;
+import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Strings.lenientFormat;
 import static com.google.common.truth.Fact.fact;
 import static com.google.common.truth.Fact.simpleFact;
@@ -300,7 +301,7 @@ public void isInstanceOf(Class<?> clazz) {
       failWithActual("expected instance of", clazz.getName());
       return;
     }
-    if (!Platform.isInstanceOfType(actual, clazz)) {
+    if (!isInstanceOfType(actual, clazz)) {
       if (classMetadataUnsupported()) {
         throw new UnsupportedOperationException(
             actualCustomStringRepresentation()
@@ -329,7 +330,7 @@ public void isNotInstanceOf(Class<?> clazz) {
     if (actual == null) {
       return; // null is not an instance of clazz.
     }
-    if (Platform.isInstanceOfType(actual, clazz)) {
+    if (isInstanceOfType(actual, clazz)) {
       failWithActual("expected not to be an instance of", clazz.getName());
       /*
        * TODO(cpovirk): Consider including actual.getClass() if it's not clazz itself but only a
@@ -338,6 +339,18 @@ public void isNotInstanceOf(Class<?> clazz) {
     }
   }
 
+  private static boolean isInstanceOfType(Object instance, Class<?> clazz) {
+    checkArgument(
+        !clazz.isPrimitive(),
+        "Cannot check instanceof for primitive type %s. Pass the wrapper class instead.",
+        clazz.getSimpleName());
+    /*
+     * TODO(cpovirk): Make the message include `Primitives.wrap(clazz).getSimpleName()` once that
+     * method is available in a public guava-gwt release that we depend on.
+     */
+    return Platform.isInstanceOfType(instance, clazz);
+  }
+
   /** Fails unless the subject is equal to any element in the given iterable. */
   public void isIn(Iterable<?> iterable) {
     if (!Iterables.contains(iterable, actual)) {

core/src/test/java/com/google/common/truth/SubjectTest.java

@@ -553,6 +553,15 @@ public void isInstanceOfInterfaceForNull() {
     expectFailure.whenTesting().that((Object) null).isInstanceOf(CharSequence.class);
   }
 
+  @Test
+  public void isInstanceOfPrimitiveType() {
+    try {
+      assertThat(1).isInstanceOf(int.class);
+      fail();
+    } catch (IllegalArgumentException expected) {
+    }
+  }
+
   @Test
   public void isNotInstanceOfUnrelatedClass() {
     assertThat("a").isNotInstanceOf(Long.class);
@@ -598,6 +607,15 @@ public void isNotInstanceOfImplementedInterface() {
     expectFailure.whenTesting().that("a").isNotInstanceOf(CharSequence.class);
   }
 
+  @Test
+  public void isNotInstanceOfPrimitiveType() {
+    try {
+      assertThat(1).isNotInstanceOf(int.class);
+      fail();
+    } catch (IllegalArgumentException expected) {
+    }
+  }
+
   @Test
   public void isIn() {
     assertThat("b").isIn(oneShotIterable("a", "b", "c"));