Add isSubtypeOf() "pre"-condition check before getSubtype() returns.

We have to call it before return because before type variable substitution, isSubtypeOf() can't apply.

See #3048

RELNOTES=Validate `TypeToken.getSubtype()`.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187777105

android/guava-tests/test/com/google/common/reflect/TypeTokenSubtypeTest.java

@@ -87,6 +87,15 @@ public void testSubtypeOfNonStaticAnonymousClass() {
             .isSubtypeOf(superclass));
   }
 
+  public void testGetSubtypeOf_impossibleWildcard() {
+    TypeToken<List<? extends Number>> numberList = new TypeToken<List<? extends Number>>() {};
+    abstract class StringList implements List<String> {}
+    try {
+      numberList.getSubtype(StringList.class);
+      fail();
+    } catch (IllegalArgumentException expected) {}
+  }
+
   private static class OwnerTypeSubtypingTests extends SubtypeTester {
     @TestSubtype
     public Mall<Outdoor>.Shop<Electronics> innerTypeIsSubtype(

android/guava/src/com/google/common/reflect/TypeToken.java

@@ -422,6 +422,8 @@ public final TypeToken<? extends T> getSubtype(Class<?> subclass) {
     Type resolvedTypeArgs = resolveTypeArgsForSubclass(subclass);
     @SuppressWarnings("unchecked") // guarded by the isAssignableFrom() statement above
     TypeToken<? extends T> subtype = (TypeToken<? extends T>) of(resolvedTypeArgs);
+    checkArgument(
+        subtype.isSubtypeOf(this), "%s does not appear to be a subtype of %s", subtype, this);
     return subtype;
   }
 

guava-tests/test/com/google/common/reflect/TypeTokenSubtypeTest.java

@@ -87,6 +87,15 @@ public void testSubtypeOfNonStaticAnonymousClass() {
             .isSubtypeOf(superclass));
   }
 
+  public void testGetSubtypeOf_impossibleWildcard() {
+    TypeToken<List<? extends Number>> numberList = new TypeToken<List<? extends Number>>() {};
+    abstract class StringList implements List<String> {}
+    try {
+      numberList.getSubtype(StringList.class);
+      fail();
+    } catch (IllegalArgumentException expected) {}
+  }
+
   private static class OwnerTypeSubtypingTests extends SubtypeTester {
     @TestSubtype
     public Mall<Outdoor>.Shop<Electronics> innerTypeIsSubtype(

guava/src/com/google/common/reflect/TypeToken.java

@@ -422,6 +422,8 @@ public final TypeToken<? extends T> getSubtype(Class<?> subclass) {
     Type resolvedTypeArgs = resolveTypeArgsForSubclass(subclass);
     @SuppressWarnings("unchecked") // guarded by the isAssignableFrom() statement above
     TypeToken<? extends T> subtype = (TypeToken<? extends T>) of(resolvedTypeArgs);
+    checkArgument(
+        subtype.isSubtypeOf(this), "%s does not appear to be a subtype of %s", subtype, this);
     return subtype;
   }