Throw a plain AssertionError if ComparisonFailure is unavailable, for…

… real this time.

This should *really* let users exclude the JUnit 4 dependency -- again, as long as they don't use Expect, ExpectFailure, or TruthJUnit (i.e., assume()).

This change makes the previously failing test procedure described in #333 (comment) pass.

RELNOTES=Made it possible for users to exclude our JUnit 4 dependency and still use standard Truth assertions -- *really* this time. (JUnit 4 is still required for some advanced features, like `Expect`, `ExpectFailure`, and `TruthJUnit.assume()`.)
PiperOrigin-RevId: 353318639

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

@@ -23,7 +23,7 @@
 import static com.google.common.truth.LazyMessage.evaluateAll;
 import static com.google.common.truth.Platform.cleanStackTrace;
 import static com.google.common.truth.Platform.inferDescription;
-import static com.google.common.truth.Platform.isLinkageError;
+import static com.google.common.truth.Platform.makeComparisonFailure;
 import static com.google.common.truth.SubjectUtils.append;
 import static com.google.common.truth.SubjectUtils.concat;
 
@@ -171,31 +171,17 @@ void failEqualityCheck(
       ImmutableList<Fact> tailFacts,
       String expected,
       String actual) {
-    ImmutableList<String> messages = evaluateAll(this.messages);
-    ImmutableList<Fact> facts =
-        makeComparisonFailureFacts(
-            concat(description(), headFacts),
-            concat(tailFacts, rootUnlessThrowable()),
+    doFail(
+        makeComparisonFailure(
+            evaluateAll(messages),
+            makeComparisonFailureFacts(
+                concat(description(), headFacts),
+                concat(tailFacts, rootUnlessThrowable()),
+                expected,
+                actual),
             expected,
-            actual);
-    Throwable cause = rootCause();
-
-    /*
-     * We perform as much work as possible outside the `try`. That way, we minimize the chance that
-     * we'll catch and swallow a LinkageError from any problem except the specific problem that
-     * JUnit is not on the classpath.
-     */
-    AssertionError failure;
-    try {
-      failure = new ComparisonFailureWithFacts(messages, facts, expected, actual, cause);
-    } catch (Error probablyJunitNotOnClasspath) {
-      // We can't catch LinkageError directly because of GWT/j2cl.
-      if (!isLinkageError(probablyJunitNotOnClasspath)) {
-        throw probablyJunitNotOnClasspath;
-      }
-      failure = new AssertionErrorWithFacts(messages, facts, cause);
-    }
-    doFail(failure);
+            actual,
+            rootCause()));
   }
 
   void fail(ImmutableList<Fact> facts) {

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

@@ -15,13 +15,15 @@
  */
 package com.google.common.truth;
 
+import static com.google.common.base.Throwables.throwIfUnchecked;
 import static com.google.common.truth.DiffUtils.generateUnifiedDiff;
 import static com.google.common.truth.Fact.fact;
 
 import com.google.common.base.Joiner;
 import com.google.common.base.Splitter;
 import com.google.common.base.Throwables;
 import com.google.common.collect.ImmutableList;
+import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.util.List;
@@ -231,7 +233,65 @@ private static boolean isInferDescriptionDisabled() {
     }
   }
 
-  static boolean isLinkageError(Error e) {
-    return e instanceof LinkageError;
+  static AssertionError makeComparisonFailure(
+      ImmutableList<String> messages,
+      ImmutableList<Fact> facts,
+      String expected,
+      String actual,
+      @Nullable Throwable cause) {
+    Class<?> comparisonFailureClass;
+    try {
+      comparisonFailureClass = Class.forName("com.google.common.truth.ComparisonFailureWithFacts");
+    } catch (LinkageError | ClassNotFoundException probablyJunitNotOnClasspath) {
+      /*
+       * LinkageError makes sense, but ClassNotFoundException shouldn't happen:
+       * ComparisonFailureWithFacts should be there, even if its JUnit 4 dependency is not. But it's
+       * harmless to catch an "impossible" exception, and if someone decides to strip the class out
+       * (perhaps along with Platform.PlatformComparisonFailure, to satisfy a tool that is unhappy
+       * because it can't find the latter's superclass because JUnit 4 is also missing?), presumably
+       * we should still fall back to a plain AssertionError.
+       *
+       * TODO(cpovirk): Consider creating and using yet another class like AssertionErrorWithFacts,
+       * not actually extending ComparisonFailure but still exposing getExpected() and getActual()
+       * methods.
+       */
+      return new AssertionErrorWithFacts(messages, facts, cause);
+    }
+    Class<? extends AssertionError> asAssertionErrorSubclass =
+        comparisonFailureClass.asSubclass(AssertionError.class);
+
+    Constructor<? extends AssertionError> constructor;
+    try {
+      constructor =
+          asAssertionErrorSubclass.getDeclaredConstructor(
+              ImmutableList.class,
+              ImmutableList.class,
+              String.class,
+              String.class,
+              Throwable.class);
+    } catch (NoSuchMethodException e) {
+      // That constructor exists.
+      throw newLinkageError(e);
+    }
+
+    try {
+      return constructor.newInstance(messages, facts, expected, actual, cause);
+    } catch (InvocationTargetException e) {
+      throwIfUnchecked(e.getCause());
+      // That constructor has no `throws` clause.
+      throw newLinkageError(e);
+    } catch (InstantiationException e) {
+      // The class is a concrete class.
+      throw newLinkageError(e);
+    } catch (IllegalAccessException e) {
+      // We're accessing a class from within its package.
+      throw newLinkageError(e);
+    }
+  }
+
+  private static LinkageError newLinkageError(Throwable cause) {
+    LinkageError error = new LinkageError(cause.toString());
+    error.initCause(cause);
+    return error;
   }
 }

core/src/main/java/com/google/common/truth/super/com/google/common/truth/Platform.java

@@ -50,10 +50,6 @@ static boolean isInstanceOfType(Object instance, Class<?> clazz) {
     return false;
   }
 
-  static boolean isLinkageError(Error e) {
-    return false;
-  }
-
   abstract static class PlatformComparisonFailure extends AssertionError {
     PlatformComparisonFailure(
         String message,
@@ -214,4 +210,27 @@ public boolean getUseGrouping() {
       return false;
     }
   }
+
+  static AssertionError makeComparisonFailure(
+      ImmutableList<String> messages,
+      ImmutableList<Fact> facts,
+      String expected,
+      String actual,
+      @Nullable Throwable cause) {
+    /*
+     * Despite the name, the class we're creating extends AssertionError but not ComparisonFailure
+     * under GWT: See its supertype, PlatformComparisonFailure, above.
+     *
+     * We're actually creating the same class as the non-GWT version of this method does. So why do
+     * we have supersource for this method? It's because we can't run (and, fortunately, don't need
+     * to run) the reflective code we have for non-GWT users, who might or might not choose to
+     * exclude JUnit 4 from their classpath.
+     *
+     * TODO(cpovirk): Remove ComparisonFailureWithFacts and PlatformComparisonFailure entirely under
+     * GWT? That would let us merge them into a single class on the server. And as noted in the
+     * non-GWT copy of Platform, we could consider another custom type that exposes getExpected()
+     * and getActual(), even in the absence of ComparisonFailure. That type would work under GWT.
+     */
+    return new ComparisonFailureWithFacts(messages, facts, expected, actual, cause);
+  }
 }