TruthIncompatibleType support for MultisetSubject#hasCount.

I propose not flagging this given the spectacularly small number of uses _at all_.

PiperOrigin-RevId: 781583086

Author: graememorgan

core/src/main/java/com/google/errorprone/bugpatterns/collectionincompatibletype/TruthIncompatibleType.java

@@ -131,6 +131,11 @@ public class TruthIncompatibleType extends BugChecker implements MethodInvocatio
               "com.google.common.truth.MapSubject", "com.google.common.truth.MultimapSubject")
           .namedAnyOf("containsKey", "doesNotContainKey");
 
+  private static final Matcher<ExpressionTree> HAS_COUNT =
+      instanceMethod()
+          .onDescendantOfAny("com.google.common.truth.MultisetSubject")
+          .named("hasCount");
+
   private static final Matcher<ExpressionTree> MAP_VECTOR_CONTAINS =
       instanceMethod()
           .onDescendantOfAny(
@@ -175,7 +180,8 @@ public Description matchMethodInvocation(MethodInvocationTree tree, VisitorState
             matchCorrespondence(tree, state),
             matchMapVectorContains(tree, state),
             matchMapScalarContains(tree, state),
-            matchMapContainsKey(tree, state))
+            matchMapContainsKey(tree, state),
+            matchHasCount(tree, state))
         .forEach(state::reportMatch);
     return NO_MATCH;
   }
@@ -441,6 +447,25 @@ private Stream<Description> matchMapScalarContains(
         .flatMap(x -> x);
   }
 
+  private Stream<Description> matchHasCount(MethodInvocationTree tree, VisitorState state) {
+    if (!HAS_COUNT.matches(tree, state)) {
+      return Stream.empty();
+    }
+    ExpressionTree receiver = getReceiver(tree);
+    if (!START_OF_ASSERTION.matches(receiver, state)) {
+      return Stream.empty();
+    }
+
+    ExpressionTree assertee = getOnlyElement(((MethodInvocationTree) receiver).getArguments());
+    TypeSymbol assertionType =
+        getOnlyElement(getSymbol((MethodInvocationTree) receiver).getParameters()).type.tsym;
+    Type targetKeyType =
+        extractTypeArgAsMemberOfSupertype(
+            ignoringCasts(assertee, state), assertionType, /* typeArgIndex= */ 0, state.getTypes());
+    var argument = tree.getArguments().get(0);
+    return checkCompatibility(argument, targetKeyType, getType(argument), state);
+  }
+
   /** Whether this is a varargs method being invoked in a non-varargs way. */
   private static boolean isNonVarargsCall(
       MethodSymbol methodSymbol, long index, Type argumentType) {

core/src/test/java/com/google/errorprone/bugpatterns/collectionincompatibletype/TruthIncompatibleTypeTest.java

@@ -695,6 +695,43 @@ public void f(int a, long b, long c) {
         .doTest();
   }
 
+  @Test
+  public void multiset_hasCount_match() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            """
+            import static com.google.common.truth.Truth.assertThat;
+            import com.google.common.collect.Multiset;
+
+            public class Test {
+              public void f(Multiset<String> a, String b) {
+                assertThat(a).hasCount(b, 1);
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void multiset_hasCount_mismatch() {
+    compilationHelper
+        .addSourceLines(
+            "Test.java",
+            """
+            import static com.google.common.truth.Truth.assertThat;
+            import com.google.common.collect.Multiset;
+
+            public class Test {
+              public void f(Multiset<String> a, Long b) {
+                // BUG: Diagnostic contains:
+                assertThat(a).hasCount(b, 1);
+              }
+            }
+            """)
+        .doTest();
+  }
+
   @Test
   public void subjectExhaustiveness(
       @TestParameter(valuesProvider = SubjectMethods.class) Method method) {