Index: clang/lib/StaticAnalyzer/Checkers/WebKit/NoUncountedMembersChecker.cpp
===================================================================
--- clang/lib/StaticAnalyzer/Checkers/WebKit/NoUncountedMembersChecker.cpp
+++ clang/lib/StaticAnalyzer/Checkers/WebKit/NoUncountedMembersChecker.cpp
@@ -75,7 +75,8 @@
         continue;
 
       if (auto *MemberCXXRD = MemberType->getPointeeCXXRecordDecl()) {
-        if (isRefCountable(MemberCXXRD))
+        // If we don't see the definition we just don't know.
+        if (MemberCXXRD->hasDefinition() && isRefCountable(MemberCXXRD))
           reportBug(Member, MemberType, MemberCXXRD, RD);
       }
     }
Index: clang/test/Analysis/Checkers/WebKit/uncounted-members-regression-46142.cpp
===================================================================
--- /dev/null
+++ clang/test/Analysis/Checkers/WebKit/uncounted-members-regression-46142.cpp
@@ -0,0 +1,9 @@
+// regression test for https://bugs.llvm.org/show_bug.cgi?id=46142
+
+// RUN: %clang_analyze_cc1 -analyzer-checker=webkit.WebKitNoUncountedMemberChecker -verify %s
+// expected-no-diagnostics
+
+class ClassWithoutADefinition;
+class Foo {
+    const ClassWithoutADefinition *foo;
+};