Index: clang-tidy/readability/IdentifierNamingCheck.cpp
===================================================================
--- clang-tidy/readability/IdentifierNamingCheck.cpp
+++ clang-tidy/readability/IdentifierNamingCheck.cpp
@@ -395,6 +395,9 @@
     if (Decl->isAnonymousStructOrUnion())
       return SK_Invalid;
 
+    if (!Decl->getCanonicalDecl()->isThisDeclarationADefinition())
+      return SK_Invalid;
+
     if (Decl->hasDefinition() && Decl->isAbstract() &&
         NamingStyles[SK_AbstractClass].isSet())
       return SK_AbstractClass;
Index: test/clang-tidy/readability-identifier-naming.cpp
===================================================================
--- test/clang-tidy/readability-identifier-naming.cpp
+++ test/clang-tidy/readability-identifier-naming.cpp
@@ -190,6 +190,11 @@
 // CHECK-MESSAGES: :[[@LINE-1]]:16: warning: invalid case style for class member 'ClassMember_2'
 // CHECK-FIXES: {{^}}    static int ClassMember2;{{$}}
 };
+class my_class;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for class 'my_class'
+// CHECK-FIXES: {{^}}class CMyClass;{{$}}
+
+class my_forward_declared_class; // No warning should be triggered.
 
 const int my_class::classConstant = 4;
 // CHECK-MESSAGES: :[[@LINE-1]]:21: warning: invalid case style for class constant 'classConstant'