[clang-tools-extra] [clang-tidy] Fix DefaultHungarianPrefix for incomplete classes (PR #215734)

[Zeyi Xu via cfe-commits]

https://github.com/zeyi2 created https://github.com/llvm/llvm-project/pull/215734

None

>From 8c73cf600e2d780f16d7ecf1e844e622823610ff Mon Sep 17 00:00:00 2001
From: Zeyi Xu <mitchell.xu2 at gmail.com>
Date: Wed, 12 Aug 2026 14:17:27 +0800
Subject: [PATCH] [clang-tidy] Fix DefaultHungarianPrefix for incomplete
 classes

---
 .../clang-tidy/readability/IdentifierNamingCheck.cpp     | 5 ++++-
 clang-tools-extra/docs/ReleaseNotes.md                   | 9 +++++++++
 .../identifier-naming/hungarian-notation1/.clang-tidy    | 1 +
 .../readability/identifier-naming-hungarian-notation.cpp | 4 ++++
 4 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp b/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp
index dfbd061986128..4ac23948c5e01 100644
--- a/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp
@@ -416,6 +416,9 @@ IdentifierNamingCheck::~IdentifierNamingCheck() = default;
 
 bool IdentifierNamingCheck::HungarianNotation::checkOptionValid(
     int StyleKindIndex) const {
+  if (StyleKindIndex == SK_Default)
+    return true;
+
   if ((StyleKindIndex >= SK_EnumConstant) &&
       (StyleKindIndex <= SK_ConstantParameter))
     return true;
@@ -643,7 +646,7 @@ StringRef IdentifierNamingCheck::HungarianNotation::getClassPrefix(
       !isOptionEnabled("TreatStructAsClass", HNOption.General))
     return {};
 
-  return CRD->isAbstract() ? "I" : "C";
+  return CRD->hasDefinition() && CRD->isAbstract() ? "I" : "C";
 }
 
 std::string IdentifierNamingCheck::HungarianNotation::getEnumPrefix(
diff --git a/clang-tools-extra/docs/ReleaseNotes.md b/clang-tools-extra/docs/ReleaseNotes.md
index 5ded07934d906..c803ef9b82477 100644
--- a/clang-tools-extra/docs/ReleaseNotes.md
+++ b/clang-tools-extra/docs/ReleaseNotes.md
@@ -121,6 +121,15 @@ infrastructure are described first, followed by tool-specific sections.
   `std::initializer_list` constructor, as the braced form could select a
   different constructor.
 
+- Improved {doc}`readability-identifier-naming
+  <clang-tidy/checks/readability/identifier-naming>` check:
+
+  - Fixed a crash when checking forward-declared classes with
+    {option}`DefaultHungarianPrefix` enabled.
+
+  - Fixed {option}`DefaultHungarianPrefix` being incorrectly diagnosed as an
+    invalid option.
+
 - Improved {doc}`readability-named-parameter
   <clang-tidy/checks/readability/named-parameter>` check by ignoring
   standard tag types (e.g. `std::in_place_t`, `std::allocator_arg_t`,
diff --git a/clang-tools-extra/test/clang-tidy/checkers/readability/Inputs/identifier-naming/hungarian-notation1/.clang-tidy b/clang-tools-extra/test/clang-tidy/checkers/readability/Inputs/identifier-naming/hungarian-notation1/.clang-tidy
index ff41479e2e96d..9852a5ff56587 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/readability/Inputs/identifier-naming/hungarian-notation1/.clang-tidy
+++ b/clang-tools-extra/test/clang-tidy/checkers/readability/Inputs/identifier-naming/hungarian-notation1/.clang-tidy
@@ -30,6 +30,7 @@ CheckOptions:
   readability-identifier-naming.StaticConstantCase: CamelCase
   readability-identifier-naming.StaticVariableCase: CamelCase
   readability-identifier-naming.VariableCase: CamelCase
+  readability-identifier-naming.DefaultHungarianPrefix: On
   readability-identifier-naming.AbstractClassHungarianPrefix: On
   readability-identifier-naming.ClassHungarianPrefix: On
   readability-identifier-naming.ClassConstantHungarianPrefix: On
diff --git a/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-hungarian-notation.cpp b/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-hungarian-notation.cpp
index 083a7f2400c17..57cec325e0bcc 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-hungarian-notation.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-hungarian-notation.cpp
@@ -581,6 +581,10 @@ INDEX iIndex = 0;
 //===----------------------------------------------------------------------===//
 // Class
 //===----------------------------------------------------------------------===//
+class Incomplete;
+// CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for identifier 'Incomplete'
+// CHECK-FIXES: class CIncomplete;
+
 class ClassCase { int Func(); };
 // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: invalid case style for class 'ClassCase' [readability-identifier-naming]
 // CHECK-FIXES: class CClassCase { int Func(); };