[clang-tools-extra] [clang-tidy] Improved readability redundant casting with enums (PR #111424)

[Félix-Antoine Constantin via cfe-commits]

https://github.com/felix642 updated https://github.com/llvm/llvm-project/pull/111424

>From a786f626beb418cf6b8847c6250f0490b60affdb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?F=C3=A9lix-Antoine=20Constantin?=
 <felix-antoine.constantin at bidgroup.ca>
Date: Mon, 7 Oct 2024 15:27:36 -0400
Subject: [PATCH] [clang-tidy] Improved readability redundant casting with
 enums

Fixed false negatives with readability-redundant-casting when the underlying
types are the same and the option IgnoreTypeAliases is set to true.

Fixes #111137
---
 .../readability/RedundantCastingCheck.cpp     |  9 ++++++++-
 clang-tools-extra/docs/ReleaseNotes.rst       |  6 ++++++
 .../readability/redundant-casting.cpp         | 20 +++++++++++++++++++
 3 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/clang-tools-extra/clang-tidy/readability/RedundantCastingCheck.cpp b/clang-tools-extra/clang-tidy/readability/RedundantCastingCheck.cpp
index b9ff0e81cbc522..612a5a91593359 100644
--- a/clang-tools-extra/clang-tidy/readability/RedundantCastingCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/RedundantCastingCheck.cpp
@@ -40,8 +40,15 @@ static bool areTypesEqual(QualType S, QualType D) {
 
 static bool areTypesEqual(QualType TypeS, QualType TypeD,
                           bool IgnoreTypeAliases) {
-  const QualType CTypeS = TypeS.getCanonicalType();
   const QualType CTypeD = TypeD.getCanonicalType();
+
+  QualType CTypeS;
+  const auto *const EnumTypeS = TypeS->getAs<EnumType>();
+  if (EnumTypeS != nullptr && !EnumTypeS->getDecl()->isScoped())
+    CTypeS = EnumTypeS->getDecl()->getIntegerType();
+  else
+    CTypeS = TypeS.getCanonicalType();
+
   if (CTypeS != CTypeD)
     return false;
 
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 3e051c7db6adcc..32bae30821092f 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -222,6 +222,12 @@ Changes in existing checks
   by adding the option `UseUpperCaseLiteralSuffix` to select the
   case of the literal suffix in fixes.
 
+- Improved :doc:`readability-redundant-casting
+  <clang-tidy/checks/readability/redundant-casting>` check by fixing
+  false negatives related to ``enum`` when setting ``IgnoreTypeAliases``
+  to true.
+
+
 - Improved :doc:`readability-redundant-smartptr-get
   <clang-tidy/checks/readability/redundant-smartptr-get>` check to
   remove `->`, when redundant `get()` is removed.
diff --git a/clang-tools-extra/test/clang-tidy/checkers/readability/redundant-casting.cpp b/clang-tools-extra/test/clang-tidy/checkers/readability/redundant-casting.cpp
index 30cac6bd5cca06..ed49f32364cba8 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/readability/redundant-casting.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/readability/redundant-casting.cpp
@@ -221,3 +221,23 @@ void testRedundantDependentNTTPCasting() {
   // CHECK-MESSAGES: :[[@LINE-4]]:25: note: source type originates from referencing this non-type template parameter
   // CHECK-FIXES: {{^}}  T a = V;
 }
+
+enum E1 : char {};
+enum class E2 : char {};
+enum E3 {};
+
+void testEnum(E1 e1, E2 e2, E3 e3){
+  char a = static_cast<char>(e1);
+  // CHECK-MESSAGES-ALIASES: :[[@LINE-1]]:12: warning: redundant explicit casting to the same type 'char' as the sub-expression, remove this casting [readability-redundant-casting]
+  // CHECK-MESSAGES-ALIASES: :[[@LINE-3]]:18: note: source type originates from referencing this parameter
+  // CHECK-FIXES-ALIASES: {{^}}  char a = e1;
+
+  unsigned int d = static_cast<unsigned int>(e3);
+  // CHECK-MESSAGES-ALIASES: :[[@LINE-1]]:20: warning: redundant explicit casting to the same type 'unsigned int' as the sub-expression, remove this casting [readability-redundant-casting]
+  // CHECK-MESSAGES-ALIASES: :[[@LINE-8]]:32: note: source type originates from referencing this parameter
+  // CHECK-FIXES-ALIASES: {{^}}  unsigned int d = e3;
+
+  char b = static_cast<char>(e2);
+  char c = static_cast<char>(e3);
+  E1 e = static_cast<E1>('0');
+}