[clang-tools-extra] [clang-tidy] Fix readability-identifier-naming FP on function templates (PR #198517)

[via cfe-commits]

https://github.com/CarvedCoder updated https://github.com/llvm/llvm-project/pull/198517

>From c492fa7130c284acb0594b6813a4d10957672ada Mon Sep 17 00:00:00 2001
From: CarvedCoder <shreyansbehera84 at gmail.com>
Date: Tue, 19 May 2026 19:10:26 +0530
Subject: [PATCH] [clang-tidy] Fix readability-identifier-naming FP on function
 templates

---
 .../readability/IdentifierNamingCheck.cpp     |  5 +++++
 .../identifier-naming-function-template.cpp   | 19 +++++++++++++++++++
 2 files changed, 24 insertions(+)
 create mode 100644 clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-function-template.cpp

diff --git a/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp b/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp
index a356e1390b62d..76068127e50a5 100644
--- a/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/IdentifierNamingCheck.cpp
@@ -11,6 +11,7 @@
 #include "../GlobList.h"
 #include "../utils/ASTUtils.h"
 #include "clang/AST/CXXInheritance.h"
+#include "clang/AST/DeclTemplate.h"
 #include "clang/Lex/PPCallbacks.h"
 #include "clang/Lex/Preprocessor.h"
 #include "llvm/ADT/ArrayRef.h"
@@ -1289,6 +1290,10 @@ StyleKind IdentifierNamingCheck::findStyleKind(
     return undefinedStyle(NamingStyles);
   }
 
+  if (const auto *FTD = dyn_cast<FunctionTemplateDecl>(D))
+    return findStyleKind(FTD->getTemplatedDecl(), NamingStyles,
+                         IgnoreMainLikeFunctions, CheckAnonFieldInParentScope);
+
   if (const auto *Decl = dyn_cast<FunctionDecl>(D)) {
     if (Decl->isMain())
       return SK_Invalid;
diff --git a/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-function-template.cpp b/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-function-template.cpp
new file mode 100644
index 0000000000000..de5848cbcf44e
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/readability/identifier-naming-function-template.cpp
@@ -0,0 +1,19 @@
+// RUN: %check_clang_tidy -std=c++20 %s readability-identifier-naming %t -- \
+// RUN:   -config='{CheckOptions: { \
+// RUN:     readability-identifier-naming.DefaultCase: lower_case, \
+// RUN:     readability-identifier-naming.ConstexprFunctionCase: UPPER_CASE, \
+// RUN:     readability-identifier-naming.TemplateParameterCase: CamelCase \
+// RUN:   }}'
+
+// Regression test for function template classification with DefaultCase.
+// Previously FunctionTemplateDecl fell through to DefaultCase instead of
+// delegating to the underlying FunctionDecl.
+
+template <typename T>
+constexpr void GOOD_NAME(const T &) noexcept {}
+
+// No warning expected for TEST_FUNC.
+
+template <typename T>
+constexpr void bad_name(const T &) noexcept {}
+// CHECK-MESSAGES: :[[@LINE-1]]:16: warning: invalid case style for constexpr function 'bad_name'