[clang-tools-extra] [clang-tidy]: `misc-unused-using-decls`: add correct handling of `operator""` with template parametes (PR #129392)

[Baranov Victor via cfe-commits]

https://github.com/vbvictor created https://github.com/llvm/llvm-project/pull/129392

Fixes false-positives when operator"" has template paremetes, e.g.
```cpp
template <char... Ts>
  int operator""_r() {
    return {};
  }
```




>From eac5105574e6144eb702e75b21d00203706c6ada Mon Sep 17 00:00:00 2001
From: Victor Baranov <bar.victor.2002 at gmail.com>
Date: Sat, 1 Mar 2025 19:38:43 +0300
Subject: [PATCH] add correct handling of template operator""

---
 .../clang-tidy/misc/UnusedUsingDeclsCheck.cpp    | 12 ++++++++++--
 clang-tools-extra/docs/ReleaseNotes.rst          |  4 ++++
 .../checkers/misc/unused-using-decls.cpp         | 16 ++++++++++++++++
 3 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/misc/UnusedUsingDeclsCheck.cpp b/clang-tools-extra/clang-tidy/misc/UnusedUsingDeclsCheck.cpp
index 5d74907aa9fab..0b501b9c9d6a3 100644
--- a/clang-tools-extra/clang-tidy/misc/UnusedUsingDeclsCheck.cpp
+++ b/clang-tools-extra/clang-tidy/misc/UnusedUsingDeclsCheck.cpp
@@ -184,8 +184,16 @@ void UnusedUsingDeclsCheck::check(const MatchFinder::MatchResult &Result) {
     return;
   }
   // Check user-defined literals
-  if (const auto *UDL = Result.Nodes.getNodeAs<UserDefinedLiteral>("used"))
-    removeFromFoundDecls(UDL->getCalleeDecl());
+  if (const auto *UDL = Result.Nodes.getNodeAs<UserDefinedLiteral>("used")) {
+    const Decl *CalleeDecl = UDL->getCalleeDecl();
+    if (const auto *FD = dyn_cast<FunctionDecl>(UDL->getCalleeDecl())) {
+      if (const FunctionTemplateDecl *FPT = FD->getPrimaryTemplate()) {
+        removeFromFoundDecls(FPT);
+        return;
+      }
+    }
+    removeFromFoundDecls(CalleeDecl);
+  }
 }
 
 void UnusedUsingDeclsCheck::removeFromFoundDecls(const Decl *D) {
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 07a79d6bbe807..ccdb741ef5879 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -137,6 +137,10 @@ Changes in existing checks
   <clang-tidy/checks/performance/move-const-arg>` check by fixing false negatives
   on ternary operators calling ``std::move``.
 
+- Improved :doc:`misc-unused-using-decls
+  <clang-tidy/checks/misc/unused-using-decls>` check by fixing false positives
+  on ``operator""`` with template parameters.
+
 Removed checks
 ^^^^^^^^^^^^^^
 
diff --git a/clang-tools-extra/test/clang-tidy/checkers/misc/unused-using-decls.cpp b/clang-tools-extra/test/clang-tidy/checkers/misc/unused-using-decls.cpp
index 12fc18f340f21..62aa17b0b1c22 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/misc/unused-using-decls.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/misc/unused-using-decls.cpp
@@ -222,3 +222,19 @@ using gh69714::StructGH69714_1;
 using gh69714::StructGH69714_2;
 struct StructGH69714_1 a;
 struct StructGH69714_2 *b;
+
+namespace gh53444 {
+namespace my_literals {
+  template <char... Ts>
+  int operator""_r() {
+    return {};
+  }
+}
+
+using my_literals::operator"" _r;
+
+int foo() {
+  auto x2 = 123_r;
+}
+
+}