[clang-tools-extra] [clang-tidy][cppcoreguidelines-missing-std-forward] Fix false positive for constrained template parameters (PR #182038)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-tidy

Author: Aditya Singh (Aditya26189)

<details>
<summary>Changes</summary>

A type-constrained parameter (e.g., `std::derived_from<A> Self`) is not a forwarding reference. Check `hasTypeConstraint()` before treating `T&&` as one.

Fixes #<!-- -->180362

---
Full diff: https://github.com/llvm/llvm-project/pull/182038.diff


3 Files Affected:

- (modified) clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp (+6) 
- (modified) clang-tools-extra/docs/ReleaseNotes.rst (+4) 
- (added) clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward-cxx23.cpp (+49) 


``````````diff
diff --git a/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp b/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
index d1d81d510c8fb..02ea045868791 100644
--- a/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
+++ b/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
@@ -46,6 +46,12 @@ AST_MATCHER(ParmVarDecl, isTemplateTypeParameter) {
 
   const QualType ParamType =
       Node.getType().getNonPackExpansionType()->getPointeeType();
+
+  // A type-constrained parameter is not a forwarding reference.
+  if (const auto *TTPT = ParamType->getAs<TemplateTypeParmType>())
+    if (const auto *Decl = TTPT->getDecl(); Decl && Decl->hasTypeConstraint())
+      return false;
+
   const auto *TemplateType = ParamType->getAsCanonical<TemplateTypeParmType>();
   if (!TemplateType)
     return false;
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index ee057d9bf0444..5fcf1c2788af8 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -182,6 +182,10 @@ Changes in existing checks
   <clang-tidy/checks/cppcoreguidelines/init-variables>` check by ensuring that
   member pointers are correctly flagged as uninitialized.
 
+- Improved :doc:`cppcoreguidelines-missing-std-forward
+  <clang-tidy/checks/cppcoreguidelines/missing-std-forward>` check by fixing
+  a false positive for constrained template parameters.
+
 - Improved :doc:`cppcoreguidelines-pro-type-vararg
   <clang-tidy/checks/cppcoreguidelines/pro-type-vararg>` check by no longer
   warning on builtins with custom type checking (e.g., type-generic builtins
diff --git a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward-cxx23.cpp b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward-cxx23.cpp
new file mode 100644
index 0000000000000..2bf328ae7984e
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward-cxx23.cpp
@@ -0,0 +1,49 @@
+// RUN: %check_clang_tidy -std=c++23 %s cppcoreguidelines-missing-std-forward %t -- -- -fno-delayed-template-parsing
+
+// NOLINTBEGIN
+namespace std {
+
+template <typename T> struct remove_reference      { using type = T; };
+template <typename T> struct remove_reference<T&>  { using type = T; };
+template <typename T> struct remove_reference<T&&> { using type = T; };
+
+template <typename T> using remove_reference_t = typename remove_reference<T>::type;
+
+template <typename T> constexpr T &&forward(remove_reference_t<T> &t) noexcept;
+template <typename T> constexpr T &&forward(remove_reference_t<T> &&t) noexcept;
+template <typename T> constexpr remove_reference_t<T> &&move(T &&x);
+
+template <class T, class U>
+concept derived_from = __is_base_of(U, T);
+
+template <class T>
+concept integral = __is_integral(T);
+
+} // namespace std
+// NOLINTEND
+
+// Tests for constrained template parameters (GH#180362).
+
+class GH180362_A {
+public:
+  template <std::derived_from<GH180362_A> Self>
+  auto operator|(this Self &&self, int) -> void {}
+};
+
+template <std::integral T>
+void gh180362_takes_integral(T &&t) {}
+
+template <typename T>
+void gh180362_unconstrained(T &&t) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:{{.*}}: warning: forwarding reference parameter 't' is never forwarded inside the function body [cppcoreguidelines-missing-std-forward]
+
+template <typename T>
+requires std::integral<T>
+void gh180362_requires_clause(T &&t) {}
+// CHECK-MESSAGES: :[[@LINE-1]]:{{.*}}: warning: forwarding reference parameter 't' is never forwarded inside the function body [cppcoreguidelines-missing-std-forward]
+
+struct GH180362_B {
+  template <typename Self>
+  void foo(this Self &&self) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:{{.*}}: warning: forwarding reference parameter 'self' is never forwarded inside the function body [cppcoreguidelines-missing-std-forward]
+};

``````````

</details>


https://github.com/llvm/llvm-project/pull/182038