[clang-tools-extra] [clang-tidy][cppcoreguidelines-missing-std-forward] Do not warn when the parameter is used in a `static_cast`. (PR #99477)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-tidy

Author: Clement Courbet (legrosbuffle)

<details>
<summary>Changes</summary>

This provides a way to inform the check that we're intending to use an the forwarding reference as a specific reference category

Fixes #<!-- -->99474.

---
Full diff: https://github.com/llvm/llvm-project/pull/99477.diff


4 Files Affected:

- (modified) clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp (+6-2) 
- (modified) clang-tools-extra/docs/ReleaseNotes.rst (+5) 
- (modified) clang-tools-extra/docs/clang-tidy/checks/cppcoreguidelines/missing-std-forward.rst (+3) 
- (modified) clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp (+17) 


``````````diff
diff --git a/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp b/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
index bbb35228ce47f..726e9fffc628b 100644
--- a/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
+++ b/clang-tools-extra/clang-tidy/cppcoreguidelines/MissingStdForwardCheck.cpp
@@ -129,6 +129,9 @@ void MissingStdForwardCheck::registerMatchers(MatchFinder *Finder) {
       unless(anyOf(hasAncestor(typeLoc()),
                    hasAncestor(expr(hasUnevaluatedContext())))));
 
+  auto StaticCast = cxxStaticCastExpr(
+      hasSourceExpression(declRefExpr(to(equalsBoundNode("param")))));
+
   Finder->addMatcher(
       parmVarDecl(
           parmVarDecl().bind("param"), hasIdentifier(),
@@ -136,8 +139,9 @@ void MissingStdForwardCheck::registerMatchers(MatchFinder *Finder) {
           hasAncestor(functionDecl().bind("func")),
           hasAncestor(functionDecl(
               isDefinition(), equalsBoundNode("func"), ToParam,
-              unless(anyOf(isDeleted(),
-                           hasDescendant(std::move(ForwardCallMatcher))))))),
+              unless(anyOf(isDeleted(), hasDescendant(expr(
+                                            anyOf(std::move(ForwardCallMatcher),
+                                                  std::move(StaticCast))))))))),
       this);
 }
 
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index a23483e6df6d2..c3f41811fa7d7 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -518,6 +518,11 @@ Changes in existing checks
   usages of ``std::string_view::compare``. Added a `StringLikeClasses` option
   to detect usages of ``compare`` method in custom string-like classes.
 
+- Improved :doc:`cppcoreguidelines-missing-std-forward
+  <clang-tidy/checks/cppcoreguidelines/missing-std-forward>` check to allow
+  using ``static_cast<T&>`` to explicitly convey the intention of using a
+  forwarding reference as an lvalue reference.
+
 Removed checks
 ^^^^^^^^^^^^^^
 
diff --git a/clang-tools-extra/docs/clang-tidy/checks/cppcoreguidelines/missing-std-forward.rst b/clang-tools-extra/docs/clang-tidy/checks/cppcoreguidelines/missing-std-forward.rst
index 0c311b59a5d5a..12765b45088de 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/cppcoreguidelines/missing-std-forward.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/cppcoreguidelines/missing-std-forward.rst
@@ -38,3 +38,6 @@ Example:
 This check implements `F.19
 <http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-forward>`_
 from the C++ Core Guidelines.
+
+Users who want to use the forwarding reference as an lvalue reference can convey
+the intention by using ``static_cast<T&>(t)``.
diff --git a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
index 8116db58c937d..519d2822948bb 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/missing-std-forward.cpp
@@ -211,3 +211,20 @@ template<typename F>
 void unused_argument3(F&& _) {}
 
 } // namespace unused_arguments
+
+namespace escape_hatch {
+
+template<typename T>
+void used_as_lvalue_on_purpose(T&& t) {
+  static_cast<T&>(t);
+  static_cast<const T&>(t);
+}
+
+template<typename T>
+void used_as_rvalue_on_purpose(T&& t) {
+  static_cast<const T&&>(t);
+  // Typically used as another spelling for `std::forward`.
+  static_cast<T&&>(t);
+}
+
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/99477