[clang] [alpha.webkit.UncountedCallArgsChecker] A return value can be erroneously treated as unsafe if it's a template parameter (PR #157993)

[Ryosuke Niwa via cfe-commits]

https://github.com/rniwa created https://github.com/llvm/llvm-project/pull/157993

When a template class takes Ref<T> as a template parameter and this template parameter is used as the return value of a member function, the return value can be treated as unsafe (i.e. emits a false positive). The issue was caused by getCanonicalType sometimes converting Ref<T> to T. Workaround this problem by avoid emitting a warning when the original, non-canonical type is a safe pointer type.

>From e4334be7318658f8530747d3c275499157a41149 Mon Sep 17 00:00:00 2001
From: Ryosuke Niwa <rniwa at webkit.org>
Date: Wed, 10 Sep 2025 22:15:10 -0700
Subject: [PATCH] [alpha.webkit.UncountedCallArgsChecker] A return value can be
 erroneously treated as unsafe if it's a template parameter

When a template class takes Ref<T> as a template parameter and this template parameter is used as the return value of
a member function, the return value can be treated as unsafe (i.e. emits a false positive). The issue was caused by
getCanonicalType sometimes converting Ref<T> to T. Workaround this problem by avoid emitting a warning when the original,
non-canonical type is a safe pointer type.
---
 .../WebKit/RawPtrRefCallArgsChecker.cpp       | 17 +++++++++++++--
 .../WebKit/template-wrapper-call-arg.cpp      | 21 +++++++++++++++++++
 2 files changed, 36 insertions(+), 2 deletions(-)
 create mode 100644 clang/test/Analysis/Checkers/WebKit/template-wrapper-call-arg.cpp

diff --git a/clang/lib/StaticAnalyzer/Checkers/WebKit/RawPtrRefCallArgsChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/WebKit/RawPtrRefCallArgsChecker.cpp
index e80f1749d595b..47bd13bea4abb 100644
--- a/clang/lib/StaticAnalyzer/Checkers/WebKit/RawPtrRefCallArgsChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/WebKit/RawPtrRefCallArgsChecker.cpp
@@ -122,9 +122,22 @@ class RawPtrRefCallArgsChecker
             return;
         }
         auto *E = MemberCallExpr->getImplicitObjectArgument();
-        QualType ArgType = MemberCallExpr->getObjectType().getCanonicalType();
+        auto MemberCallQT = MemberCallExpr->getObjectType();
+        QualType ArgType = MemberCallQT.getCanonicalType();
         std::optional<bool> IsUnsafe = isUnsafeType(ArgType);
-        if (IsUnsafe && *IsUnsafe && !isPtrOriginSafe(E))
+
+        // Sometimes, canonical type erroneously turns Ref<T> into T.
+        // Workaround this problem by checking again if the original type was
+        // a SubstTemplateTypeParmType of a safe smart pointer type (e.g. Ref).
+        bool IsSafePtr = false;
+        if (auto *Subst = dyn_cast<SubstTemplateTypeParmType>(MemberCallQT)) {
+          if (auto *Decl = Subst->getAssociatedDecl()) {
+            if (auto *CXXRD = dyn_cast<CXXRecordDecl>(Decl))
+              IsSafePtr = isSafePtr(CXXRD);
+          }
+        }
+
+        if (IsUnsafe && *IsUnsafe && !IsSafePtr && !isPtrOriginSafe(E))
           reportBugOnThis(E, D);
       }
 
diff --git a/clang/test/Analysis/Checkers/WebKit/template-wrapper-call-arg.cpp b/clang/test/Analysis/Checkers/WebKit/template-wrapper-call-arg.cpp
new file mode 100644
index 0000000000000..b0ff210f9415e
--- /dev/null
+++ b/clang/test/Analysis/Checkers/WebKit/template-wrapper-call-arg.cpp
@@ -0,0 +1,21 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=alpha.webkit.UncountedCallArgsChecker -verify %s
+// expected-no-diagnostics
+
+#include "mock-types.h"
+
+struct Obj {
+  void ref() const;
+  void deref() const;
+
+  void someFunction();
+};
+
+template<typename T> class Wrapper {
+public:
+  T obj();
+};
+
+static void foo(Wrapper<Ref<Obj>>&& wrapper)
+{
+  wrapper.obj()->someFunction();
+}