[clang] Allow default arguments to be evaluated like other arguments. (PR #80956)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-static-analyzer-1

Author: Ryosuke Niwa (rniwa)

<details>
<summary>Changes</summary>

This PR aligns the evaluation of default arguments with other kinds of arguments by extracting the expressions within them as argument values to be evaluated.

---
Full diff: https://github.com/llvm/llvm-project/pull/80956.diff


2 Files Affected:

- (modified) clang/lib/StaticAnalyzer/Checkers/WebKit/UncountedCallArgsChecker.cpp (+3) 
- (added) clang/test/Analysis/Checkers/WebKit/ref-countable-default-arg-nullptr.cpp (+45) 


``````````diff
diff --git a/clang/lib/StaticAnalyzer/Checkers/WebKit/UncountedCallArgsChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/WebKit/UncountedCallArgsChecker.cpp
index 31ccae8b097b89..7cb0e4680d9e9e 100644
--- a/clang/lib/StaticAnalyzer/Checkers/WebKit/UncountedCallArgsChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/WebKit/UncountedCallArgsChecker.cpp
@@ -91,6 +91,9 @@ class UncountedCallArgsChecker
 
         const auto *Arg = CE->getArg(ArgIdx);
 
+        if (auto *defaultArg = dyn_cast<CXXDefaultArgExpr>(Arg))
+          Arg = defaultArg->getExpr();
+
         std::pair<const clang::Expr *, bool> ArgOrigin =
             tryToFindPtrOrigin(Arg, true);
 
diff --git a/clang/test/Analysis/Checkers/WebKit/ref-countable-default-arg-nullptr.cpp b/clang/test/Analysis/Checkers/WebKit/ref-countable-default-arg-nullptr.cpp
new file mode 100644
index 00000000000000..cd38b335dcf85e
--- /dev/null
+++ b/clang/test/Analysis/Checkers/WebKit/ref-countable-default-arg-nullptr.cpp
@@ -0,0 +1,45 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=alpha.webkit.UncountedCallArgsChecker -verify %s
+
+template <typename T>
+class RefPtr {
+public:
+  RefPtr(T* ptr)
+    : m_ptr(ptr)
+  {
+    if (m_ptr)
+      m_ptr->ref();
+  }
+
+  ~RefPtr()
+  {
+    if (m_ptr)
+      m_ptr->deref();
+  }
+
+  T* get() { return m_ptr; }
+
+private:
+  T* m_ptr;
+};
+
+class Obj {
+public:
+  static Obj* get();
+  static RefPtr<Obj> create();
+  void ref() const;
+  void deref() const;
+};
+
+void someFunction(Obj*, Obj* = nullptr);
+void otherFunction(Obj*, Obj* = Obj::get());
+// expected-warning at -1{{Call argument is uncounted and unsafe [alpha.webkit.UncountedCallArgsChecker]}}
+void anotherFunction(Obj*, Obj* = Obj::create().get());
+
+void otherFunction() {
+  someFunction(nullptr);
+  someFunction(Obj::get());
+  // expected-warning at -1{{Call argument is uncounted and unsafe [alpha.webkit.UncountedCallArgsChecker]}}
+  someFunction(Obj::create().get());
+  otherFunction(nullptr);
+  anotherFunction(nullptr);
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/80956