[analyzer] Allow default arguments to be evaluated like other arguments. (llvm#80956)

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.

Author: rniwa

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);
 

clang/test/Analysis/Checkers/WebKit/ref-countable-default-arg-nullptr.cpp

@@ -0,0 +1,25 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=alpha.webkit.UncountedCallArgsChecker -verify %s
+
+#include "mock-types.h"
+
+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@-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@-1{{Call argument is uncounted and unsafe [alpha.webkit.UncountedCallArgsChecker]}}
+  someFunction(Obj::create().get());
+  otherFunction(nullptr);
+  anotherFunction(nullptr);
+}