[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.

(cherry picked from commit 2dbfa84)

Author: rniwa

clang/lib/StaticAnalyzer/Checkers/WebKit/UncountedCallArgsChecker.cpp

@@ -92,6 +92,9 @@ 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);
+}