[clang] [analyzer] Accept C library functions from the `std` namespace (PR #84469)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: None (NagyDonat)

<details>
<summary>Changes</summary>

Previously, the function `isCLibraryFunction()` and logic relying on it only accepted functions that are declared directly within a TU (i.e. not in a namespace or a class). However C++ headers like <cstdlib> declare many C standard library functions within the namespace `std`, so this commit ensures that functions within the namespace `std` are also accepted.

After this commit it will be possible to match functions like `malloc` or `free` with `CallDescription::Mode::CLibrary`.

---
Full diff: https://github.com/llvm/llvm-project/pull/84469.diff


2 Files Affected:

- (modified) clang/include/clang/StaticAnalyzer/Core/PathSensitive/CallDescription.h (+2-6) 
- (modified) clang/lib/StaticAnalyzer/Core/CheckerContext.cpp (+6-3) 


``````````diff
diff --git a/clang/include/clang/StaticAnalyzer/Core/PathSensitive/CallDescription.h b/clang/include/clang/StaticAnalyzer/Core/PathSensitive/CallDescription.h
index 3432d2648633c2..7da65734a44cf3 100644
--- a/clang/include/clang/StaticAnalyzer/Core/PathSensitive/CallDescription.h
+++ b/clang/include/clang/StaticAnalyzer/Core/PathSensitive/CallDescription.h
@@ -41,12 +41,8 @@ class CallDescription {
     ///  - We also accept calls where the number of arguments or parameters is
     ///    greater than the specified value.
     /// For the exact heuristics, see CheckerContext::isCLibraryFunction().
-    /// Note that functions whose declaration context is not a TU (e.g.
-    /// methods, functions in namespaces) are not accepted as C library
-    /// functions.
-    /// FIXME: If I understand it correctly, this discards calls where C++ code
-    /// refers a C library function through the namespace `std::` via headers
-    /// like <cstdlib>.
+    /// (This mode only matches functions that are declared either directly
+    /// within a TU or in the `std::` namespace.)
     CLibrary,
 
     /// Matches "simple" functions that are not methods. (Static methods are
diff --git a/clang/lib/StaticAnalyzer/Core/CheckerContext.cpp b/clang/lib/StaticAnalyzer/Core/CheckerContext.cpp
index d6d4cec9dd3d4d..c1ae9b441d98d1 100644
--- a/clang/lib/StaticAnalyzer/Core/CheckerContext.cpp
+++ b/clang/lib/StaticAnalyzer/Core/CheckerContext.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
+#include "clang/Analysis/AnalysisDeclContext.h"
 #include "clang/Basic/Builtins.h"
 #include "clang/Lex/Lexer.h"
 #include "llvm/ADT/StringExtras.h"
@@ -87,9 +88,11 @@ bool CheckerContext::isCLibraryFunction(const FunctionDecl *FD,
   if (!II)
     return false;
 
-  // Look through 'extern "C"' and anything similar invented in the future.
-  // If this function is not in TU directly, it is not a C library function.
-  if (!FD->getDeclContext()->getRedeclContext()->isTranslationUnit())
+  // C library functions are either declared directly within a TU (the common
+  // case) or they are accessed through the namespace `std::` (when they are
+  // used in C++ via headers like <cstdlib>).
+  if (!FD->getDeclContext()->getRedeclContext()->isTranslationUnit() &&
+          !AnalysisDeclContext::isInStdNamespace(FD))
     return false;
 
   // If this function is not externally visible, it is not a C library function.

``````````

</details>


https://github.com/llvm/llvm-project/pull/84469