diff --git a/clang/include/clang/StaticAnalyzer/Core/PathSensitive/CallDescription.h b/clang/include/clang/StaticAnalyzer/Core/PathSensitive/CallDescription.h
index 3432d2648633c..b4e1636130ca7 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 namespace `std`.)
     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 d6d4cec9dd3d4..1a9bff529e9bb 100644
--- a/clang/lib/StaticAnalyzer/Core/CheckerContext.cpp
+++ b/clang/lib/StaticAnalyzer/Core/CheckerContext.cpp
@@ -87,9 +87,11 @@ 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>).
+  const DeclContext *DC = FD->getDeclContext()->getRedeclContext();
+  if (!(DC->isTranslationUnit() || DC->isStdNamespace()))
     return false;
 
   // If this function is not externally visible, it is not a C library function.
diff --git a/clang/unittests/StaticAnalyzer/CMakeLists.txt b/clang/unittests/StaticAnalyzer/CMakeLists.txt
index 775f0f8486b8f..db56e77331b82 100644
--- a/clang/unittests/StaticAnalyzer/CMakeLists.txt
+++ b/clang/unittests/StaticAnalyzer/CMakeLists.txt
@@ -11,6 +11,7 @@ add_clang_unittest(StaticAnalysisTests
   CallEventTest.cpp
   ConflictingEvalCallsTest.cpp
   FalsePositiveRefutationBRVisitorTest.cpp
+  IsCLibraryFunctionTest.cpp
   NoStateChangeFuncVisitorTest.cpp
   ParamRegionTest.cpp
   RangeSetTest.cpp
diff --git a/clang/unittests/StaticAnalyzer/IsCLibraryFunctionTest.cpp b/clang/unittests/StaticAnalyzer/IsCLibraryFunctionTest.cpp
new file mode 100644
index 0000000000000..19c66cc6bee1e
--- /dev/null
+++ b/clang/unittests/StaticAnalyzer/IsCLibraryFunctionTest.cpp
@@ -0,0 +1,89 @@
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Analysis/AnalysisDeclContext.h"
+#include "clang/Frontend/ASTUnit.h"
+#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
+#include "clang/Tooling/Tooling.h"
+#include "gtest/gtest.h"
+
+#include <memory>
+
+using namespace clang;
+using namespace ento;
+using namespace ast_matchers;
+
+testing::AssertionResult extractFunctionDecl(StringRef Code,
+                                             const FunctionDecl *&Result) {
+  auto ASTUnit = tooling::buildASTFromCode(Code);
+  if (!ASTUnit)
+    return testing::AssertionFailure() << "AST construction failed";
+
+  ASTContext &Context = ASTUnit->getASTContext();
+  if (Context.getDiagnostics().hasErrorOccurred())
+    return testing::AssertionFailure() << "Compilation error";
+
+  auto Matches = ast_matchers::match(functionDecl().bind("fn"), Context);
+  if (Matches.empty())
+    return testing::AssertionFailure() << "No function declaration found";
+
+  if (Matches.size() > 1)
+    return testing::AssertionFailure()
+           << "Multiple function declarations found";
+
+  Result = Matches[0].getNodeAs<FunctionDecl>("fn");
+  return testing::AssertionSuccess();
+}
+
+TEST(IsCLibraryFunctionTest, AcceptsGlobal) {
+  const FunctionDecl *Result;
+  ASSERT_TRUE(extractFunctionDecl(R"cpp(void fun();)cpp", Result));
+  EXPECT_TRUE(CheckerContext::isCLibraryFunction(Result));
+}
+
+TEST(IsCLibraryFunctionTest, AcceptsExternCGlobal) {
+  const FunctionDecl *Result;
+  ASSERT_TRUE(
+      extractFunctionDecl(R"cpp(extern "C" { void fun(); })cpp", Result));
+  EXPECT_TRUE(CheckerContext::isCLibraryFunction(Result));
+}
+
+TEST(IsCLibraryFunctionTest, RejectsNoInlineNoExternalLinkage) {
+  // Functions that are neither inlined nor externally visible cannot be C library functions.
+  const FunctionDecl *Result;
+  ASSERT_TRUE(extractFunctionDecl(R"cpp(static void fun();)cpp", Result));
+  EXPECT_FALSE(CheckerContext::isCLibraryFunction(Result));
+}
+
+TEST(IsCLibraryFunctionTest, RejectsAnonymousNamespace) {
+  const FunctionDecl *Result;
+  ASSERT_TRUE(
+      extractFunctionDecl(R"cpp(namespace { void fun(); })cpp", Result));
+  EXPECT_FALSE(CheckerContext::isCLibraryFunction(Result));
+}
+
+TEST(IsCLibraryFunctionTest, AcceptsStdNamespace) {
+  const FunctionDecl *Result;
+  ASSERT_TRUE(
+      extractFunctionDecl(R"cpp(namespace std { void fun(); })cpp", Result));
+  EXPECT_TRUE(CheckerContext::isCLibraryFunction(Result));
+}
+
+TEST(IsCLibraryFunctionTest, RejectsOtherNamespaces) {
+  const FunctionDecl *Result;
+  ASSERT_TRUE(
+      extractFunctionDecl(R"cpp(namespace stdx { void fun(); })cpp", Result));
+  EXPECT_FALSE(CheckerContext::isCLibraryFunction(Result));
+}
+
+TEST(IsCLibraryFunctionTest, RejectsClassStatic) {
+  const FunctionDecl *Result;
+  ASSERT_TRUE(
+      extractFunctionDecl(R"cpp(class A { static void fun(); };)cpp", Result));
+  EXPECT_FALSE(CheckerContext::isCLibraryFunction(Result));
+}
+
+TEST(IsCLibraryFunctionTest, RejectsClassMember) {
+  const FunctionDecl *Result;
+  ASSERT_TRUE(extractFunctionDecl(R"cpp(class A { void fun(); };)cpp", Result));
+  EXPECT_FALSE(CheckerContext::isCLibraryFunction(Result));
+}
diff --git a/llvm/utils/gn/secondary/clang/unittests/StaticAnalyzer/BUILD.gn b/llvm/utils/gn/secondary/clang/unittests/StaticAnalyzer/BUILD.gn
index 01c2b6ced3366..9c240cff18163 100644
--- a/llvm/utils/gn/secondary/clang/unittests/StaticAnalyzer/BUILD.gn
+++ b/llvm/utils/gn/secondary/clang/unittests/StaticAnalyzer/BUILD.gn
@@ -19,6 +19,7 @@ unittest("StaticAnalysisTests") {
     "CallEventTest.cpp",
     "ConflictingEvalCallsTest.cpp",
     "FalsePositiveRefutationBRVisitorTest.cpp",
+    "IsCLibraryFunctionTest.cpp",
     "NoStateChangeFuncVisitorTest.cpp",
     "ParamRegionTest.cpp",
     "RangeSetTest.cpp",