[clang] [analyzer] StdVariantChecker: fix crash when argument to `std::get` is `UnknownVal` (PR #167341)

[via cfe-commits]

https://github.com/guillem-bartrina-sonarsource created https://github.com/llvm/llvm-project/pull/167341

Although very unusual, the SVal of the argument is not checked for UnknownVal, so we may get a null pointer dereference.

>From e6a800f0aa4d040d9618da1ca98cc7a53a951520 Mon Sep 17 00:00:00 2001
From: guillem-bartrina-sonarsource <guillem.bartrina at sonarsource.com>
Date: Mon, 10 Nov 2025 17:35:54 +0100
Subject: [PATCH] StdVariantChecker: fix crash when argument to std::get is
 UnknownVal

---
 .../StaticAnalyzer/Checkers/StdVariantChecker.cpp   | 10 ++++++----
 clang/test/Analysis/std-variant-checker.cpp         | 13 ++++++++++++-
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/clang/lib/StaticAnalyzer/Checkers/StdVariantChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/StdVariantChecker.cpp
index db8bbee8761d5..805f64f4804cf 100644
--- a/clang/lib/StaticAnalyzer/Checkers/StdVariantChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/StdVariantChecker.cpp
@@ -219,10 +219,12 @@ class StdVariantChecker : public Checker<eval::Call, check::RegionChanges> {
   bool handleStdGetCall(const CallEvent &Call, CheckerContext &C) const {
     ProgramStateRef State = C.getState();
 
-    const auto &ArgType = Call.getArgSVal(0)
-                              .getType(C.getASTContext())
-                              ->getPointeeType()
-                              .getTypePtr();
+    SVal ArgSVal = Call.getArgSVal(0);
+    if (ArgSVal.isUnknown())
+      return false;
+
+    const auto &ArgType =
+        ArgSVal.getType(C.getASTContext())->getPointeeType().getTypePtr();
     // We have to make sure that the argument is an std::variant.
     // There is another std::get with std::pair argument
     if (!isStdVariant(ArgType))
diff --git a/clang/test/Analysis/std-variant-checker.cpp b/clang/test/Analysis/std-variant-checker.cpp
index 7f136c06b19cc..fbb69327e1de5 100644
--- a/clang/test/Analysis/std-variant-checker.cpp
+++ b/clang/test/Analysis/std-variant-checker.cpp
@@ -355,4 +355,15 @@ void nonInlineFunctionCallPtr() {
   char c = std::get<char> (v); // no-warning
   (void)a;
   (void)c;
-}
\ No newline at end of file
+}
+
+// ----------------------------------------------------------------------------//
+// Misc
+// ----------------------------------------------------------------------------//
+
+using uintptr_t = unsigned long long;
+
+void unknownVal() {
+  // force the argument to be UnknownVal
+  (void)std::get<int>(*(std::variant<int, float>*)(uintptr_t)3.14f); // no crash
+}