[clang] Crash when referencing capture in static lambda (PR #74661)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Ben Jackson (puremourning)

<details>
<summary>Changes</summary>

The constant evaluator could try to reference a lambda capture in a static lambda call operator. Static lambdas can't have captures, so we simply abort. Either the lambda needs to be made non-static, or the capture (and reference to it) need to be removed.

Fixes: https://github.com/llvm/llvm-project/issues/74608

---
Full diff: https://github.com/llvm/llvm-project/pull/74661.diff


2 Files Affected:

- (modified) clang/lib/AST/ExprConstant.cpp (+12-2) 
- (modified) clang/test/Parser/cxx2b-lambdas.cpp (+12) 


``````````diff
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 986302e1fd225f..e806318efd8a5e 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -8492,14 +8492,24 @@ bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
       return false;
 
     if (auto *FD = Info.CurrentCall->LambdaCaptureFields.lookup(VD)) {
+      auto *MD = cast<CXXMethodDecl>(Info.CurrentCall->Callee);
+
+      // Static lambda function call operators can't have captures. We already
+      // diagnosed this, so bail out here.
+      if (MD->isStatic()) {
+        assert(Info.CurrentCall->This == nullptr &&
+               "This should not be set for a static call operator");
+        return false;
+      }
+
       // Start with 'Result' referring to the complete closure object...
-      if (auto *MD = cast<CXXMethodDecl>(Info.CurrentCall->Callee);
-          MD->isExplicitObjectMemberFunction()) {
+      if (MD->isExplicitObjectMemberFunction()) {
         APValue *RefValue =
             Info.getParamSlot(Info.CurrentCall->Arguments, MD->getParamDecl(0));
         Result.setFrom(Info.Ctx, *RefValue);
       } else
         Result = *Info.CurrentCall->This;
+
       // ... then update it to refer to the field of the closure object
       // that represents the capture.
       if (!HandleLValueMember(Info, E, Result, FD))
diff --git a/clang/test/Parser/cxx2b-lambdas.cpp b/clang/test/Parser/cxx2b-lambdas.cpp
index bb9ed226afffaf..ad975a17b6e476 100644
--- a/clang/test/Parser/cxx2b-lambdas.cpp
+++ b/clang/test/Parser/cxx2b-lambdas.cpp
@@ -66,3 +66,15 @@ void static_captures() {
     }
   };
 }
+
+constexpr auto static_capture_constexpr() {
+  char n = 'n';
+  return [n] static { return n; }(); // expected-error {{a static lambda cannot have any captures}}
+}
+static_assert(static_capture_constexpr()); // expected-error {{static assertion expression is not an integral constant expression}}
+
+constexpr auto capture_constexpr() {
+  char n = 'n';
+  return [n] { return n; }();
+}
+static_assert(capture_constexpr());

``````````

</details>


https://github.com/llvm/llvm-project/pull/74661