[Clang] Fix constant evaluating a captured variable in a lambda

[cor3ntin]

with an explicit parameter.

We tried to read a pointer to a non-existent This APValue when constant-evaluating an explicit object lambda call operator (the this pointer is never set in explicit object member functions)

Fixes #68070

[llvmbot]

@llvm/pr-subscribers-clang

Changes

with an explicit parameter.

Fixes #68070

---

Full diff: https://github.com/llvm/llvm-project/pull/68090.diff

2 Files Affected:

• (modified) clang/lib/AST/ExprConstant.cpp (+7-1)
• (modified) clang/test/SemaCXX/cxx2b-deducing-this-constexpr.cpp (+18)

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index a142ea7c47a4730..e8bf037c879c640 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -8366,8 +8366,14 @@ 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);
       // Start with 'Result' referring to the complete closure object...
-      Result = *Info.CurrentCall->This;
+      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/SemaCXX/cxx2b-deducing-this-constexpr.cpp b/clang/test/SemaCXX/cxx2b-deducing-this-constexpr.cpp
index 44de0d711674ba8..9dbea17dd2cae34 100644
--- a/clang/test/SemaCXX/cxx2b-deducing-this-constexpr.cpp
+++ b/clang/test/SemaCXX/cxx2b-deducing-this-constexpr.cpp
@@ -54,3 +54,21 @@ consteval void test() {
     static_assert(*s == 42);
     static_assert((s << 11) == 31);
 }
+
+namespace GH68070 {
+
+constexpr auto f = [x = 3]<typename Self>(this Self&& self) {
+    return x;
+};
+
+auto g = [x = 3]<typename Self>(this Self&& self) {
+    return x;
+};
+
+int test() {
+  constexpr int a = f();
+  static_assert(a == 3);
+  return f() + g();
+}
+
+}

[tbaederr]

Suggested change

	if (MD->isExplicitObjectMemberFunction()) {
	if (const auto *MD = cast<CXXMethodDecl>(Info.CurrentCall->Callee);
	MD && MD->isExplicitObjectMemberFunction()) {

Does that work?

[tbaederr]

Shouldn't the CurrentCall->this be set to the first argument for explicit member functions, somewhere where the stack frame is created?

[cor3ntin]

I'd rather not pretend that there is a this object even though it would not correspond to an actual this pointer (and evaluating this is ill formed. I imagine it might make things more difficult to maintain in the future

[shafik]

Can you write a more detailed description explaining what the problem is what the fix is.

This is what usually ends up in the git log and we want that to be as descriptive as possible for folks who use it to understand changes quickly without digging into details.

[shafik]

So what is the difference between calling setFrom and assigning to Result using assignment here?

[cor3ntin]

LValue has no assignment operator from APValue (because sometimes you need an ASTContext)

[Fznamznon]

Probably needs a release note.

[Fznamznon]

I think LLVM code style suggests curleys in case other branch had them

Suggested change

	} else
	Result = *Info.CurrentCall->This;
	} else {
	Result = *Info.CurrentCall->This;
	}

[cor3ntin]

Will fix, thanks!

[cor3ntin]

Probably needs a release note.

Nah, this fixes a bug with deducing this I introduced on Monday :)

[shafik]

LGTM