From 49666ec0386174b9fcf637560951b8ee29ca82c7 Mon Sep 17 00:00:00 2001
From: cor3ntin <corentinjabot@gmail.com>
Date: Thu, 5 Oct 2023 10:17:50 +0200
Subject: [PATCH] [Clang] Fix constant evaluating a captured variable in a
 lambda (#68090)

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
---
 clang/lib/AST/ExprConstant.cpp                 |  8 +++++++-
 .../SemaCXX/cxx2b-deducing-this-constexpr.cpp  | 18 ++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index d0e27de74360..5a33e918db8e 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -8370,7 +8370,13 @@ bool LValueExprEvaluator::VisitVarDecl(const Expr *E, const VarDecl *VD) {
 
     if (auto *FD = Info.CurrentCall->LambdaCaptureFields.lookup(VD)) {
       // Start with 'Result' referring to the complete closure object...
-      Result = *Info.CurrentCall->This;
+      if (auto *MD = cast<CXXMethodDecl>(Info.CurrentCall->Callee);
+          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 44de0d711674..9dbea17dd2ca 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();
+}
+
+}