[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

Author: cor3ntin

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))

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();
+}
+
+}