[Clang] [Sema] Always rebuild this if captured by value in a lambda with a dependent explicit object parameter (#154276)

We have a flag that tracks whether a `CXXThisExpr` refers to a `*this`
capture in a lambda with a dependent explicit object parameter; this is
to mark it and member accesses involving it as dependent because there
is no other way to track that (DREs have a similar flag); when
instantiating the lambda, we need to always rebuild the `CXXThisExpr` to
potentially clear that flag if the explicit object parameter is no
longer dependent.

Fixes #154054.

Author: Sirraide

clang/docs/ReleaseNotes.rst

@@ -232,6 +232,8 @@ Bug Fixes to C++ Support
   "intializing multiple members of union" coincide (#GH149985).
 - Fix a crash when using ``explicit(bool)`` in pre-C++11 language modes. (#GH152729)
 - Fix the parsing of variadic member functions when the ellipis immediately follows a default argument.(#GH153445)
+- Fixed a bug that caused ``this`` captured by value in a lambda with a dependent explicit object parameter to not be
+  instantiated properly. (#GH154054)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/TreeTransform.h

@@ -14326,7 +14326,9 @@ TreeTransform<Derived>::TransformCXXThisExpr(CXXThisExpr *E) {
   // for type deduction, so we need to recompute it.
   //
   // Always recompute the type if we're in the body of a lambda, and
-  // 'this' is dependent on a lambda's explicit object parameter.
+  // 'this' is dependent on a lambda's explicit object parameter; we
+  // also need to always rebuild the expression in this case to clear
+  // the flag.
   QualType T = [&]() {
     auto &S = getSema();
     if (E->isCapturedByCopyInLambdaWithExplicitObjectParameter())
@@ -14336,7 +14338,8 @@ TreeTransform<Derived>::TransformCXXThisExpr(CXXThisExpr *E) {
     return S.getCurrentThisType();
   }();
 
-  if (!getDerived().AlwaysRebuild() && T == E->getType()) {
+  if (!getDerived().AlwaysRebuild() && T == E->getType() &&
+      !E->isCapturedByCopyInLambdaWithExplicitObjectParameter()) {
     // Mark it referenced in the new context regardless.
     // FIXME: this is a bit instantiation-specific.
     getSema().MarkThisReferenced(E);

clang/test/CodeGenCXX/cxx2b-deducing-this.cpp

@@ -264,3 +264,55 @@ void test() {
 // CHECK: call void @_ZNH5P27971C1cERKS0_
 // CHECK: call void @_ZN5P27971C1cEi
 }
+
+// This used to crash because we weren’t instantiating a dependent 'this'.
+namespace GH154054 {
+struct S {
+  int x;
+  auto byval() {
+    return [*this](this auto) { return this->x; };
+  }
+};
+
+// CHECK-LABEL: define {{.*}} void @_ZN8GH1540544mainEv
+void main() {
+  S s{ 42 };
+
+  // CHECK: call {{.*}} i32 @_ZZN8GH1540541S5byvalEvENHUlT_E_clIS2_EEDaS1_
+  if ( s.byval()() != 42)
+    __builtin_abort();
+}
+
+// CHECK-LABEL: define {{.*}} i32 @_ZZN8GH1540541S5byvalEvENHUlT_E_clIS2_EEDaS1_(i32 %.coerce)
+// CHECK: entry:
+// CHECK:   %0 = alloca %class.anon.11, align 4
+// CHECK:   %coerce.dive = getelementptr inbounds nuw %class.anon.11, ptr %0, i32 0, i32 0
+// CHECK:   %coerce.dive1 = getelementptr inbounds nuw %"struct.GH154054::S", ptr %coerce.dive, i32 0, i32 0
+// CHECK:   store i32 %.coerce, ptr %coerce.dive1, align 4
+// CHECK:   %1 = getelementptr inbounds nuw %class.anon.11, ptr %0, i32 0, i32 0
+// CHECK:   %x = getelementptr inbounds nuw %"struct.GH154054::S", ptr %1, i32 0, i32 0
+// CHECK:   %2 = load i32, ptr %x, align 4
+// CHECK:   ret i32 %2
+
+struct s {
+  int q;
+  auto f() {
+    return [*this](this auto) { return this; };
+  }
+};
+
+// CHECK-LABEL: define {{.*}} void @_ZN8GH1540541fEv
+void f() {
+  // CHECK: call {{.*}} ptr @_ZZN8GH1540541s1fEvENHUlT_E_clIS2_EEDaS1_
+  s{}.f()();
+}
+
+// CHECK-LABEL: define {{.*}} ptr @_ZZN8GH1540541s1fEvENHUlT_E_clIS2_EEDaS1_(i32 %.coerce)
+// CHECK: entry:
+// CHECK:   %0 = alloca %class.anon.12, align 4
+// CHECK:   %coerce.dive = getelementptr inbounds nuw %class.anon.12, ptr %0, i32 0, i32 0
+// CHECK:   %coerce.dive1 = getelementptr inbounds nuw %"struct.GH154054::s", ptr %coerce.dive, i32 0, i32 0
+// CHECK:   store i32 %.coerce, ptr %coerce.dive1, align 4
+// CHECK:   %1 = getelementptr inbounds nuw %class.anon.12, ptr %0, i32 0, i32 0
+// CHECK:   ret ptr %1
+}