[clang] [Clang] [CodeGen] Perform derived-to-base conversion on explicit object parameter in lambda (PR #89828)

[via cfe-commits]

https://github.com/Sirraide created https://github.com/llvm/llvm-project/pull/89828

Consider this code:
```c++
template <typename... Ts>
struct Overloaded : Ts... { using Ts::operator()...; };

template <typename... Ts>
Overloaded(Ts...) -> Overloaded<Ts...>;

void f() {
  int x;
  Overloaded o {
    [&](this auto& self) {
      return &x;
    }
  };
  o();
}
```

To access `x` in the lambda, we need to perform derived-to-base conversion on `self` (since the type of `self` is not the lambda type, but rather `Overloaded<(lambda type)>`). We were previously missing this step, causing us to attempt to load the entire lambda (as the base class, it would end up being the ‘field’ with index `0` here), which would then assert later on in codegen.

This fixes #87210 and fixes #89541.

>From b5422012a65165f27bb31be7e9490892f663acfe Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Tue, 23 Apr 2024 22:45:29 +0200
Subject: [PATCH] [Clang] [CodeGen] Perform derived-to-base conversion on
 explicit object parameter in lambda

---
 clang/docs/ReleaseNotes.rst                   |  3 +
 clang/lib/CodeGen/CGExpr.cpp                  | 23 +++++++
 clang/test/CodeGenCXX/cxx2b-deducing-this.cpp | 63 +++++++++++++++++++
 3 files changed, 89 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index d1f7293a842bb6..34aad4abf39619 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -562,6 +562,9 @@ Bug Fixes to C++ Support
 - Fixed a crash when trying to evaluate a user-defined ``static_assert`` message whose ``size()``
   function returns a large or negative value. Fixes (#GH89407).
 - Fixed a use-after-free bug in parsing of type constraints with default arguments that involve lambdas. (#GH67235)
+- Fixed a crash when trying to emit captures in a lambda call operator with an explicit object
+  parameter that is called on a derived type of the lambda.
+  Fixes (#GH87210), (GH89541).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 931cb391342ea2..33795d7d4d1921 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -4684,6 +4684,29 @@ LValue CodeGenFunction::EmitLValueForLambdaField(const FieldDecl *Field,
     else
       LambdaLV = MakeAddrLValue(AddrOfExplicitObject,
                                 D->getType().getNonReferenceType());
+
+    // Make sure we have an lvalue to the lambda itself and not a derived class.
+    auto *ThisTy = D->getType().getNonReferenceType()->getAsCXXRecordDecl();
+    auto *LambdaTy = cast<CXXRecordDecl>(Field->getParent());
+    if (ThisTy != LambdaTy) {
+      CXXBasePaths Paths(/*FindAmbiguities=*/false, /*RecordPaths=*/true,
+                         /*DetectVirtual=*/false);
+
+      [[maybe_unused]] bool Derived = ThisTy->isDerivedFrom(LambdaTy, Paths);
+      assert(Derived && "Type not derived from lambda type?");
+
+      const CXXBasePath *Path = &Paths.front();
+      CXXCastPath BasePathArray;
+      for (unsigned I = 0, E = Path->size(); I != E; ++I)
+        BasePathArray.push_back(
+            const_cast<CXXBaseSpecifier *>((*Path)[I].Base));
+
+      Address Base = GetAddressOfBaseClass(
+          LambdaLV.getAddress(*this), ThisTy, BasePathArray.begin(),
+          BasePathArray.end(), /*NullCheckValue=*/false, SourceLocation());
+
+      LambdaLV = MakeAddrLValue(Base, QualType{LambdaTy->getTypeForDecl(), 0});
+    }
   } else {
     QualType LambdaTagType = getContext().getTagDeclType(Field->getParent());
     LambdaLV = MakeNaturalAlignAddrLValue(ThisValue, LambdaTagType);
diff --git a/clang/test/CodeGenCXX/cxx2b-deducing-this.cpp b/clang/test/CodeGenCXX/cxx2b-deducing-this.cpp
index b755e80db35a12..649fe2afbf4e91 100644
--- a/clang/test/CodeGenCXX/cxx2b-deducing-this.cpp
+++ b/clang/test/CodeGenCXX/cxx2b-deducing-this.cpp
@@ -182,3 +182,66 @@ auto dothing(int num)
   fun();
 }
 }
+
+namespace GH87210 {
+template <typename... Ts>
+struct Overloaded : Ts... {
+  using Ts::operator()...;
+};
+
+template <typename... Ts>
+Overloaded(Ts...) -> Overloaded<Ts...>;
+
+// CHECK-LABEL: define dso_local void @_ZN7GH872101fEv()
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[X:%.*]] = alloca i32
+// CHECK-NEXT:    [[Over:%.*]] = alloca %"{{.*}}Overloaded"
+// CHECK:         call noundef ptr @"_ZZN7GH872101fEvENH3$_0clINS_10OverloadedIJS0_EEEEEDaRT_"(ptr {{.*}} [[Over]])
+void f() {
+  int x;
+  Overloaded o {
+    // CHECK: define internal noundef ptr @"_ZZN7GH872101fEvENH3$_0clINS_10OverloadedIJS0_EEEEEDaRT_"(ptr {{.*}} [[Self:%.*]])
+    // CHECK-NEXT:  entry:
+    // CHECK-NEXT:    [[SelfAddr:%.*]] = alloca ptr
+    // CHECK-NEXT:    store ptr [[Self]], ptr [[SelfAddr]]
+    // CHECK-NEXT:    [[SelfPtr:%.*]] = load ptr, ptr [[SelfAddr]]
+    // CHECK-NEXT:    [[XRef:%.*]] = getelementptr inbounds %{{.*}}, ptr [[SelfPtr]], i32 0, i32 0
+    // CHECK-NEXT:    [[X:%.*]] = load ptr, ptr [[XRef]]
+    // CHECK-NEXT:    ret ptr [[X]]
+    [&](this auto& self) {
+      return &x;
+    }
+  };
+  o();
+}
+
+void g() {
+  int x;
+  Overloaded o {
+    [=](this auto& self) {
+      return x;
+    }
+  };
+  o();
+}
+}
+
+namespace GH89541 {
+// Same as above; just check that this doesn't crash.
+int one = 1;
+auto factory(int& x = one) {
+  return [&](this auto self) {
+    x;
+  };
+};
+
+using Base = decltype(factory());
+struct Derived : Base {
+  Derived() : Base(factory()) {}
+};
+
+void f() {
+  Derived d;
+  d();
+}
+}