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[cxx-interop] Avoid unchecked recursion when importing C++ classes with circular inheritance #81188

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Merged
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May 1, 2025
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[cxx-interop] Avoid unchecked recursion when importing C++ classes with circular inheritance #81188

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16 changes: 16 additions & 0 deletions include/swift/ClangImporter/ClangImporter.h
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Expand Up @@ -786,6 +786,22 @@ bool declIsCxxOnly(const Decl *decl);

/// Is this DeclContext an `enum` that represents a C++ namespace?
bool isClangNamespace(const DeclContext *dc);

/// For some \a templatedClass that inherits from \a base, whether they are
/// derived from the same class template.
///
/// This kind of circular inheritance can happen when a templated class inherits
/// from a specialization of itself, e.g.:
///
/// template <typename T> class C;
/// template <> class C<void> { /* ... */ };
/// template <typename T> class C<T> : C<void> { /* ... */ };
///
/// Checking for this kind of scenario is necessary for avoiding infinite
/// recursion during symbolic imports (importSymbolicCXXDecls), where
/// specialized class templates are instead imported as unspecialized.
bool isSymbolicCircularBase(const clang::CXXRecordDecl *templatedClass,
const clang::RecordDecl *base);
} // namespace importer

struct ClangInvocationFileMapping {
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20 changes: 20 additions & 0 deletions lib/ClangImporter/ClangImporter.cpp
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Expand Up @@ -6364,6 +6364,11 @@ TinyPtrVector<ValueDecl *> ClangRecordMemberLookup::evaluate(
continue;

auto *baseRecord = baseType->getAs<clang::RecordType>()->getDecl();

if (isSymbolicCircularBase(cxxRecord, baseRecord))
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My last force push updated the test case to cover one more case (a class that inherits from a circularly inheriting class), and fixed this code to check for the right base. It used to check the C++ class of inheritingDecl, but now it checks the C++ class of recordDecl, where:

template <typename T> class Circular;
template <> class Circular<void> { ... };
template <typename T> class Circular<T> : Circular<void> { ... };

typedef Circular<SomeType> RecordDecl;
class InheritingDecl : RecordDecl { ... };

Neither check is complete, but heuristically checking recordDecl is probably better (at least that's what matches the circular inheritance case in std::exception).

// Skip circular bases to avoid unbounded recursion
continue;

if (auto import = clangModuleLoader->importDeclDirectly(baseRecord)) {
// If we are looking up the base class, go no further. We will have
// already found it during the other lookup.
Expand Down Expand Up @@ -8774,3 +8779,18 @@ bool importer::isClangNamespace(const DeclContext *dc) {

return false;
}

bool importer::isSymbolicCircularBase(const clang::CXXRecordDecl *symbolicClass,
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Should we only do this when we are importing something in symbolic mode?

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Yeah that's a good point. I'm not entirely sure whether we should, because the symbolic mode is only the trigger for this issue rather than the actual problem itself (which is that we're not handling CxxRecordDecls that describe uninstantiated templates).

In any case, I can add the check just for this to be an extra conservative patch, since I presume we'll need to cherry-pick this.

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I think we should be consistent here in terms of symbolic vs non-symbolic mode, i.e. bailing in both modes, like this patch does currently.

const clang::RecordDecl *base) {
auto *classTemplate = symbolicClass->getDescribedClassTemplate();
if (!classTemplate)
return false;

auto *specializedBase =
dyn_cast<clang::ClassTemplateSpecializationDecl>(base);
if (!specializedBase)
return false;

return classTemplate->getCanonicalDecl() ==
specializedBase->getSpecializedTemplate()->getCanonicalDecl();
}
6 changes: 5 additions & 1 deletion lib/Sema/TypeCheckInvertible.cpp
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Expand Up @@ -17,11 +17,12 @@
//===----------------------------------------------------------------------===//

#include "TypeCheckInvertible.h"
#include "TypeChecker.h"
#include "swift/AST/ASTContext.h"
#include "swift/AST/ClangModuleLoader.h"
#include "swift/AST/GenericEnvironment.h"
#include "swift/Basic/Assertions.h"
#include "TypeChecker.h"
#include "swift/ClangImporter/ClangImporter.h"

using namespace swift;

Expand Down Expand Up @@ -316,6 +317,9 @@ bool StorageVisitor::visit(NominalTypeDecl *nominal, DeclContext *dc) {
dyn_cast_or_null<clang::CXXRecordDecl>(nominal->getClangDecl())) {
for (auto cxxBase : cxxRecordDecl->bases()) {
if (auto cxxBaseDecl = cxxBase.getType()->getAsCXXRecordDecl()) {
if (importer::isSymbolicCircularBase(cxxRecordDecl, cxxBaseDecl))
// Skip circular bases to avoid unbounded recursion
continue;
auto clangModuleLoader = dc->getASTContext().getClangModuleLoader();
auto importedDecl =
clangModuleLoader->importDeclDirectly(cxxBaseDecl);
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32 changes: 32 additions & 0 deletions test/Interop/Cxx/class/inheritance/Inputs/circular-inheritance.h
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@@ -0,0 +1,32 @@
#ifndef CIRCULAR_INHERITANCE_H
#define CIRCULAR_INHERITANCE_H

struct DinoEgg {
void dinoEgg(void) const {}
};

template <typename Chicken>
struct Egg;

template <>
struct Egg<void> : DinoEgg {
Egg() {}
void voidEgg(void) const {}
};

template <typename Chicken>
struct Egg : Egg<void> {
Egg(Chicken _chicken) {}
Chicken chickenEgg(Chicken c) const { return c; }
};

typedef Egg<void> VoidEgg;
typedef Egg<bool> BoolEgg;
typedef Egg<Egg<void>> EggEgg;

struct NewEgg : Egg<int> {
NewEgg() : Egg<int>(555) {}
void newEgg() const {}
};

#endif // !CIRCULAR_INHERITANCE_H
5 changes: 5 additions & 0 deletions test/Interop/Cxx/class/inheritance/Inputs/module.modulemap
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Expand Up @@ -48,3 +48,8 @@ module InheritedLookup {
header "inherited-lookup.h"
requires cplusplus
}

module CircularInheritance {
header "circular-inheritance.h"
requires cplusplus
}
27 changes: 27 additions & 0 deletions test/Interop/Cxx/class/inheritance/circular-inheritance-typechecker.swift
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@@ -0,0 +1,27 @@
// RUN: %empty-directory(%t/index)
// RUN: %target-typecheck-verify-swift -verify-ignore-unknown -I %S/Inputs -cxx-interoperability-mode=default -index-store-path %t/index
//
// Note that we specify an -index-store-path to ensure we also test importing symbolic C++ decls,
// to exercise code that handles importing unspecialized class templates.

import CircularInheritance

let voidEgg = VoidEgg()
voidEgg.dinoEgg()
voidEgg.voidEgg()

let boolEgg = BoolEgg(false)
boolEgg.dinoEgg()
boolEgg.voidEgg()
boolEgg.chickenEgg(true)

let eggEgg = EggEgg(VoidEgg())
eggEgg.dinoEgg()
eggEgg.voidEgg()
eggEgg.chickenEgg(VoidEgg())

let newEgg = NewEgg()
newEgg.dinoEgg()
newEgg.voidEgg()
newEgg.chickenEgg(555)
newEgg.newEgg()