[clang-tools-extra] [clang-tidy] Unsafe CRTP check (PR #82403)

[Piotr Zegar via cfe-commits]

================
@@ -0,0 +1,177 @@
+//===--- CrtpConstructorAccessibilityCheck.cpp - clang-tidy
+//---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "CrtpConstructorAccessibilityCheck.h"
+#include "../utils/LexerUtils.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::bugprone {
+
+static bool hasPrivateConstructor(const CXXRecordDecl *RD) {
+  return llvm::any_of(RD->ctors(), [](const CXXConstructorDecl *Ctor) {
+    return Ctor->getAccess() == AS_private;
+  });
+}
+
+static bool isDerivedParameterBefriended(const CXXRecordDecl *CRTP,
+                                         const NamedDecl *Param) {
+  return llvm::any_of(CRTP->friends(), [&](const FriendDecl *Friend) {
+    const auto *TTPT =
+        dyn_cast<TemplateTypeParmType>(Friend->getFriendType()->getType());
+
+    return TTPT && TTPT->getDecl() == Param;
+  });
+}
+
+static bool isDerivedClassBefriended(const CXXRecordDecl *CRTP,
+                                     const CXXRecordDecl *Derived) {
+  return llvm::any_of(CRTP->friends(), [&](const FriendDecl *Friend) {
+    return Friend->getFriendType()->getType()->getAsCXXRecordDecl() == Derived;
+  });
+}
+
+static const NamedDecl *
+getDerivedParameter(const ClassTemplateSpecializationDecl *CRTP,
+                    const CXXRecordDecl *Derived) {
+  size_t Idx = 0;
+  const bool AnyOf = llvm::any_of(
+      CRTP->getTemplateArgs().asArray(), [&](const TemplateArgument &Arg) {
+        ++Idx;
+        return Arg.getKind() == TemplateArgument::Type &&
+               Arg.getAsType()->getAsCXXRecordDecl() == Derived;
+      });
+
+  return AnyOf ? CRTP->getSpecializedTemplate()
+                     ->getTemplateParameters()
+                     ->getParam(Idx - 1)
+               : nullptr;
+}
+
+static std::vector<FixItHint>
+hintMakeCtorPrivate(const CXXConstructorDecl *Ctor,
+                    const std::string &OriginalAccess) {
+  std::vector<FixItHint> Hints;
+
+  Hints.emplace_back(FixItHint::CreateInsertion(
+      Ctor->getBeginLoc().getLocWithOffset(-1), "private:\n"));
+
+  const ASTContext &ASTCtx = Ctor->getASTContext();
+  const SourceLocation CtorEndLoc =
+      Ctor->isExplicitlyDefaulted()
+          ? utils::lexer::findNextTerminator(Ctor->getEndLoc(),
+                                             ASTCtx.getSourceManager(),
+                                             ASTCtx.getLangOpts())
+          : Ctor->getEndLoc();
+  Hints.emplace_back(FixItHint::CreateInsertion(
+      CtorEndLoc.getLocWithOffset(1), '\n' + OriginalAccess + ':' + '\n'));
+
+  return Hints;
+}
+
+void CrtpConstructorAccessibilityCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      classTemplateSpecializationDecl(
+          decl().bind("crtp"),
+          hasAnyTemplateArgument(refersToType(recordType(hasDeclaration(
+              cxxRecordDecl(
+                  isDerivedFrom(cxxRecordDecl(equalsBoundNode("crtp"))))
+                  .bind("derived")))))),
+      this);
+}
+
+void CrtpConstructorAccessibilityCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  const auto *CRTPInstantiation =
+      Result.Nodes.getNodeAs<ClassTemplateSpecializationDecl>("crtp");
+  const auto *DerivedRecord = Result.Nodes.getNodeAs<CXXRecordDecl>("derived");
+  const CXXRecordDecl *CRTPDeclaration =
+      CRTPInstantiation->getSpecializedTemplate()->getTemplatedDecl();
+
+  const auto *DerivedTemplateParameter =
+      getDerivedParameter(CRTPInstantiation, DerivedRecord);
+
+  assert(DerivedTemplateParameter &&
+         "No template parameter corresponds to the derived class of the CRTP.");
+
+  bool NeedsFriend = !isDerivedParameterBefriended(CRTPDeclaration,
+                                                   DerivedTemplateParameter) &&
+                     !isDerivedClassBefriended(CRTPDeclaration, DerivedRecord);
+
+  const FixItHint HintFriend = FixItHint::CreateInsertion(
+      CRTPDeclaration->getBraceRange().getEnd(),
+      "friend " + DerivedTemplateParameter->getNameAsString() + ';' + '\n');
+
+  if (hasPrivateConstructor(CRTPDeclaration) && NeedsFriend) {
+    diag(CRTPDeclaration->getLocation(),
+         "the CRTP cannot be constructed from the derived class")
+        << CRTPDeclaration << HintFriend;
+    diag(CRTPDeclaration->getLocation(),
----------------
PiotrZSL wrote:

just merge this diagnostic with previous one:
`the CRTP cannot be constructed from the derived class; consider declaring the derived class as friend`

https://github.com/llvm/llvm-project/pull/82403