[clang-tools-extra] [clang-tidy] Add new check bugprone-tagged-union-member-count (PR #89925)

[Piotr Zegar via cfe-commits]

================
@@ -0,0 +1,125 @@
+//===--- TaggedUnionMemberCountCheck.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 "TaggedUnionMemberCountCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/Expr.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+#include "clang/AST/PrettyPrinter.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/Casting.h"
+#include <limits>
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::bugprone {
+
+void TaggedUnionMemberCountCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(
+      recordDecl(
+          allOf(isStruct(),
+                has(fieldDecl(hasType(recordDecl(isUnion()).bind("union")))),
+                has(fieldDecl(hasType(enumDecl().bind("tags"))))))
+          .bind("root"),
+      this);
+}
+
+static bool hasMultipleUnionsOrEnums(const RecordDecl *rec) {
+  int tags = 0;
+  int unions = 0;
+  for (const FieldDecl *r : rec->fields()) {
+    TypeSourceInfo *info = r->getTypeSourceInfo();
+    QualType qualtype = info->getType();
+    const Type *type = qualtype.getTypePtr();
+    if (type->isUnionType())
+      unions += 1;
+    else if (type->isEnumeralType())
+      tags += 1;
+    if (tags > 1 || unions > 1)
+      return true;
+  }
+  return false;
+}
+
+static int64_t getNumberOfValidEnumValues(const EnumDecl *ed) {
+  int64_t maxTagValue = std::numeric_limits<int64_t>::min();
+  int64_t minTagValue = std::numeric_limits<int64_t>::max();
+
+  // Heuristic for counter enum constants.
+  //
+  //   enum tag_with_counter {
+  //     tag1,
+  //     tag2,
+  //     tag_count, <-- Searching for these enum constants
+  //   };
+  //
+  // The 'ce' prefix is used to abbreviate counterEnum.
+  // The final tag count is decreased by 1 if and only if:
+  // 1. The number of counting enum constants = 1,
+  int ceCount = 0;
+  // 2. The counting enum constant is the last enum constant that is defined,
+  int ceFirstIndex = 0;
+  // 3. The value of the counting enum constant is the largest out of every enum constant.
+  int64_t ceValue = 0;
+
+  int64_t enumConstantsCount = 0;
+  for (auto En : llvm::enumerate(ed->enumerators())) {
+    enumConstantsCount += 1;
+
+    int64_t enumValue = En.value()->getInitVal().getExtValue();
+    StringRef enumName = En.value()->getName();
+
+    if (enumValue > maxTagValue)
+      maxTagValue = enumValue;
+    if (enumValue < minTagValue)
+      minTagValue = enumValue;
+
+    if (enumName.ends_with_insensitive("count")) {
+      if (ceCount == 0) {
+        ceFirstIndex = En.index();
+      }
+      ceValue = enumValue;
+      ceCount += 1;
+    }
+  }
+
+  int64_t validValuesCount = maxTagValue - minTagValue + 1;
+  if (ceCount == 1 &&
+      ceFirstIndex == enumConstantsCount - 1 &&
+      ceValue == maxTagValue) {
+    validValuesCount -= 1;
+  }
+  return validValuesCount;
+}
+
+void TaggedUnionMemberCountCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  const auto *root = Result.Nodes.getNodeAs<RecordDecl>("root");
+  const auto *unionMatch = Result.Nodes.getNodeAs<RecordDecl>("union");
+  const auto *tagMatch = Result.Nodes.getNodeAs<EnumDecl>("tags");
+
+  if (hasMultipleUnionsOrEnums(root))
+    return;
+
+  int64_t unionMemberCount = llvm::range_size(unionMatch->fields());
+  int64_t tagCount = getNumberOfValidEnumValues(tagMatch);
+
+  // FIXME: Maybe a emit a note when a counter enum constant was found.
+  if (unionMemberCount > tagCount) {
+    diag(root->getLocation(), "Tagged union has more data members than tags! "
+                              "Data members: %0 Tags: %1")
----------------
PiotrZSL wrote:

should be more a:
"tagged union has more data members (%0) than tags (%1)"
some suggestion what to do would be also nice.

https://github.com/llvm/llvm-project/pull/89925