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

[via cfe-commits]

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@@ -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);
----------------
isuckatcs wrote:

This only matches the following form, right?
```c++
struct {
    <enum> e;
    <union> u;
};
```

What happens if an anonymous union is used, as it can be seen in the [wikipedia article](https://en.wikipedia.org/wiki/Tagged_union#1970s_&_1980s) for tagged union?

```c++
enum Tags { INT, FLOAT };

struct TU {
  Tags tags;
  union {
    int i;
    float f;
  };
};
```

Also `isStruct()` won't match `class`. Is it intentional?

I think the checker would benefit from more matchers, or a more generalized one, so that the above cases are detected too.

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