[clang-tidy] Fix bugprone-tagged-union-member-count false-positive

Types from system headers and the std namespace are no longer considered as
the enum part or the union part of a user-defined tagged union.

Fixes #134840

Author: tigbr

clang-tools-extra/clang-tidy/bugprone/TaggedUnionMemberCountCheck.cpp

@@ -105,11 +105,15 @@ void TaggedUnionMemberCountCheck::storeOptions(
 
 void TaggedUnionMemberCountCheck::registerMatchers(MatchFinder *Finder) {
 
-  auto UnionField = fieldDecl(hasType(qualType(
-      hasCanonicalType(recordType(hasDeclaration(recordDecl(isUnion())))))));
+  auto NotFromSystemHeaderOrStdNamespace =
+      unless(anyOf(isExpansionInSystemHeader(), isInStdNamespace()));
 
-  auto EnumField = fieldDecl(hasType(
-      qualType(hasCanonicalType(enumType(hasDeclaration(enumDecl()))))));
+  auto UnionField =
+      fieldDecl(hasType(qualType(hasCanonicalType(recordType(hasDeclaration(
+          recordDecl(isUnion(), NotFromSystemHeaderOrStdNamespace)))))));
+
+  auto EnumField = fieldDecl(hasType(qualType(hasCanonicalType(
+      enumType(hasDeclaration(enumDecl(NotFromSystemHeaderOrStdNamespace)))))));
 
   auto HasOneUnionField = fieldCountOfKindIsOne(UnionField, UnionMatchBindName);
   auto HasOneEnumField = fieldCountOfKindIsOne(EnumField, TagMatchBindName);

clang-tools-extra/docs/clang-tidy/checks/bugprone/tagged-union-member-count.rst

@@ -9,6 +9,9 @@ different from the number of data members inside the union.
 A struct or a class is considered to be a tagged union if it has
 exactly one union data member and exactly one enum data member and
 any number of other data members that are neither unions or enums.
+The union and enum data members that are from system header files or
+the std namespace are not considered to make up the tagged union part
+of a user-defined tagged union type.
 
 Example:
 
@@ -28,6 +31,25 @@ Example:
     } Data;
   };
 
+The following example illustrates the exception for unions and enums from
+system header files and the std namespace.
+
+.. code-block:: c++
+
+  #include <pthread.h>
+
+  struct NotTaggedUnion {
+    enum MyEnum { MyEnumConstant1, MyEnumConstant2 } En;
+    pthread_mutex_t Mutex;
+  };
+
+The pthread_mutex_t type may be defined as a union behind a typedef,
+in which case the check could mistake this type as a user-defined tagged union.
+After all it has exactly one enum data member and exactly one union data member.
+To avoid false-positive cases originating from this, unions and enums from
+system headers and the std namespace are ignored when pinpointing the
+union part and the enum part of a potential user-defined tagged union.
+
 How enum constants are counted
 ------------------------------
 

clang-tools-extra/test/clang-tidy/checkers/bugprone/tagged-union-member-count.c

@@ -147,3 +147,16 @@ struct Name {\
 
 // CHECK-MESSAGES: :[[@LINE+1]]:44: warning: tagged union has more data members (4) than tags (3)
 DECLARE_TAGGED_UNION_STRUCT(Tags3, Union4, TaggedUnionStructFromMacro);
+
+// Typedefed unions from system header files should be ignored when
+// we are trying to pinpoint the union part in a user-defined tagged union.
+#include "pthread.h"
+
+// This should not be analyzed as a user-defined tagged union,
+// even though pthread_mutex_t may be declared as a typedefed union.
+struct SystemTypedefedUnionDataMemberShouldBeIgnored {
+  pthread_mutex_t Mutex;
+  enum {
+    MyEnum
+  } EnumField;
+};

clang-tools-extra/test/clang-tidy/checkers/bugprone/tagged-union-member-count.cpp

@@ -308,3 +308,16 @@ void DoNotMatchLambdas() {
 	} u;
     auto L = [e, u] () {};
 }
+
+// Typedefed unions from system header files should be ignored when
+// we are trying to pinpoint the union part in a user-defined tagged union.
+#include "pthread.h"
+
+// This should not be analyzed as a user-defined tagged union,
+// even though pthread_mutex_t may be declared as a typedefed union.
+struct SystemTypedefedUnionDataMemberShouldBeIgnored {
+  pthread_mutex_t Mutex;
+  enum {
+    MyEnum
+  } EnumField;
+};

clang-tools-extra/test/clang-tidy/checkers/bugprone/tagged-union-member-count.m

@@ -147,3 +147,16 @@
 
 // CHECK-MESSAGES: :[[@LINE+1]]:44: warning: tagged union has more data members (4) than tags (3)
 DECLARE_TAGGED_UNION_STRUCT(Tags3, Union4, TaggedUnionStructFromMacro);
+
+// Typedefed unions from system header files should be ignored when
+// we are trying to pinpoint the union part in a user-defined tagged union.
+#include "pthread.h"
+
+// This should not be analyzed as a user-defined tagged union,
+// even though pthread_mutex_t may be declared as a typedefed union.
+struct SystemTypedefedUnionDataMemberShouldBeIgnored {
+  pthread_mutex_t Mutex;
+  enum {
+    MyEnum
+  } EnumField;
+};

clang-tools-extra/test/clang-tidy/checkers/bugprone/tagged-union-member-count.mm

@@ -307,3 +307,16 @@ void DoNotMatchLambdas() {
 	} u;
     auto L = [e, u] () {};
 }
+
+// Typedefed unions from system header files should be ignored when
+// we are trying to pinpoint the union part in a user-defined tagged union.
+#include "pthread.h"
+
+// This should not be analyzed as a user-defined tagged union,
+// even though pthread_mutex_t may be declared as a typedefed union.
+struct SystemTypedefedUnionDataMemberShouldBeIgnored {
+  pthread_mutex_t Mutex;
+  enum {
+    MyEnum
+  } EnumField;
+};