Fix wrong FixIt about union in cppcoreguidelines-pro-type-member-init

At most one variant member of a union may have a default member
initializer. The case of anonymous records with multiple levels of
nesting like the following also needs to meet this rule. The original
logic is to horizontally obtain all the member variables in a record
that need to be initialized and then filter to the variables that need
to be fixed. Obviously, it is impossible to correctly initialize the
desired variables according to the nesting relationship.

See Example 3 in class.union

union U {
  U() {}
  int x;  // int x{};
  union {
    int k;  // int k{};  <==  wrong fix
  };
  union {
    int z;  // int z{};  <== wrong fix
    int y;
  };
};

clang-tools-extra/clang-tidy/cppcoreguidelines/ProTypeMemberInitCheck.cpp

@@ -44,6 +44,23 @@ void forEachField(const RecordDecl &Record, const T &Fields, Func &&Fn) {
   }
 }
 
+template <typename T, typename Func>
+void forEachFieldWithFilter(const RecordDecl &Record, const T &Fields,
+                            bool &AnyMemberHasInitPerUnion, Func &&Fn) {
+  for (const FieldDecl *F : Fields) {
+    if (F->isAnonymousStructOrUnion()) {
+      if (const CXXRecordDecl *R = F->getType()->getAsCXXRecordDecl()) {
+        AnyMemberHasInitPerUnion = false;
+        forEachFieldWithFilter(*R, R->fields(), AnyMemberHasInitPerUnion, Fn);
+      }
+    } else {
+      Fn(F);
+    }
+    if (Record.isUnion() && AnyMemberHasInitPerUnion)
+      break;
+  }
+}
+
 void removeFieldsInitializedInBody(
     const Stmt &Stmt, ASTContext &Context,
     SmallPtrSetImpl<const FieldDecl *> &FieldDecls) {
@@ -461,8 +478,9 @@ void ProTypeMemberInitCheck::checkMissingMemberInitializer(
   // Collect all fields but only suggest a fix for the first member of unions,
   // as initializing more than one union member is an error.
   SmallPtrSet<const FieldDecl *, 16> FieldsToFix;
-  SmallPtrSet<const RecordDecl *, 4> UnionsSeen;
-  forEachField(ClassDecl, OrderedFields, [&](const FieldDecl *F) {
+  bool AnyMemberHasInitPerUnion = false;
+  forEachFieldWithFilter(ClassDecl, ClassDecl.fields(),
+                         AnyMemberHasInitPerUnion, [&](const FieldDecl *F) {
     if (!FieldsToInit.count(F))
       return;
     // Don't suggest fixes for enums because we don't know a good default.
@@ -471,8 +489,8 @@ void ProTypeMemberInitCheck::checkMissingMemberInitializer(
     if (F->getType()->isEnumeralType() ||
         (!getLangOpts().CPlusPlus20 && F->isBitField()))
       return;
-    if (!F->getParent()->isUnion() || UnionsSeen.insert(F->getParent()).second)
-      FieldsToFix.insert(F);
+    FieldsToFix.insert(F);
+    AnyMemberHasInitPerUnion = true;
   });
   if (FieldsToFix.empty())
     return;

clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines-pro-type-member-init.cpp

@@ -516,3 +516,39 @@ struct PositiveDefaultConstructorOutOfDecl {
 
 PositiveDefaultConstructorOutOfDecl::PositiveDefaultConstructorOutOfDecl() = default;
 // CHECK-MESSAGES: :[[@LINE-1]]:1: warning: constructor does not initialize these fields: F
+
+union U1 {
+  U1() {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: union constructor should initialize one of these fields: X, K, Z, Y
+  int X;
+  // CHECK-FIXES: int X{};
+  union {
+    int K;
+    // CHECK-FIXES-NOT: int K{};
+  };
+  union {
+    int Z;
+    // CHECK-FIXES-NOT: int Z{};
+    int Y;
+    // CHECK-FIXES-NOT: int Y{};
+  };
+};
+
+union U2 {
+  U2() {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: union constructor should initialize one of these fields: B, C, A
+  struct {
+    int B;
+    // CHECK-FIXES: int B{};
+    union {
+      struct {
+        PositiveMultipleConstructors Value;
+        // CHECK-FIXES-NOT: PositiveMultipleConstructors Value{};
+      };
+      int C;
+      // CHECK-FIXES: int C{};
+    };
+  };
+  int A;
+  // CHECK-FIXES-NOT: int A{};
+};