[clang-tidy] Handle unions with existing default-member-init

[malcolm.parsons]

Summary

clang-tidy's modernize-use-default-member-init was crashing for unions
with an existing default member initializer.

Fixes PR40492

[riccibruno]

Just to make sure I understand the issue correctly (and so please correct me if I am wrong):

The union member e leads to an IndirectFieldDecl in the record for UnionExisting. Then CXXCtorInitializer::getMember returns null since the initialized member is not a FieldDecl, but an IndirectFieldDecl. Using CXXCtorInitializer::getAnyMember handles both FieldDecls and IndirectFieldDecls, which solves the issue.

Looks OK but probably better to wait for someone else to give the OK ?

[JonasToth]

LGTM, please close the bug report and notify for the fix with the request to check in the real-project if it actually fixes the issue. Thanks :)

clang-tidy/modernize/UseDefaultMemberInitCheck.cpp

void UseDefaultMemberInitCheck::checkDefaultInit(
   if (!UseAssignment)
     Diag << FixItHint::CreateInsertion(FieldEnd, "}");
 
   Diag << FixItHint::CreateRemoval(Init->getSourceRange());
 }
 
 void UseDefaultMemberInitCheck::checkExistingInit(
     const MatchFinder::MatchResult &Result, const CXXCtorInitializer *Init) {
-  const FieldDecl *Field = Init->getMember();
+  const FieldDecl *Field = Init->getAnyMember();
 
   if (!sameValue(Field->getInClassInitializer(), Init->getInit()))
     return;
 
   diag(Init->getSourceLocation(), "member initializer for %0 is redundant")
       << Field
       << FixItHint::CreateRemoval(Init->getSourceRange());
 }
 
 } // namespace modernize
 } // namespace tidy
 } // namespace clang

test/clang-tidy/modernize-use-default-member-init.cpp

struct ExistingString {
   // CHECK-MESSAGES: :[[@LINE-1]]:48: warning: member initializer for 'e3' is redundant
   // CHECK-FIXES: ExistingString(char) : e1("foo"), e2("foo"),  e4("foo") {}
   const char *e1{};
   const char *e2 = nullptr;
   const char *e3 = "foo";
   const char *e4 = "bar";
 };
 
+struct UnionExisting {
+  UnionExisting() : e(5.0) {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:21: warning: member initializer for 'e' is redundant
+  // CHECK-FIXES: UnionExisting()  {}
+  union {
+    int i;
+    double e = 5.0;
+  };
+};
+
 template <typename T>
 struct NegativeTemplateExisting {
   NegativeTemplateExisting(int) : t(0) {}
   T t{};
 };
 
 NegativeTemplateExisting<int> ntei(0);
 NegativeTemplateExisting<double> nted(0);