[clang-tidy] Handle bitfields in modernize-use-default-member-init if using C++2a

[malcolm.parsons]

Summary

C++2a allows bitfields to have default member initializers.
Add support for this to clang-tidy's modernize-use-default-member-init check.

[aaron.ballman]

clang-tidy/modernize/UseDefaultMemberInitCheck.cpp

165 ↗

(On Diff #131032)

I wonder if we could add a nothing() matcher (sibling to the anything() matcher) so that this could be rewritten as:

unless(anyOf(getLangOpts().CPlusPlus2a ? nothing() : isBitField()))

The duplication here adds complexity if we decide to change the matchers in the future.

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

1 ↗

(On Diff #131032)

I'd like to see a test where C++2a is not enabled to ensure that the fix is not suggested in that case.

[malcolm.parsons]

Use ternary operator.

clang-tidy/modernize/UseDefaultMemberInitCheck.cpp

165 ↗

(On Diff #131032)

unless(anything()) is equivalent to nothing().

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

1 ↗

(On Diff #131032)

See modernize-use-default-member-init.cpp:222

[aaron.ballman]

LGTM!

clang-tidy/modernize/UseDefaultMemberInitCheck.cpp

165 ↗

(On Diff #131032)

Ah, good point! Nice, this is much more clean.

clang-tools-extra/trunk/clang-tidy/modernize/UseDefaultMemberInitCheck.cpp

auto Init =
             cxxBoolLiteral(), cxxNullPtrLiteralExpr(), implicitValueInitExpr(),
             declRefExpr(to(enumConstantDecl())));
 
   Finder->addMatcher(
       cxxConstructorDecl(
           isDefaultConstructor(), unless(isInstantiated()),
           forEachConstructorInitializer(
               cxxCtorInitializer(
-                  forField(unless(anyOf(isBitField(),
+                  forField(unless(anyOf(getLangOpts().CPlusPlus2a
+                                            ? unless(anything())
+                                            : isBitField(),
                                         hasInClassInitializer(anything()),
                                         hasParent(recordDecl(isUnion()))))),
                   isWritten(), withInitializer(ignoringImplicit(Init)))
                   .bind("default"))),
       this);
 
   Finder->addMatcher(
       cxxConstructorDecl(

clang-tools-extra/trunk/test/clang-tidy/modernize-use-default-member-init-bitfield.cpp

+// RUN: %check_clang_tidy %s modernize-use-default-member-init %t -- -- -std=c++2a
+
+struct PositiveBitField
+{
+  PositiveBitField() : i(6) {}
+  // CHECK-FIXES: PositiveBitField()  {}
+  int i : 5;
+  // CHECK-MESSAGES: :[[@LINE-1]]:7: warning: use default member initializer for 'i' [modernize-use-default-member-init]
+  // CHECK-FIXES: int i : 5{6};
+};