[llvm-bugs] [Bug 42196] New: -Wunused-member-function default move

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Sat Jun 8 12:29:08 PDT 2019


            Bug ID: 42196
           Summary: -Wunused-member-function default move
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: m-llvm at bodyfour.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Note this is similar to #25084, but I believe it is distinct, since it involves
-Wunused-member-function instead of -Wunneeded-member-function

Take this code for example:

namespace {

class F {
        explicit F(int x) : x_(x)
        F(const F&) = default;
        F& operator=(const F&) = default;
        F(F&&) = default;
        F& operator=(F&&) = default;

        int get() const { return x_; }

        int x_;


int main(void)
        F f(123);
        return f.get();

Compiling it reveals these warnings:

$ bin/clang --std=c++11 -fsyntax-only -Wunused-member-function
rvalue-moveop-unused.cpp:10:2: warning: unused member function 'F'
        F(F&&) = default;
rvalue-moveop-unused.cpp:11:5: warning: unused member function 'operator='
        F& operator=(F&&) = default;

So it complains about the "=default" move operator/ctor being unused. 
Interestingly, it does not do the same for the "=default" *copy* operator/ctor
even though it's just as unused.

Ultimately this warning isn't very useful, since it's complaining about a
function that isn't actually defined but just indicates that the same copy/move
that the class gets by default can be used.

This is especially annoying when these annotations are added via compiler
macros, since it means you get warnings whenever a class is put into the
anonymous namespace.

I'm far from an expert on clang internals, but I can see that the warning is
generated by lib/Sema/Sema.cpp which does:

  if (!Diags.hasErrorOccurred() && TUKind != TU_Module) {
      if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(*I)) {
        const FunctionDecl *DiagD;
        if (!FD->hasBody(DiagD))
          DiagD = FD;
        if (DiagD->isDeleted())
          continue; // Deleted functions are supposed to be unused.

So maybe it should also include ->isDeleted() there?  Or maybe ->isTrivial()
too?  But it's not clear to me why this affects move-ctor and not copy-ctor.

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