[llvm-bugs] [Bug 25901] New: "cannot define the implicit copy assignment operator" error could be more useful

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25901

            Bug ID: 25901
           Summary: "cannot define the implicit copy assignment operator"
                    error could be more useful
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: hfinkel at anl.gov
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

Consider the following:

$ cat /tmp/ao.cpp 
struct Foo;
struct Bar {
  Foo &R;

  Bar(Foo &R) : R(R) {}
};

void test(Bar &B, Foo &F) {
  B = Bar(F);
}


We currently produce this:

$ clang++ -fsyntax-only /tmp/ao.cpp 
/tmp/ao.cpp:2:8: error: cannot define the implicit copy assignment operator for
'Bar', because non-static reference member 'R' cannot use copy assignment
operator
struct Bar {
       ^
/tmp/ao.cpp:3:8: note: declared here
  Foo &R;
       ^
/tmp/ao.cpp:9:5: note: implicit copy assignment operator for 'Bar' first
required here
  B = Bar(F);
    ^
1 error generated.

g++ (4.8.2) produces this:

$ g++ -fsyntax-only /tmp/ao.cpp
/tmp/ao.cpp: In member function ‘Bar& Bar::operator=(const Bar&)’:
/tmp/ao.cpp:2:8: error: non-static reference member ‘Foo& Bar::R’, can’t use
default assignment operator
 struct Bar {
        ^
/tmp/ao.cpp: In function ‘void test(Bar&, Foo&)’:
/tmp/ao.cpp:9:5: note: synthesized method ‘Bar& Bar::operator=(const Bar&)’
first required here 
   B = Bar(F);
     ^

In this case I find g++'s error more useful, primarily because it tells me
exactly what method cannot be implicitly defined ('Bar& Bar::operator=(const
Bar&)' in this case). This information is directly useful to the user (the user
can define exactly that operator method as he or she sees fit in order to solve
the problem). I suggest we do the same (provide the signature of the missing
method).

Second, it might also be useful to:

 1. Explain the source of the problem (it is not possible to rebind a
reference).

 2. Suggest that the user might want to change the reference member to a
pointer member (in my experience, this is the most common solution).

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