[Bug c++/97584] New: ADL inconsistency when calling the stream operator with x << y or with operator<<(x,y)

[andrea_iob at hotmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97584

            Bug ID: 97584
           Summary: ADL inconsistency when calling the stream operator
                    with x << y or with operator<<(x,y)
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andrea_iob at hotmail dot com
  Target Milestone: ---

Created attachment 49442
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49442&action=edit
Example code that shows the problem.

This started as a possible bug in clang, but it seems it may be a bug in gcc
instead. 

In the attached example I'm declaring a template stream (<<) operator, then I'm
declaring a non-template operator in a different namespace.

Depending on how I call the stream operator, x << y vs operator<<(x, y), I get
two different behaviours: in the first case the non-template operator is called
whereas in the latter the template operator is called.

    # g++ -o test test.cpp 
    # ./test 
     Using x << y syntax:
      > NON-TEMPLATE STREAM OPERATOR FOR CLASS data::Data
     Using operator<<(x, y) syntax:
      > TEMPLATE STREAM OPERATOR

(I've tried gcc 10.2.1, gcc 7.5.0, and gcc 4.8.5.)

Running the same example with clang I get a consistent behaviour: the template
stream operator is selected.

    # clang++ -o test test.cpp 
    # ./test 
     Using x << y syntax:
      > TEMPLATE STREAM OPERATOR
     Using operator<<(x, y) syntax:
      > TEMPLATE STREAM OPERATOR

This is the clang bugzilla entry: https://bugs.llvm.org/show_bug.cgi?id=47967

Let me know if further details are required.
Thanks.