[Bug c++/106837] New: False compilation error "inconsistent begin/end types in range-based 'for' statement"

[ofekshilon at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106837

            Bug ID: 106837
           Summary: False compilation error "inconsistent begin/end types
                    in range-based 'for' statement"
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ofekshilon at gmail dot com
  Target Milestone: ---

The following fails to build with all gcc versions at least since 6.1, when
built with "-std=c++17" or (for supporting versions) with "-std=c++20".  

  struct myDat {
    myDat() {}
    struct Iter {
      Iter &operator++();
      int operator*();
      bool operator!=(int& other);
    };
    Iter begin() ;
    int end() ;
  };

int main() { 
    for (int pos : myDat()) 
        ; 
}

Here's a godbolt link, with comparison to a successful build by clang:
https://godbolt.org/z/1fsfvxzb7


Note this patch by Jason Merrill:
https://gcc.gnu.org/legacy-ml/gcc-patches/2016-03/msg00792.html

+           {
+             if (cxx_dialect >= cxx1z
+                 && (build_x_binary_op (input_location, NE_EXPR,
+                                        *begin, ERROR_MARK,
+                                        *end, ERROR_MARK,
+                                        NULL, tf_none)
+                     != error_mark_node))
+               /* P08184R0 allows __begin and __end to have different types,
+                  but make sure they are comparable so we can give a better
+                  diagnostic.  */;
+             else
+               error ("inconsistent begin/end types in range-based %<for%> "
+                      "statement: %qT and %qT",
+                      TREE_TYPE (*begin), TREE_TYPE (*end));
+           }

In this case the begin/end iter types *are* comparable, but the
build_x_binary_op test fails.