Fix operator warning

[sandro-elsweijer]

Describe your changes here:
Fixes an ambiguity warning which prevents me from compiling this.

ISO C++20 considers use of overloaded operator '==' (with operand types 'const DummyInt' (aka 'const T8Type<int, dummy_int, Addable, Subtractable, AddAssignable, Multipliable, Dividable, PrefixDecrementable, PrefixIncrementable, EqualityComparable, Printable>') and 'const DummyInt') to be ambiguous despite there being a unique best viable function

Stackoverflow explanation:

The specific issue is that in C++20, comparison operators add a new notion of rewritten and reversed candidates. So lookup for the expression a == b will also end up matching operators like b == a. In the typical case, this means you have to write fewer operators, since we know equality is commutative.

But if you have a const-mismatch, what happens is you end up with these two candidates:

bool operator==(/* this*/ A&, A const&); // member function
bool operator==(A const&, /* this*/ A&); // reversed member function

With two arguments of type A. The first candidate is better in the first argument, and the second candidate is better in the second argument. Neither candidate is better than the other, hence ambiguous.

All these boxes must be checked by the reviewers before merging the pull request:

As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually

• The code follows the t8code coding guidelines

• New source/header files are properly added to the Makefiles

• The code is well documented

• All function declarations, structs/classes and their members have a proper doxygen documentation

• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue)

Tests

• The code is covered in an existing or new test case using Google Test

Github action

• The code compiles without warning in debugging and release mode, with and without MPI (this should be executed automatically in a github action)

• All tests pass (in various configurations, this should be executed automatically in a github action)

  If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

  • Should this use case be added to the github action?
  • If not, does the specific use case compile and all tests pass (check manually)

Scripts and Wiki

• If a new directory with source-files is added, it must be covered by the script/find_all_source_files.scp to check the indentation of these files.
• If this PR introduces a new feature, it must be covered in an example/tutorial and a Wiki article.

License

• The author added a BSD statement to doc/ (or already has one)

Tag Label

• The author added the patch/minor/major label in accordance to semantic versioning.

[Davknapp]

Thank you!