"override members" idea as a gateway to UFCS (language evolution)

(UFCS backgrounder: https://isocpp.org/files/papers/N4174.pdf )

I have two functions that tell me if a string contains the
characters of a particular integer. They're called hasInt and intIn.
(intIn is inspired by the python keyword in.)
They looks like this:

bool hasInt(const string s, int n)// does s have n?
{
return s.contains(tostring(n));
}

bool intIn(int n, const string s)// is n in s?
{
return s.contains(tostring(n));
}

It would be convenient if I could add hasInt as a member function to std::string:

bool string::hasInt(int n)
{
return ::hasInt(this, n);
}

Then I could use "member syntax" to call the function, like `text.hasInt(123)`.

Of course, that's not possible, because then I'd be changing the header files
in the standard libraries.

Here's an idea for a new language feature:
let's use the `override` keyword to allow us to "inject" member functions
into an existing class, without modifying the class definition. So the code:

override bool string::hasInt(int n)
{
return ::hasInt(this, n);
}

will (in effect) add hasInt as a member function to string.

Thus, this "override member function" feature has a syntax like:

ReturnType ClassName::function(args){...etc...}

HOWEVER..... what if ClassName doesn't necessarily need to be a class, and could be
other types? Then you open the door to override members like:

override bool int::intIn(const string s)
{
return ::intIn(this, s);
}

Which allows code like `(123).intIn(text)`.

This is halfway to UFCS!

Using some macro magic and helper templates, we could define a
MAKE_UFCS macro to convert a non-member function into a member function:

#define MAKE_UFCS(f) \
override \
retType(f) argType1(f)::f(argType2(f) x)\
{ \
return f(this, x); \
}

Thus the non-member functions hasInt and intIn could be "opted in" to UFCS
by the macro calls:

MAKEUFCS(hasInt);
MAKEUFCS(intIn);

Or maybe, if this override-to-UFCS is useful enough, the override feature can be
applied to a collection of functions at once, like:

override hasInt, intIn;

or

override {
#include <cstdlib>
}

To UFCS-ify an entire header file at the same time.

EDIT: this idea would be similar to Scala's "Extension Methods": https://docs.scala-lang.org/scala3/book/ca-extension-methods.html

or C#'s "Extension Members": https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods

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