It is not really related to property initialization, simpler repro:

> let x =$"123";;

  let x =$"123";;
  ------^^

stdin(2,7): error FS0035: This construct is deprecated: '$' is not permitted as a character in operator names and is reserved for future use

and not so much to interpolated strings either:

> let x =-1;;

  let x =-1;;
  ------^^

stdin(3,7): error FS0010: Unexpected infix operator in binding. Expected '=' or other token.

Well, the last one I think is a different thing, and correctly reported as error.
Actually, if I think about it, also =$ is probably according to F# syntax recognized as an operator, and correctly rejected.
Same thing as let x = 1+-2.
A better error message might be in order.
(In any case, following the formatting guidelines and using Fantomas is helpful ;-))

@Martin521 it is kind of the same thing, because in both cases lexer thinks it sees INFIX_COMPARE_OP, in one case it throws an error there because $ is not allowed there, in the other case, I am actually not sure when the error is thrown (after lexing I think), but it stems from the way this code got lexed IMO.

And yeah, not quite sure if this is a bug tbh.

@abonie Property initialisation is a case where skipping spaces around = makes the most sense. I've met it when writing html dsl, so it looks like <img src="img_girl.jpg"> , note no spaces around equals operator.

I think this can be fixed following the same approach than https://github.com/dotnet/fsharp/pull/15923/files

This is really bad TBH. another glitch that should be addressed

This is really bad TBH. another glitch that should be addressed

Easier said than done. It is lexed and parsed like an operator, so it needs be "special cased" for binding, fields and properties assignments (also, how should $"foo"=$"bar" be treated in this case?), but I'm pretty sure by the time we parsed it and produced this diagnostics, there's no "way back" to treating it as assignment.

One possible solution is for us to "deprecate" $ for good, removing it from lexing, so it's never treated as part of the operator, but this will need suggestion/discussion.

@auduchinok thoughts?

Also, I think message in Rider is outdated: