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Showing content from https://docs.scala-lang.org/scala3/reference/changed-features/operators.html below:

Rules for Operators

The rules for infix operators have changed in some parts:

First, an alphanumeric method can be used as an infix operator only if its definition carries an infix modifier.

Second, it is recommended (but not enforced) to augment definitions of symbolic operators with @targetName annotations.

Finally, a syntax change allows infix operators to be written on the left in a multi-line expression.

An infix modifier on a method definition allows using the method as an infix operation. Example:

import scala.annotation.targetName

trait MultiSet[T]:

  infix def union(other: MultiSet[T]): MultiSet[T]

  def difference(other: MultiSet[T]): MultiSet[T]

  @targetName("intersection")
  def *(other: MultiSet[T]): MultiSet[T]

end MultiSet

val s1, s2: MultiSet[Int]

s1 union s2         // OK
s1 `union` s2       // also OK but unusual
s1.union(s2)        // also OK

s1.difference(s2)   // OK
s1 `difference` s2  // OK
s1 difference s2    // gives a deprecation warning

s1 * s2             // OK
s1 `*` s2           // also OK, but unusual
s1.*(s2)            // also OK, but unusual

Infix operations involving alphanumeric operators are deprecated, unless one of the following conditions holds:

• the operator definition carries an infix modifier, or
• the operator was compiled with Scala 2, or
• the operator is followed by an opening brace.

An alphanumeric operator is an operator consisting entirely of letters, digits, the $ and _ characters, or any Unicode character c for which java.lang.Character.isIdentifierPart(c) returns true.

Infix operations involving symbolic operators are always allowed, so infix is redundant for methods with symbolic names.

The infix modifier can also be given to a type:

infix type or[X, Y]
val x: String or Int = ...

The purpose of the infix modifier is to achieve consistency across a code base in how a method or type is applied. The idea is that the author of a method decides whether that method should be applied as an infix operator or in a regular application. Use sites then implement that decision consistently.

1. infix is a soft modifier. It is treated as a normal identifier except when in modifier position.

2. If a method overrides another, their infix annotations must agree. Either both are annotated with infix, or none of them are.

3. infix modifiers can be given to method definitions. The first non-receiver parameter list of an infix method must define exactly one parameter. Examples:

   infix def op1(x: S): R             // ok
   infix def op2[T](x: T)(y: S): R    // ok
   infix def op3[T](x: T, y: S): R    // error: two parameters
   
   extension (x: A)
     infix def op4(y: B): R          // ok
     infix def op5(y1: B, y2: B): R  // error: two parameters
   

4. infix modifiers can also be given to type, trait or class definitions that have exactly two type parameters. An infix type like

   can be applied using infix syntax, i.e. A op B.

5. To smooth migration to Scala 3.0, alphanumeric operators will only be deprecated from Scala 3.1 onwards, or if the -source future option is given in Dotty/Scala 3.

It is recommended that definitions of symbolic operators carry a @targetName annotation that provides an encoding of the operator with an alphanumeric name. This has several benefits:

• It helps interoperability between Scala and other languages. One can call a Scala-defined symbolic operator from another language using its target name, which avoids having to remember the low-level encoding of the symbolic name.
• It helps legibility of stacktraces and other runtime diagnostics, where the user-defined alphanumeric name will be shown instead of the low-level encoding.
• It serves as a documentation tool by providing an alternative regular name as an alias of a symbolic operator. This makes the definition also easier to find in a search.

Infix operators can now appear at the start of lines in a multi-line expression. Examples:

val str = "hello"
  ++ " world"
  ++ "!"

def condition =
  x > 0
  ||
  xs.exists(_ > 0)
  || xs.isEmpty

Previously, those expressions would have been rejected, since the compiler's semicolon inference would have treated the continuations ++ " world" or || xs.isEmpty as separate statements.

To make this syntax work, the rules are modified to not infer semicolons in front of leading infix operators. A leading infix operator is

• a symbolic identifier such as +, or approx_==, or an identifier in backticks that
• starts a new line, and
• is not following a blank line, and
• is followed by at least one whitespace character and a token that can start an expression.
• Furthermore, if the operator appears on its own line, the next line must have at least the same indentation width as the operator.

Example:

This is recognized as a single infix operation. Compare with:

This is seen as two statements, freezing and !boiling. The difference is that only the operator in the first example is followed by a space.

Another example:

  println("hello")
  ???
  ??? match { case 0 => 1 }

This code is recognized as three different statements. ??? is syntactically a symbolic identifier, but neither of its occurrences is followed by a space and a token that can start an expression.

A unary operator must not have explicit parameter lists even if they are empty. A unary operator is a method named "unary_op" where op is one of +, -, !, or ~.

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