automatic eta-expansion should work for SAM expected type and function with single empty parameter clauses

[unkarjedy]

(same as https://github.com/lampepfl/dotty/issues/11414)

reproduction steps

using Scala "3.0.0-M3"

Example

abstractclassSam00{deffoo():Int } abstractclassSam1{deffoo(s: String):Int } deffoo00():Int=1deffoo1(s: String):Int=1vala:Sam00= foo00 valb: () =>Int= foo00 valc:Sam1= foo1 vald:String=>Int= foo1 vale:Sam1= foo1 _ valf:String=>Int= foo1 _

Problem

I would expect automatic eta-expansion work the same way for SAM expected type and Function expected type for functions with empty parameters: whether allow it or disallow
This is also taking into account that it works unified for 1-arity (currently it compiles OK for functions with non-empty parameters, for SAM and Function expected type)

[liufengyun]

Here is the specification for automatic eta-expansion in Scala 3:

• If m has an argument list with one or more parameters, we always eta-expand
• If m is has an empty argument list (i.e. has type ()R):
  1. If the expected type is of the form () => T, we eta expand.
  2. If m is defined by Java, or overrides a Java defined method, we insert ().
  3. Otherwise we issue an error of the form:

Thus, an unapplied method with an empty argument list is only converted to a function when a function type is expected. It is considered best practice to either explicitly apply the method to (), or convert it to a function with () => m().

The nullary methods are explicitly treated differently, due to possible interaction with auto-application.