Update Function0 syntax

[travisbrown]

Another Dotty-friendliness change. To be honest I don't think I even realized that eta-expansion syntax could be used for Function0 in Scala 2. It doesn't work on Dotty:

4 |  def mk1(x: => String): Thunk = new Thunk(x _)
  |                                           ^^^
  |Only function types can be followed by _ but the current expression has type String

I think it makes sense to change these now. For me at least the () => x syntax is less bizarre and surprising anyway.

I double-checked that they compile to the same thing.

class Thunk(val value: () => String)

object Thunk {
  def mk1(x: => String): Thunk = new Thunk(x _)
  def mk2(x: => String): Thunk = new Thunk(() => x)
}

Becomes:

  public static Thunk mk2(scala.Function0<java.lang.String>);
    descriptor: (Lscala/Function0;)LThunk;
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #21                 // Field Thunk$.MODULE$:LThunk$;
         3: aload_0
         4: invokevirtual #23                 // Method Thunk$.mk2:(Lscala/Function0;)LThunk;
         7: areturn
    Signature: #14                          // (Lscala/Function0<Ljava/lang/String;>;)LThunk;
    MethodParameters:
      Name                           Flags
      x                              final

  public static Thunk mk1(scala.Function0<java.lang.String>);
    descriptor: (Lscala/Function0;)LThunk;
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #21                 // Field Thunk$.MODULE$:LThunk$;
         3: aload_0
         4: invokevirtual #26                 // Method Thunk$.mk1:(Lscala/Function0;)LThunk;
         7: areturn
    Signature: #14                          // (Lscala/Function0<Ljava/lang/String;>;)LThunk;
    MethodParameters:
      Name                           Flags
      x                              final

[codecov-io]

Codecov Report

Merging #3185 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3185   +/-   ##
=======================================
  Coverage   92.99%   92.99%           
=======================================
  Files         376      376           
  Lines        7381     7381           
  Branches      195      195           
=======================================
  Hits         6864     6864           
  Misses        517      517

Flag	Coverage Δ
#scala_version_212	93.31% <100%> (-0.02%)	⬇️
#scala_version_213	90.59% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
core/src/main/scala/cats/Eval.scala	98.83% <100%> (ø)	⬆️

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