Elide unit binding when beta-reducing

See #20082 (comment)

compiler/src/dotty/tools/dotc/transform/BetaReduce.scala

@@ -8,6 +8,7 @@ import MegaPhase.*
 import Symbols.*, Contexts.*, Types.*, Decorators.*
 import StdNames.nme
 import ast.TreeTypeMap
+import Constants.Constant
 
 import scala.collection.mutable.ListBuffer
 
@@ -131,6 +132,7 @@ object BetaReduce:
             val tpe =
               if arg.tpe.isBottomType then param.tpe.widenTermRefExpr
               else if arg.tpe.dealias.isInstanceOf[ConstantType] then arg.tpe.dealias
+              else if arg.tpe.dealias =:= defn.UnitType then ConstantType(Constant(()))
               else arg.tpe.widen
             val binding = ValDef(newSymbol(ctx.owner, param.name, flags, tpe, coord = arg.span), arg).withSpan(arg.span)
             if !(tpe.isInstanceOf[ConstantType] && isPureExpr(arg)) then

compiler/test/dotty/tools/backend/jvm/InlineBytecodeTests.scala

@@ -765,4 +765,24 @@ class InlineBytecodeTests extends DottyBytecodeTest {
           diffInstructions(instructions1, instructions2))
     }
   }
+
+  @Test def beta_reduce_elide_unit_binding = {
+    val source = """class Test:
+                   |  def test = ((u: Unit) => u).apply(())
+                 """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn      = dir.lookupName("Test.class", directory = false).input
+      val clsNode    = loadClassNode(clsIn)
+
+      val fun = getMethod(clsNode, "test")
+      val instructions = instructionsFromMethod(fun)
+      val expected = List(Op(RETURN))
+
+      assert(instructions == expected,
+        "`i was not properly beta-reduced in `test`\n" + diffInstructions(instructions, expected))
+
+    }
+  }
+
 }