Support command overloading in a binary compatible fashion (#5376)

Fixes  #5281

The previous code already collected all command overloads in the
`Discover` data structure, but it returned one arbitrarily when queried.
This PR makes it return the one with the longer argument list, which
supports `@unroll`-style signature evolution of adding parameters with
default values to the right side of a parameter list.

Added a unit test that fails on main, passes on this PR

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: lihaoyi

core/define/src/mill/define/Discover.scala

@@ -23,7 +23,11 @@ class Discover(val classInfo: Map[Class[?], Discover.ClassInfo]) {
       if ep.mainName.getOrElse(ep.defaultName) == name
     } yield ep
 
-    res.headOption
+    // When there are multiple entrypoints matching the class and name,
+    // return the one with the longest argument lists, since users may
+    // add additional arguments to a command (with default values) to
+    // preserve binary compatibility while evolving the method signature
+    res.maxByOption(_.argSigs0.length)
   }
 }
 

core/exec/test/src/mill/exec/ExecutionTests.scala

@@ -45,6 +45,12 @@ object ExecutionTests extends TestSuite {
     lazy val millDiscover = Discover[this.type]
   }
 
+  object overloads extends TestRootModule {
+    def overloaded(x: Int) = Task.Command { x + 1 }
+    def overloaded(x: Int, y: Int = 0) = Task.Command { x + y + 1 }
+    lazy val millDiscover = Discover[this.type]
+  }
+
   class Checker[T <: mill.testkit.TestRootModule](module: T)
       extends exec.Checker(module)
 
@@ -448,5 +454,20 @@ object ExecutionTests extends TestSuite {
         assert(res.executionResults.transitiveFailing.keySet == Set(anonTaskFailure.task))
       }
     }
+    test("overloaded") {
+      UnitTester(overloads, null).scoped { tester =>
+        val res = tester.apply(Seq(overloads.overloaded(1)))
+        assert(res == Right(UnitTester.Result(Vector(2), 1)))
+
+        val res2 = tester.apply(Seq(overloads.overloaded(1, 2)))
+        assert(res2 == Right(UnitTester.Result(Vector(4), 1)))
+
+        val res3 = tester.apply("overloaded", "-x", "1")
+        assert(res3 == Right(UnitTester.Result(Vector(2), 1)))
+
+        val res4 = tester.apply("overloaded", "-x", "1", "-y", "2")
+        assert(res4 == Right(UnitTester.Result(Vector(4), 1)))
+      }
+    }
   }
 }