underscores for routine parameters (#21192)

* underscores for routine parameters

fixes #13443, fixes #13804, refs #21121

* add changelog + more tests

* support generics and ensure inferred lambdas work

tests/proc/tunderscoreparam.nim

@@ -0,0 +1,109 @@
+discard """
+  targets: "c cpp js"
+"""
+
+import std/[assertions, sequtils]
+
+proc test() =
+  block:
+    proc ok(_, _, a: int): int =
+      doAssert not compiles(_)
+      a
+    doassert ok(4, 2, 5) == 5
+
+  block:
+    proc ok(_: int, _: int, a: int): int = a
+    doAssert ok(4, 2, 5) == 5
+
+  block:
+    proc ok(_: int, _: float, a: int): int = a
+    doAssert ok(1, 2.0, 5) == 5
+
+  block:
+    proc ok(_: int, _: float, _: string, a: int): int = a
+    doAssert ok(1, 2.6, "5", 5) == 5
+    
+  block:
+    proc ok[T](_, _, a: T): T =
+      doAssert not compiles(_)
+      a
+    doAssert ok(4, 2, 5) == 5
+    doAssert ok("a", "b", "c") == "c"
+    doAssert not compiles(ok(1, 2, "a"))
+  
+  block:
+    let ok = proc (_, _, a: int): int =
+      doAssert not compiles(_)
+      a
+    doAssert ok(4, 2, 5) == 5
+  
+  block:
+    proc foo(lam: proc (_, _, a: int): int): int =
+      lam(4, 2, 5)
+    doAssert foo(proc (_, _, a: auto): auto =
+      doAssert not compiles(_)
+      a) == 5
+    
+  block:
+    iterator fn(_, _: int, c: int): int = yield c
+    doAssert toSeq(fn(1,2,3)) == @[3]
+
+  block:
+    template ok(_, _, a: int): int = a
+    doAssert ok(4, 2, 5) == 5
+
+  block:
+    doAssert not (compiles do:
+      template bad(_: int): int = _
+      discard bad(3))
+
+  block:
+    template ok(_: int, _: int, a: int): int = a
+    doAssert ok(4, 2, 5) == 5
+
+  block:
+    template ok(_: int, _: float, a: int): int = a
+    doAssert ok(1, 2.0, 5) == 5
+
+  block:
+    template ok(_: int, _: float, _: string, a: int): int = a
+    doAssert ok(1, 2.6, "5", 5) == 5
+  
+  block:
+    template main2() =
+      iterator fn(_, _: int, c: int): int = yield c
+    main2()
+
+  block:
+    template main =
+      proc foo(_: int) =
+        let a = _
+    doAssert not compiles(main())
+
+proc closureTest() =
+  var x = 0
+
+  block:
+    proc foo(_, _: int) = x += 5
+
+    foo(1, 2)
+    doAssert x == 5
+
+  block:
+    proc foo(_: int, _: float) = x += 5
+
+    foo(1, 2)
+    doAssert x == 10
+
+  block:
+    proc foo(_: int, _: float, _: string) = x += 5
+
+    foo(1, 2, "5")
+    doAssert x == 15
+
+static: test()
+test()
+
+when not defined(js):
+  static: closureTest()
+closureTest()