Generalize ValidArgs: use it implicitly with any validator

5 of the commits in this PR are about refactoring args_test.go.
The sixth one (feat: generalize ValidArgs; use it implicitly with
any validator) moves the validation of ValidArgs from args.go to
command.go. As a result:

- Any validator can be used along with ValidArgs. ValidArgs is
  checked first, and then the defined validator.
- OnlyValidArgs and ExactValidArgs are deprecated.

args_test.go is updated accordingly:

```
=== RUN   TestNoArgs
--- PASS: TestNoArgs (0.00s)
=== RUN   TestNoArgsWithArgs
--- PASS: TestNoArgsWithArgs (0.00s)
=== RUN   TestNoArgsWithArgsWithValid
--- PASS: TestNoArgsWithArgsWithValid (0.00s)
=== RUN   TestArbitraryArgs
--- PASS: TestArbitraryArgs (0.00s)
=== RUN   TestArbitraryArgsWithValid
--- PASS: TestArbitraryArgsWithValid (0.00s)
=== RUN   TestArbitraryArgsWithValidWithInvalidArgs
--- PASS: TestArbitraryArgsWithValidWithInvalidArgs (0.00s)
=== RUN   TestMinimumNArgs
--- PASS: TestMinimumNArgs (0.00s)
=== RUN   TestMinimumNArgsWithValid
--- PASS: TestMinimumNArgsWithValid (0.00s)
=== RUN   TestMinimumNArgsWithValidWithInvalidArgs
--- PASS: TestMinimumNArgsWithValidWithInvalidArgs (0.00s)
=== RUN   TestMinimumNArgsWithLessArgs
--- PASS: TestMinimumNArgsWithLessArgs (0.00s)
=== RUN   TestMinimumNArgsWithLessArgsWithValid
--- PASS: TestMinimumNArgsWithLessArgsWithValid (0.00s)
=== RUN   TestMinimumNArgsWithLessArgsWithValidWithInvalidArgs
--- PASS: TestMinimumNArgsWithLessArgsWithValidWithInvalidArgs (0.00s)
=== RUN   TestMaximumNArgs
--- PASS: TestMaximumNArgs (0.00s)
=== RUN   TestMaximumNArgsWithValid
--- PASS: TestMaximumNArgsWithValid (0.00s)
=== RUN   TestMaximumNArgsWithValidWithInvalidArgs
--- PASS: TestMaximumNArgsWithValidWithInvalidArgs (0.00s)
=== RUN   TestMaximumNArgsWithMoreArgs
--- PASS: TestMaximumNArgsWithMoreArgs (0.00s)
=== RUN   TestMaximumNArgsWithMoreArgsWithValid
--- PASS: TestMaximumNArgsWithMoreArgsWithValid (0.00s)
=== RUN   TestMaximumNArgsWithMoreArgsWithValidWithInvalidArgs
--- PASS: TestMaximumNArgsWithMoreArgsWithValidWithInvalidArgs (0.00s)
=== RUN   TestExactArgs
--- PASS: TestExactArgs (0.00s)
=== RUN   TestExactArgsWithValid
--- PASS: TestExactArgsWithValid (0.00s)
=== RUN   TestExactArgsWithValidWithInvalidArgs
--- PASS: TestExactArgsWithValidWithInvalidArgs (0.00s)
=== RUN   TestExactArgsWithInvalidCount
--- PASS: TestExactArgsWithInvalidCount (0.00s)
=== RUN   TestExactArgsWithInvalidCountWithValid
--- PASS: TestExactArgsWithInvalidCountWithValid (0.00s)
=== RUN   TestExactArgsWithInvalidCountWithValidWithInvalidArgs
--- PASS: TestExactArgsWithInvalidCountWithValidWithInvalidArgs (0.00s)
=== RUN   TestRangeArgs
--- PASS: TestRangeArgs (0.00s)
=== RUN   TestRangeArgsWithValid
--- PASS: TestRangeArgsWithValid (0.00s)
=== RUN   TestRangeArgsWithValidWithInvalidArgs
--- PASS: TestRangeArgsWithValidWithInvalidArgs (0.00s)
=== RUN   TestRangeArgsWithInvalidCount
--- PASS: TestRangeArgsWithInvalidCount (0.00s)
=== RUN   TestRangeArgsWithInvalidCountWithValid
--- PASS: TestRangeArgsWithInvalidCountWithValid (0.00s)
=== RUN   TestRangeArgsWithInvalidCountWithValidWithInvalidArgs
--- PASS: TestRangeArgsWithInvalidCountWithValidWithInvalidArgs (0.00s)
=== RUN   TestRootTakesNoArgs
--- PASS: TestRootTakesNoArgs (0.00s)
=== RUN   TestRootTakesArgs
--- PASS: TestRootTakesArgs (0.00s)
=== RUN   TestChildTakesNoArgs
--- PASS: TestChildTakesNoArgs (0.00s)
=== RUN   TestChildTakesArgs
--- PASS: TestChildTakesArgs (0.00s)
```

Merge spf13/cobra#841

Fix #838 and Fix #745

args.go

@@ -7,6 +7,25 @@ import (
 
 type PositionalArgs func(cmd *Command, args []string) error
 
+// validateArgs returns an error if there are any positional args that are not in
+// the `ValidArgs` field of `Command`
+func validateArgs(cmd *Command, args []string) error {
+	if len(cmd.ValidArgs) > 0 {
+		// Remove any description that may be included in ValidArgs.
+		// A description is following a tab character.
+		var validArgs []string
+		for _, v := range cmd.ValidArgs {
+			validArgs = append(validArgs, strings.Split(v, "\t")[0])
+		}
+		for _, v := range args {
+			if !stringInSlice(v, validArgs) {
+				return fmt.Errorf("invalid argument %q for %q%s", v, cmd.CommandPath(), cmd.findSuggestions(args[0]))
+			}
+		}
+	}
+	return nil
+}
+
 // Legacy arg validation has the following behaviour:
 // - root commands with no subcommands can take arbitrary arguments
 // - root commands with subcommands will do subcommand validity checking
@@ -32,25 +51,6 @@ func NoArgs(cmd *Command, args []string) error {
 	return nil
 }
 
-// OnlyValidArgs returns an error if any args are not in the list of ValidArgs.
-func OnlyValidArgs(cmd *Command, args []string) error {
-	if len(cmd.ValidArgs) > 0 {
-		// Remove any description that may be included in ValidArgs.
-		// A description is following a tab character.
-		var validArgs []string
-		for _, v := range cmd.ValidArgs {
-			validArgs = append(validArgs, strings.Split(v, "\t")[0])
-		}
-
-		for _, v := range args {
-			if !stringInSlice(v, validArgs) {
-				return fmt.Errorf("invalid argument %q for %q%s", v, cmd.CommandPath(), cmd.findSuggestions(args[0]))
-			}
-		}
-	}
-	return nil
-}
-
 // ArbitraryArgs never returns an error.
 func ArbitraryArgs(cmd *Command, args []string) error {
 	return nil
@@ -86,18 +86,6 @@ func ExactArgs(n int) PositionalArgs {
 	}
 }
 
-// ExactValidArgs returns an error if
-// there are not exactly N positional args OR
-// there are any positional args that are not in the `ValidArgs` field of `Command`
-func ExactValidArgs(n int) PositionalArgs {
-	return func(cmd *Command, args []string) error {
-		if err := ExactArgs(n)(cmd, args); err != nil {
-			return err
-		}
-		return OnlyValidArgs(cmd, args)
-	}
-}
-
 // RangeArgs returns an error if the number of args is not within the expected range.
 func RangeArgs(min int, max int) PositionalArgs {
 	return func(cmd *Command, args []string) error {
@@ -119,3 +107,18 @@ func MatchAll(pargs ...PositionalArgs) PositionalArgs {
 		return nil
 	}
 }
+
+// ExactValidArgs returns an error if there are not exactly N positional args OR
+// there are any positional args that are not in the `ValidArgs` field of `Command`
+//
+// Deprecated: now `ExactArgs` honors `ValidArgs`, when defined and not empty
+func ExactValidArgs(n int) PositionalArgs {
+	return ExactArgs(n)
+}
+
+// OnlyValidArgs returns an error if any args are not in the list of `ValidArgs`.
+//
+// Deprecated: now `ArbitraryArgs` honors `ValidArgs`, when defined and not empty
+func OnlyValidArgs(cmd *Command, args []string) error {
+	return ArbitraryArgs(cmd, args)
+}