Produce a more useful error when flag-like values are used for flag values.

eg.  myapp: error: --log-level: expected string value but got "--foo" (long flag); perhaps try --log-level="--foo"?

context.go

@@ -5,6 +5,8 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
+
+	"github.com/pkg/errors"
 )
 
 // Path records the nodes and parsed values from the current command-line.
@@ -504,6 +506,9 @@ func (c *Context) parseFlag(flags []*Flag, match string) (err error) {
 		c.scan.Pop()
 		err := flag.Parse(c.scan, c.getValue(flag.Value))
 		if err != nil {
+			if e, ok := errors.Cause(err).(*expectedError); ok && e.token.InferredType().IsAny(FlagToken, ShortFlagToken) {
+				return errors.Errorf("%s; perhaps try %s=%q?", err, flag.ShortSummary(), e.token)
+			}
 			return err
 		}
 		c.Path = append(c.Path, &Path{Flag: flag})

kong_test.go

@@ -715,7 +715,7 @@ func TestNumericParamErrors(t *testing.T) {
 	}
 	parser := mustNew(t, &cli)
 	_, err := parser.Parse([]string{"--name", "-10"})
-	require.EqualError(t, err, `--name: expected string value but got "-10" (short flag)`)
+	require.EqualError(t, err, `--name: expected string value but got "-10" (short flag); perhaps try --name="-10"?`)
 }
 
 func TestDefaultValueIsHyphen(t *testing.T) {

model.go

@@ -7,6 +7,8 @@ import (
 	"reflect"
 	"strconv"
 	"strings"
+
+	"github.com/pkg/errors"
 )
 
 // A Visitable component in the model.
@@ -289,7 +291,7 @@ func (v *Value) Parse(scan *Scanner, target reflect.Value) (err error) {
 		if rerr := recover(); rerr != nil {
 			switch rerr := rerr.(type) {
 			case Error:
-				err = fmt.Errorf("%s: %s", v.ShortSummary(), rerr)
+				err = errors.Wrap(rerr, v.ShortSummary())
 			default:
 				panic(fmt.Sprintf("mapper %T failed to apply to %s: %s", v.Mapper, v.Summary(), rerr))
 			}
@@ -297,7 +299,7 @@ func (v *Value) Parse(scan *Scanner, target reflect.Value) (err error) {
 	}()
 	err = v.Mapper.Decode(&DecodeContext{Value: v, Scan: scan}, target)
 	if err != nil {
-		return fmt.Errorf("%s: %s", v.ShortSummary(), err)
+		return errors.Wrap(err, v.ShortSummary())
 	}
 	v.Set = true
 	return nil

scanner.go

@@ -3,8 +3,6 @@ package kong
 import (
 	"fmt"
 	"strings"
-
-	"github.com/pkg/errors"
 )
 
 // TokenType is the type of a token.
@@ -14,11 +12,11 @@ type TokenType int
 const (
 	UntypedToken TokenType = iota
 	EOLToken
-	FlagToken                // --<flag>
-	FlagValueToken           // =<value>
-	ShortFlagToken           // -<short>[<tail]
-	ShortFlagTailToken       // <tail>
-	PositionalArgumentToken  // <arg>
+	FlagToken               // --<flag>
+	FlagValueToken          // =<value>
+	ShortFlagToken          // -<short>[<tail]
+	ShortFlagTailToken      // <tail>
+	PositionalArgumentToken // <arg>
 )
 
 func (t TokenType) String() string {
@@ -142,13 +140,22 @@ func (s *Scanner) Pop() Token {
 	return arg
 }
 
+type expectedError struct {
+	context string
+	token   Token
+}
+
+func (e *expectedError) Error() string {
+	return fmt.Sprintf("expected %s value but got %q (%s)", e.context, e.token, e.token.InferredType())
+}
+
 // PopValue pops a value token, or returns an error.
 //
 // "context" is used to assist the user if the value can not be popped, eg. "expected <context> value but got <type>"
 func (s *Scanner) PopValue(context string) (Token, error) {
 	t := s.Pop()
 	if !t.IsValue() {
-		return t, errors.Errorf("expected %s value but got %q (%s)", context, t, t.InferredType())
+		return t, &expectedError{context, t}
 	}
 	return t, nil
 }
@@ -157,9 +164,9 @@ func (s *Scanner) PopValue(context string) (Token, error) {
 //
 // "context" is used to assist the user if the value can not be popped, eg. "expected <context> value but got <type>"
 func (s *Scanner) PopValueInto(context string, target interface{}) error {
-	t := s.Pop()
-	if !t.IsValue() {
-		return errors.Errorf("expected %s value but got %q (%s)", context, t, t.InferredType())
+	t, err := s.PopValue(context)
+	if err != nil {
+		return err
 	}
 	return jsonTranscode(t.Value, target)
 }