Preserve alias context in usage errors

binding.go

@@ -1,6 +1,7 @@
 package commandkit
 
 import (
+	"errors"
 	"fmt"
 	"strconv"
 )
@@ -22,10 +23,10 @@ func ValueFlag[I, V any](meta FlagMeta, defaultValue *V, parse func(string) (V,
 	binding.prepare = func(values []string) (preparedValue[I], error) {
 		value := fallback
 		supplied := len(values) > 0
-		for _, raw := range values {
+		for index, raw := range values {
 			parsed, err := parse(raw)
 			if err != nil {
-				return preparedValue[I]{}, err
+				return preparedValue[I]{}, &flagOccurrenceError{index: index, cause: err}
 			}
 			value = parsed
 		}
@@ -47,10 +48,10 @@ func BoolFlag[I any](meta FlagMeta, defaultValue bool, set func(*I, Supplied[boo
 	binding.prepare = func(values []string) (preparedValue[I], error) {
 		value := defaultValue
 		supplied := len(values) > 0
-		for _, raw := range values {
+		for index, raw := range values {
 			parsed, err := strconv.ParseBool(raw)
 			if err != nil {
-				return preparedValue[I]{}, err
+				return preparedValue[I]{}, &flagOccurrenceError{index: index, cause: err}
 			}
 			value = parsed
 		}
@@ -76,10 +77,10 @@ func RepeatFlag[I, V any](meta FlagMeta, defaults []V, parse func(string) (V, er
 		supplied := len(values) > 0
 		if supplied {
 			parsedValues = make([]V, 0, len(values))
-			for _, raw := range values {
+			for index, raw := range values {
 				parsed, err := parse(raw)
 				if err != nil {
-					return preparedValue[I]{}, err
+					return preparedValue[I]{}, &flagOccurrenceError{index: index, cause: err}
 				}
 				parsedValues = append(parsedValues, parsed)
 			}
@@ -158,6 +159,28 @@ type preparedInput[I any] struct {
 	values   []preparedValue[I]
 }
 
+type flagOccurrenceError struct {
+	index int
+	cause error
+}
+
+func (err *flagOccurrenceError) Error() string { return err.cause.Error() }
+
+func (err *flagOccurrenceError) Unwrap() error { return err.cause }
+
+type flagConversionError struct {
+	binding    string
+	spelling   string
+	occurrence int
+	cause      error
+}
+
+func (err *flagConversionError) Error() string {
+	return fmt.Sprintf("flag %s occurrence %d: %v", err.spelling, err.occurrence, err.cause)
+}
+
+func (err *flagConversionError) Unwrap() error { return err.cause }
+
 func (p preparedInput[I]) build() I {
 	var input I
 	if p.newInput != nil {
@@ -173,15 +196,36 @@ func (p preparedInput[I]) build() I {
 // creates an input value. When enforceRequired is false, missing required flags
 // and operands are allowed so metadata help can validate supplied syntax.
 func prepareInput[I any](spec CommandSpec[I], flagValues [][]string, operands []string, enforceRequired bool) (preparedInput[I], error) {
+	return prepareInputWithSpellings(spec, flagValues, nil, operands, enforceRequired)
+}
+
+func prepareInputWithSpellings[I any](spec CommandSpec[I], flagValues, flagSpellings [][]string, operands []string, enforceRequired bool) (preparedInput[I], error) {
 	if len(flagValues) != len(spec.Flags) {
 		return preparedInput[I]{}, fmt.Errorf("received %d flag value sets for %d bindings", len(flagValues), len(spec.Flags))
 	}
+	if flagSpellings == nil {
+		flagSpellings = make([][]string, len(spec.Flags))
+	}
+	if len(flagSpellings) != len(spec.Flags) {
+		return preparedInput[I]{}, fmt.Errorf("received %d flag spelling sets for %d bindings", len(flagSpellings), len(spec.Flags))
+	}
 	preparedFlags := make([]preparedValue[I], len(spec.Flags))
 	missingRequired := ""
 	for i, flag := range spec.Flags {
 		binding := flag.binding
 		value, err := binding.prepare(flagValues[i])
 		if err != nil {
+			var occurrenceErr *flagOccurrenceError
+			if errors.As(err, &occurrenceErr) {
+				spelling := "--" + binding.meta.Name
+				if occurrenceErr.index >= 0 && occurrenceErr.index < len(flagSpellings[i]) && flagSpellings[i][occurrenceErr.index] != "" {
+					spelling = flagSpellings[i][occurrenceErr.index]
+				}
+				return preparedInput[I]{}, &flagConversionError{
+					binding: binding.meta.Name, spelling: spelling,
+					occurrence: occurrenceErr.index + 1, cause: occurrenceErr.cause,
+				}
+			}
 			return preparedInput[I]{}, fmt.Errorf("flag --%s: %w", binding.meta.Name, err)
 		}
 		preparedFlags[i] = value