Improve error messages when unmarshaling map values

decoder.go

@@ -2,6 +2,7 @@ package maxminddb
 
 import (
 	"encoding/binary"
+	"fmt"
 	"math"
 	"math/big"
 	"reflect"
@@ -418,7 +419,7 @@ func (d *decoder) unmarshalMap(
 	result = indirect(result)
 	switch result.Kind() {
 	default:
-		return 0, newUnmarshalTypeError("map", result.Type())
+		return 0, newUnmarshalTypeStrError("map", result.Type())
 	case reflect.Struct:
 		return d.decodeStruct(size, offset, result, depth)
 	case reflect.Map:
@@ -430,7 +431,7 @@ func (d *decoder) unmarshalMap(
 			result.Set(rv)
 			return newOffset, err
 		}
-		return 0, newUnmarshalTypeError("map", result.Type())
+		return 0, newUnmarshalTypeStrError("map", result.Type())
 	}
 }
 
@@ -465,7 +466,7 @@ func (d *decoder) unmarshalSlice(
 			return newOffset, err
 		}
 	}
-	return 0, newUnmarshalTypeError("array", result.Type())
+	return 0, newUnmarshalTypeStrError("array", result.Type())
 }
 
 func (d *decoder) unmarshalString(size, offset uint, result reflect.Value) (uint, error) {
@@ -615,7 +616,7 @@ func (d *decoder) decodeMap(
 
 		offset, err = d.decode(offset, elemValue, depth)
 		if err != nil {
-			return 0, err
+			return 0, fmt.Errorf("decoding value for %s: %w", key, err)
 		}
 
 		keyValue.SetString(string(key))
@@ -772,7 +773,7 @@ func (d *decoder) decodeStruct(
 
 		offset, err = d.decode(offset, result.Field(j), depth)
 		if err != nil {
-			return 0, err
+			return 0, fmt.Errorf("decoding value for %s: %w", key, err)
 		}
 	}
 	return offset, nil

errors.go

@@ -30,13 +30,17 @@ type UnmarshalTypeError struct {
 	Value string
 }
 
-func newUnmarshalTypeError(value any, rType reflect.Type) UnmarshalTypeError {
+func newUnmarshalTypeStrError(value string, rType reflect.Type) UnmarshalTypeError {
 	return UnmarshalTypeError{
-		Value: fmt.Sprintf("%v", value),
 		Type:  rType,
+		Value: value,
 	}
 }
 
+func newUnmarshalTypeError(value any, rType reflect.Type) UnmarshalTypeError {
+	return newUnmarshalTypeStrError(fmt.Sprintf("%v (%T)", value, value), rType)
+}
+
 func (e UnmarshalTypeError) Error() string {
-	return fmt.Sprintf("maxminddb: cannot unmarshal %s into type %s", e.Value, e.Type.String())
+	return fmt.Sprintf("maxminddb: cannot unmarshal %s into type %s", e.Value, e.Type)
 }

reader_test.go

@@ -594,7 +594,7 @@ func TestBrokenDoubleDatabase(t *testing.T) {
 	expected := newInvalidDatabaseError(
 		"the MaxMind DB file's data section contains bad data (float 64 size of 2)",
 	)
-	assert.Equal(t, expected, err)
+	require.ErrorAs(t, err, &expected)
 	require.NoError(t, reader.Close(), "error on close")
 }