decoder2: prepare for moving into json2 (#25039)

Author: Larsimusrex

vlib/x/json2/decoder2/check.v

@@ -92,7 +92,6 @@ fn (mut decoder Decoder) check_element_type_valid[T](element T, current_node &No
 				return decoder.check_struct_type_valid(element, current_node)
 			}
 		}
-		else {}
 	}
 
 	return false
@@ -304,7 +303,6 @@ fn (mut decoder Decoder) init_sumtype_by_value_kind[T](mut val T, value_info Val
 				}
 			}
 		}
-		else {}
 	}
 
 	if failed_struct {

vlib/x/json2/decoder2/decode.v

@@ -0,0 +1,145 @@
+import x.json2.decoder2 as json
+import x.json2
+
+fn test_check_if_json_match() {
+	// /* Test wrong string values */
+	mut has_error := false
+
+	json.decode[string]('{"key": "value"}') or {
+		if err is json.JsonDecodeError {
+			assert err.line == 1
+			assert err.character == 1
+			assert err.message == 'Data: Expected string, but got object'
+		}
+		has_error = true
+	}
+	assert has_error, 'Expected error'
+	has_error = false
+
+	json.decode[map[string]string]('"value"') or {
+		if err is json.JsonDecodeError {
+			assert err.line == 1
+			assert err.character == 1
+			assert err.message == 'Data: Expected object, but got string_'
+		}
+		has_error = true
+	}
+	assert has_error, 'Expected error'
+	has_error = false
+
+	json.decode[[]int]('{"key": "value"}') or {
+		if err is json.JsonDecodeError {
+			assert err.line == 1
+			assert err.character == 1
+			assert err.message == 'Data: Expected array, but got object'
+		}
+		has_error = true
+	}
+	assert has_error, 'Expected error'
+	has_error = false
+
+	json.decode[string]('[1, 2, 3]') or {
+		if err is json.JsonDecodeError {
+			assert err.line == 1
+			assert err.character == 1
+			assert err.message == 'Data: Expected string, but got array'
+		}
+		has_error = true
+	}
+	assert has_error, 'Expected error'
+	has_error = false
+
+	json.decode[int]('{"key": "value"}') or {
+		if err is json.JsonDecodeError {
+			assert err.line == 1
+			assert err.character == 1
+			assert err.message == 'Data: Expected number, but got object'
+		}
+		has_error = true
+	}
+	assert has_error, 'Expected error'
+	has_error = false
+
+	json.decode[bool]('{"key": "value"}') or {
+		if err is json.JsonDecodeError {
+			assert err.line == 1
+			assert err.character == 1
+			assert err.message == 'Data: Expected boolean, but got object'
+		}
+		has_error = true
+	}
+	assert has_error, 'Expected error'
+	has_error = false
+
+	// /* Right string values */
+	json.decode[string]('"value"') or { assert false }
+
+	json.decode[map[string]string]('{"key": "value"}') or { assert false }
+
+	json.decode[[]int]('[1, 2, 3]') or { assert false }
+
+	json.decode[string]('"string"') or { assert false }
+
+	json.decode[int]('123') or { assert false }
+
+	json.decode[bool]('true') or { assert false }
+
+	json.decode[bool]('false') or { assert false }
+}
+
+fn test_check_json_format() {
+	json_and_error_message := [
+		{
+			'json':  ']'
+			'error': 'Syntax: unknown value kind'
+		},
+		{
+			'json':  '}'
+			'error': 'Syntax: unknown value kind'
+		},
+		{
+			'json':  'truely'
+			'error': 'Syntax: invalid value. Unexpected character after boolean end'
+		},
+		{
+			'json':  '0[1]'
+			'error': 'Syntax: invalid value. Unexpected character after number end'
+		},
+		{
+			'json':  '[1, 2, g3]'
+			'error': 'Syntax: unknown value kind'
+		},
+		{
+			'json':  '[1, 2,, 3]'
+			'error': 'Syntax: unknown value kind'
+		},
+		{
+			'json':  '{"key": 123'
+			'error': 'Syntax: EOF error: braces are not closed'
+		},
+		{
+			'json':  '{"key": 123,'
+			'error': 'Syntax: EOF error: Expecting object key after `,`'
+		},
+		{
+			'json':  '{"key": 123, "key2": 456,}'
+			'error': 'Syntax: Expecting object key after `,`'
+		},
+		{
+			'json':  '[[1, 2, 3], [4, 5, 6],]'
+			'error': 'Syntax: Cannot use `,`, before `]`'
+		},
+	]
+
+	for json_and_error in json_and_error_message {
+		mut has_error := false
+
+		json.decode[json2.Any](json_and_error['json']) or {
+			if err is json.JsonDecodeError {
+				assert err.message == json_and_error['error']
+			}
+			has_error = true
+		}
+		assert has_error, 'Expected error ${json_and_error['error']}'
+	}
+}