cgen: fix struct field with default optional value (fix #11119) (#22167)

vlib/v/gen/c/struct.v

@@ -415,6 +415,14 @@ fn (mut g Gen) zero_struct_field(field ast.StructField) bool {
 					g.struct_init(default_init)
 				}
 				return true
+			} else if sym.language == .v && !field.typ.is_ptr() && sym.mod != 'builtin'
+				&& !sym.info.is_empty_struct() {
+				default_init := ast.StructInit{
+					typ: field.typ
+				}
+				g.write('.${field_name} = ')
+				g.struct_init(default_init)
+				return true
 			}
 		}
 	}

vlib/v/tests/structs/struct_field_default_value_optional_test.v

@@ -0,0 +1,27 @@
+import regex
+
+struct RegexCache {
+mut:
+	tag_script_start regex.RE = regex.regex_opt(r'^script.*>') or { panic(err) }
+}
+
+pub struct Tokenizer {
+mut:
+	regex_cache RegexCache = RegexCache{}
+}
+
+fn new_parser() &Parser {
+	mut parser := &Parser{}
+	return parser
+}
+
+pub struct Parser {
+mut:
+	tnzr Tokenizer
+}
+
+fn test_struct_field_default_value_optional() {
+	p := new_parser()
+	println(p)
+	assert true
+}