cgen: fix struct initialization bugs

vlib/v/gen/cgen.v

@@ -1980,13 +1980,14 @@ fn (var g Gen) struct_init(struct_init ast.StructInit) {
 				continue
 			}
 			field_name := c_name(field.name)
+			g.write('\t.$field_name = ')
 			if field.has_default_expr {
 				g.expr(field.default_expr)
-				g.writeln(',')
+
 			} else {
-				zero := g.type_default(field.typ)
-				g.writeln('\t.$field_name = $zero,')
+				g.write(g.type_default(field.typ))
 			}
+			g.writeln(',')
 		}
 	}
 	if struct_init.fields.len == 0 && info.fields.len == 0 {

vlib/v/tests/struct_test.v

@@ -265,3 +265,44 @@ fn test_levels() {
 		}
 	}
 }
+
+// Struct where an inizialized field is after a non-initilized field.
+struct StructWithDefaultValues1 {
+	field_required int
+	field_optional int = 5
+}
+
+// Struct where an inizialized field is before a non-initilized field.
+struct StructWithDefaultValues2 {
+	field_optional  int = 3
+	field_required  int
+}
+
+// Struct where an inizialized field is before several non-initilized fields.
+struct StructWithDefaultValues3 {
+	field_optional     int = 2
+	field_required     int
+	field_required_too int
+}
+
+fn test_struct_with_default_values_init() {
+	s1 := StructWithDefaultValues1{ field_required: 5 }
+	s2 := StructWithDefaultValues2{ field_required: 5 }
+	// Partially initialized
+	s3 := StructWithDefaultValues3{ field_required: 5 }
+
+	assert s1.field_optional == 5
+	assert s2.field_optional == 3
+	assert s3.field_optional == 2
+}
+
+fn test_struct_with_default_values_no_init() {
+	// Don't inititialize
+	s1 := StructWithDefaultValues1{}
+	s2 := StructWithDefaultValues2{}
+	s3 := StructWithDefaultValues3{}
+
+	assert s1.field_optional == 5
+	assert s2.field_optional == 3
+	assert s3.field_optional == 2
+}