Merge pull request #1884 from roim/patch-1

docs: Add ValidationPipe hint on not using `import type` for DTOs

Author: kamilmysliwiec

content/techniques/validation.md

@@ -121,6 +121,8 @@ create(@Body() createUserDto: CreateUserDto) {
 
 > info **Hint** Since TypeScript does not store metadata about **generics or interfaces**, when you use them in your DTOs, `ValidationPipe` may not be able to properly validate incoming data.  For this reason, consider using concrete classes in your DTOs.
 
+> info **Hint** When importing your DTOs, you can't use a type-only import as that would be erased at runtime, i.e. remember to `import {{ '{' }} CreateUserDto {{ '}' }}` instead of `import type {{ '{' }} CreateUserDto {{ '}' }}`.
+
 Now we can add a few validation rules in our `CreateUserDto`. We do this using decorators provided by the `class-validator` package, described in detail [here](https://github.com/typestack/class-validator#validation-decorators). In this fashion, any route that uses the `CreateUserDto` will automatically enforce these validation rules.
 
 ```typescript