`default` tag causes panic with generic wrapper types (e.g., `OmittableNullable[string]`) #814

derkan · 2025-05-03T20:46:34Z

Hi 👋

While experimenting with the OmittableNullable[T] type as shown in the official examples/omit, I encountered a panic when trying to set a default value on a field of this type.

❗️ Problem

The following struct works fine:

type MyInput struct {
    Name OmittableNullable[string] `json:"name,omitempty" maxLength:"10"`
}

However, adding a default tag causes a runtime panic:

type MyInput struct {
    Name OmittableNullable[string] `json:"name,omitempty" maxLength:"10" default:"Kari"`
}

💥 Panic Trace

panic: unable to convert string to main.OmittableNullable[string] for field 'Name': schema is invalid
github.com/danielgtaylor/huma/v2.convertType(...)
	.../huma/v2@v2.32.0/schema.go:469 +0x3bc
...

⚠️ Root Cause

From what I can see in convertType() in schema.go, the default value "Kari" is treated as a string, but the target field type is OmittableNullable[string], a custom generic struct. Go's reflection system cannot convert between these types directly, which triggers the panic.

✅ Expected Behavior

Ideally, Huma should:

Detect when a field is a wrapper struct around a basic type, like OmittableNullable[string].
Allow setting a default tag for the inner value type (T), constructing a proper wrapper instance (e.g., OmittableNullable[string]{Value: "Kari", Present: true}).

💡 Possible Solutions

Introduce support for unwrapping known nullable/wrapper types when parsing defaults.
Optionally allow custom hooks or interfaces (e.g., FromDefault(value string) (any, error)) on field types to control how default values are constructed.

Let me know if a workaround is possible in the meantime — thank you for the great library!

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`default` tag causes panic with generic wrapper types (e.g., `OmittableNullable[string]`) #814

`default` tag causes panic with generic wrapper types (e.g., `OmittableNullable[string]`) #814

default tag causes panic with generic wrapper types (e.g., OmittableNullable[string]) #814

default tag causes panic with generic wrapper types (e.g., OmittableNullable[string]) #814

Comments

❗️ Problem

💥 Panic Trace

⚠️ Root Cause

✅ Expected Behavior

💡 Possible Solutions

`default` tag causes panic with generic wrapper types (e.g., `OmittableNullable[string]`) #814

`default` tag causes panic with generic wrapper types (e.g., `OmittableNullable[string]`) #814