Default values are ignored when an inline input object field is populated from an undefined variable

When an input object field is set inline from a variable (e.g. { truthyBoolean: $truthyBoolean }) and that variable is left undefined, the schema's default value for the field isn't applied. Instead, the resolver receives nil. If the entire input object is passed as a variable, or if the field is omitted altogether, the default is applied as expected. This behavior is inconsistent.

Schema:

input DefaultInput {
    falsyBoolean:  Boolean = false
    truthyBoolean: Boolean = true
}

type Mutation {
    defaultInput(input: DefaultInput!): DefaultParametersMirror!
}

Operation (no variables provided, so $truthyBoolean is undefined):

mutation ($truthyBoolean: Boolean) {
    defaultInput(input: { truthyBoolean: $truthyBoolean }) {
        falsyBoolean
        truthyBoolean
    }
}

Expected:

• truthyBoolean = true

• falsyBoolean = false

  Actual:

• falsyBoolean = false (default applied since it's absent from the object)

• truthyBoolean = null (the resolver receives nil and the default is ignored)

The underlying issue appears to be in gqlparser. The parser materializes the inline object { truthyBoolean: $truthyBoolean } as {"truthyBoolean": nil} which means the key is present with a nil value. gqlgen's generated method unmarshalInputDefaultInput only fills a default value when the key is absent but skips it if it's present but is nil.

Reproduction: It can be added as a subtest to codegen/testserver/singlefile/defaults_test.go in the test method TestDefaults

t.Run("default input field from undefined variable", func(t *testing.T) {
    resolvers.MutationResolver.DefaultInput = func(
        ctx context.Context,
        input DefaultInput,
    ) (*DefaultParametersMirror, error) {
        return &DefaultParametersMirror{
            FalsyBoolean:  input.FalsyBoolean,
            TruthyBoolean: input.TruthyBoolean,
        }, nil
    }

    var resp struct{ DefaultInput *DefaultParametersMirror }
    // No variables are provided, so $truthyBoolean is undefined.
    err := c.Post(`mutation ($truthyBoolean: Boolean) {
        defaultInput(input: { truthyBoolean: $truthyBoolean }) {
            falsyBoolean
            truthyBoolean
        }
    }`, &resp)
    require.NoError(t, err)
    assertDefaults(t, resp.DefaultInput) // fails: truthyBoolean is nil, wants true
})

Related gqlparser issue: https://github.com/vektah/gqlparser/issues/347 However, because it appears directly in gqlgen, I'm opening this issue here for visibility and tracking. I’d be happy to work on a fix for this.