Do not use pointers on named map types

[bastianccm]

Named map types such as
type Foo map[string]interface{}
are nilable and named, which means they should not be references as a
pointer in generated code.

I am unsure if the test code is in the proper location.

This resolves #878

Describe your PR and link to any relevant issues.

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[vektah]

I think this should be calling

gqlgen/codegen/config/binder.go

Line 413 in 14dccc5

func IsNilable(t types.Type) bool {

(and make it public) to stop these falling out of sync again in the future?

[bastianccm]

Hey @vektah,

in general I agree, however updating it I stumbled upon another issue: the same issue this change fixes for maps applies to slices as well.
To have correct generated code I would use

func (o *Object) Reference() types.Type {
	if config.IsNilable(o.Type) {
		return o.Type
	}
	return types.NewPointer(o.Type)
}

for the object generation (the change you suggested), and update IsNilable to include a check for types.Slice as well

_, isSlice := t.(*types.Slice)
return isPtr || isMap || isInterface || isSlice

However this breaks TestIntrospection/enabled_by_default and TestSlices/nulls_vs_empty_slices.
I can fix the nulls_vs_empty_slices tests by using

return &Slices{
	Test1: nil,
	Test2: nil,
	Test3: make([]*string, 0),
	Test4: make([]string, 0),
}, nil

to mark explicitly what is null and what is empty.
However for the introspection I am unsure if I am on the right path doing these changes.

I pushed this as a new commit, with the failing tests, so you have a chance to see what I did.

Maybe you have an idea/opinion on this topic?

[lwc]

Replaced with #1242 thanks for the contribution!