Bug: Cannot return nil for generic type T, when T is an interface

[mwittie]

Description

For the interfaces

type Builder[T any] interface {
	Build() (T, error)
}

type Thing interface {
	DoStuff()
}

I'd like to mock Builder behavior as

thingBuilder := new(mocks.Builder[Thing])
thingBuilder.On("Build").Return(nil, errors.New("some error"))

But I get an error like:

panic: interface conversion: interface is nil, not Thing

Shouldn't mockery know that Thing is an interface and so Builder[Thing].Build() should be able to return (nil, error)?

Mockery Version

$ mockery --version
19 Oct 22 17:49 MDT INF Starting mockery dry-run=false version=v2.14.0
v2.14.0

Golang Version

$ go version
go version go1.19 linux/amd64

Installation Method

• Binary Distribution
• Docker
• brew
• go install
• Other: [specify]

Expected Behavior

Can mock generic return types as nil when they are in fact interfaces.

Actual Behavior

Tests panic

[dlwyatt]

This is a workaround to get the test working with the current version of mockery:

thingBuilder.On("Build").Return(func() Thing { return nil }, errors.New("some error"))

[dlwyatt]

Possible fix: Generate the following code in all non-error cases, instead of the current isNillable check:

			v := ret.Get(0)
			if !reflect.ValueOf(&v).Elem().IsZero() && !reflect.ValueOf(v).IsZero() {
				r0 = v.(T)
			}

r0 is already the zero value for whatever its type is. The first call to IsZero (on ValueOf(&v).Elem()) covers the case where v is a nil, invalid interface (avoiding the panic in this issue), and the second call (on ValueOf(v) looks at whatever concrete type was passed into Return, assuming it's not the invalid nil interface. If both of those IsZero checks are false, then r0 needs to have a value assigned, and I think the type assertion v.(WhateverType) should always work.

This also requires an addition of g.addPackageImportWithName(ctx, "reflect", "reflect") up in NewGenerator

(The second check may not be strictly necessary, I was just trying to match the behavior of the code that mockery currently generates. It seems to work fine with just if !reflect.ValueOf(&v).Elem().IsZero() { even if the type is a pointer, map, slice, etc and the value is nil.)

[dlwyatt]

Even just putting if v != nil in all cases (not bothering to check for nillable or using reflect) might work, tbh.