Weird nil / not-nil behavior

I have the following code in my project:

		mod := srt.PrimaryModule()
		if mod != nil {
			mods[mod.Name()] = mod
		}

When it executes, I get:

PANIC: runtime error: invalid memory address or nil pointer dereference

with the stack right at the third line of that. (Yes it's the mod dereference, the map mods is created just a line above...)

So, when is a nil value not equal to nil? And why?

 srt.PrimaryModule() 

returns an interface type, Module, with a Name() method defined returning string.

In this particular case, srt is typed StdReflectedType, which is an interface type with a function PrimaryModule() returning type Module. The actual implementation of srt is a struct that returns an uninitialized Module value from its fields.

Further, if I do

reflect.ValueOf(mod)

the result is the zero reflect.Value.

From the reflect package documentation:

 ValueOf returns a new Value initialized to the concrete value stored in the interface i. ValueOf(nil) returns the zero Value

Which seems right, except the previous line just determined mod != nil. So what gives?

I cannot seem to reproduce this in a minimal example - is there any sensible behavior that could cause those lines to do this - am I not understandig something important about interface values and nil pointers?

if any concrete value has been stored in the interface, the interface will not be nil. However, the concrete value stored in the interface can be a nil pointer.

For example, is the fmt.Println(mod) output, the concrete value, nil?

if mod != nil {
    fmt.Println(mod)
    mods[mod.Name()] = mod
}

Does srt.PrimaryModule() return a nil pointer to mod? If so, mod.Name() will panic with a nil pointer dereference.

References:

The Go Programming Language Specification: Interface types

Go Data Structures: Interfaces

Why is my nil error value not equal to nil?

What is the output from this revised version of your code?

mod := srt.PrimaryModule()
if mod != nil {
	fmt.Printf("%T %v\n", mod, mod)
	if mod.(Module) != nil {
		fmt.Println(mod.Name())
		mods[mod.Name()] = mod
	}
}