Ambiguous merging of nullable structs with StructToStruct

[uabjabborov]

The following new test case for copy.go is failing. I added it in my local copy.

I wonder what the correct behavior should be in this case.

func TestStructToStruct_NestedNilParentStruct_NonNilDst(t *testing.T) {
	type A struct {
		Field1 string
		Field2 int
	}
	type B struct {
		Field1 string
		Field2 int
		A      *A
	}
	type C struct {
		Field1 string
		B      B
	}

	// Given: src.B.A is nil, dst.B.A is not nil
	src := &C{
		Field1: "src C field1",
		B: B{
			Field1: "src StringerB field1",
			Field2: 1,
			A:      nil, // nil struct
		},
	}
	dst := &C{
		Field1: "dst C field1",
		B: B{
			Field1: "dst StringerB field1",
			Field2: 2,
			A: &A{
				Field1: "dst StringerA field1",
				Field2: 10,
			},
		},
	}

	// Given: mask contains subfield of nil field in src, i.e. src.B.A
	mask := fieldmask_utils.MaskFromString("B{Field1,A{Field2}}")

	// When: StructToStruct is called
	err := fieldmask_utils.StructToStruct(mask, src, dst)

	// Then: no error is returned
	require.NoError(t, err)

	// Then: dst B.A is not modified
	assert.Equal(t, &C{
		Field1: "dst C field1",
		B: B{
			Field1: src.B.Field1,
			Field2: 2,
			A: &A{
				Field1: "dst StringerA field1",
				Field2: 10,
			},
		},
	}, dst)
}

[mennanov]

I think the current behavior is correct: if the source is nil then the destination should also be nil as a result.

If you need to keep the non-nil destination fields then you would need smth like a MapVisitor but for the structToStruct function.
It is not implemented yet, contributions are welcome though.

If you decide to implement it then consider adding a generic visitor option that can be used in both functions: StructToMap and StructToStruct