Cannot update fields in elements of map[T1]*T2

[kortschak]

While map elements are not themselves addressable, if the element is a pointer the pointee should be modifiable directly via map access as shown in this playground snippet.

gomacro> import "reflect" // Just a package with a handy struct.
gomacro> m := make(map[string]*reflect.SliceHeader)
gomacro> m["a"] = &reflect.SliceHeader{}
gomacro> m["a"].Len = 2
repl.go:1:1: cannot assign to m["a"].Len
	repl.go:1:1: cannot take the address of a map element: m["a"]

[cosmos72]

confirmed, it's a bug.

The problem is not limited to maps - it happens with all syntaxes that return non-settable values which happen to be pointers. Another example:

gomacro> import "reflect"
gomacro> func header() *reflect.SliceHeader { return &reflect.SliceHeader{} }
gomacro> header().Len
0 // int
gomacro> header().Len = 2
repl.go:1:1: cannot assign to header().Len
        repl.go:1:1: cannot take the address of: header()

If you want to give a shot at it, I guess the problem is due to gomacro/fast/assignment.go method
(*Comp).placeOrAddress() not realizing that the implicit pointer dereference makes the expression addressable and settable.

Otherwise, I will probably be able to investigate it in 1-2 weeks (real life struck)

[cosmos72]

Solved in commit 48c8420