interface conversion: interface {} is interp.valueInterface, not string when indexing map

[bailsman]

The following program sample.go triggers an unexpected result

package main

import (
        "fmt"
)

func main() {
        var m = map[string]interface{}{"a": "a"}
        a, _ := m["a"]
        b := a.(string)
        fmt.Println(a, b)
}

Expected result

$ go run ./sample.go
a a

Got

$ yaegi ./sample.go
./sample.go:8:33: panic
run: interface conversion: interface {} is interp.valueInterface, not string
goroutine 1 [running]:
runtime/debug.Stack(0x1, 0xc0002ea800, 0x40)
        /usr/local/go/src/runtime/debug/stack.go:24 +0x9f
github.com/traefik/yaegi/interp.(*Interpreter).Execute.func1(0xc0001adaf8)
        /go/src/yaegi/interp/program.go:131 +0xc8
panic(0xe43620, 0xc00019efb0)
        /usr/local/go/src/runtime/panic.go:965 +0x1b9
github.com/traefik/yaegi/interp.runCfg.func1(0xc0000cabb0, 0xc0004078c0, 0x0)
        /go/src/yaegi/interp/run.go:193 +0x251
panic(0xe43620, 0xc00019efb0)
        /usr/local/go/src/runtime/panic.go:965 +0x1b9
github.com/traefik/yaegi/interp.typeAssert.func3(0xc0000cabb0, 0x0)
        /go/src/yaegi/interp/run.go:441 +0x76b
github.com/traefik/yaegi/interp.runCfg(0xc0004078c0, 0xc0000cabb0, 0xc0004067e0, 0x0)
        /go/src/yaegi/interp/run.go:201 +0xbe
github.com/traefik/yaegi/interp.(*Interpreter).run(0xc0000ed8c0, 0xc0004067e0, 0xc0000caa50)
        /go/src/yaegi/interp/run.go:120 +0x2d2
github.com/traefik/yaegi/interp.(*Interpreter).Execute(0xc0000ed8c0, 0xc00048c9c0, 0x0, 0x0, 0x0, 0x0, 0x0)
        /go/src/yaegi/interp/program.go:157 +0x245
github.com/traefik/yaegi/interp.(*Interpreter).eval(0xc0000ed8c0, 0xc0002d4fa0, 0x91, 0x7ffe1ea59eb4, 0xb, 0x0, 0x92, 0x0, 0x0, 0x91, ...)
        /go/src/yaegi/interp/interp.go:615 +0xd3
github.com/traefik/yaegi/interp.(*Interpreter).EvalPath(0xc0000ed8c0, 0x7ffe1ea59eb4, 0xb, 0xc0002d4e00, 0x91, 0x0, 0x0, 0xc0001adc08)
        /go/src/yaegi/interp/interp.go:508 +0x13a
main.runFile(0xc0000ed8c0, 0x7ffe1ea59eb4, 0xb, 0x0, 0x1, 0x0)
        /go/src/yaegi/cmd/yaegi/run.go:153 +0xb1
main.run(0xc000032050, 0x1, 0x1, 0xc00000a501, 0xc000001680)
        /go/src/yaegi/cmd/yaegi/run.go:116 +0xa2a
main.main()
        /go/src/yaegi/cmd/yaegi/yaegi.go:144 +0x415

Yaegi Version

fbee2ba

Additional Notes

The following patch seems to fix the problem (at least for this instance):

diff --git a/interp/run.go b/interp/run.go
index 1295807..354f976 100644
--- a/interp/run.go
+++ b/interp/run.go
@@ -1843,11 +1843,7 @@ func getIndexMap2(n *node) {
                        n.exec = func(f *frame) bltn {
                                v := value0(f).MapIndex(mi)
                                if v.IsValid() {
-                                       if e := v.Elem(); e.Type().AssignableTo(valueInterfaceType) {
-                                               dest(f).Set(e)
-                                       } else {
-                                               dest(f).Set(reflect.ValueOf(valueInterface{n, e}))
-                                       }
+                                       dest(f).Set(v.Elem())
                                }
                                if doStatus {
                                        value2(f).SetBool(v.IsValid())
@@ -1879,11 +1875,7 @@ func getIndexMap2(n *node) {
                        n.exec = func(f *frame) bltn {
                                v := value0(f).MapIndex(value1(f))
                                if v.IsValid() {
-                                       if e := v.Elem(); e.Type().AssignableTo(valueInterfaceType) {
-                                               dest(f).Set(e)
-                                       } else {
-                                               dest(f).Set(reflect.ValueOf(valueInterface{n, e}))
-                                       }
+                                       dest(f).Set(v.Elem())
                                }
                                if doStatus {
                                        value2(f).SetBool(v.IsValid())

But note that this is more trial and error than me really understanding what's going on.