interp: support conversion of runtime func into interp func

Conversion of interp func into runtime func already worked, but added a
test anyway to watch out for regressions.

Fixes #941

_test/convert1.go

@@ -0,0 +1,17 @@
+package main
+
+import "strconv"
+
+type atoidef func(s string) (int, error)
+
+func main() {
+	stdatoi := atoidef(strconv.Atoi)
+	n, err := stdatoi("7")
+	if err != nil {
+		panic(err)
+	}
+	println(n)
+}
+
+// Output:
+// 7

_test/convert2.go

@@ -0,0 +1,19 @@
+package main
+
+import "bufio"
+
+func fakeSplitFunc(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	return 7, nil, nil
+}
+
+func main() {
+	splitfunc := bufio.SplitFunc(fakeSplitFunc)
+	n, _, err := splitfunc(nil, true)
+	if err != nil {
+		panic(err)
+	}
+	println(n)
+}
+
+// Output:
+// 7

interp/run.go

@@ -405,10 +405,15 @@ func convert(n *node) {
 		return
 	}
 
+	doConvert := true
 	var value func(*frame) reflect.Value
-	if c.typ.cat == funcT {
+	switch {
+	case c.typ.cat == funcT:
 		value = genFunctionWrapper(c)
-	} else {
+	case n.child[0].typ.cat == funcT && c.typ.cat == valueT:
+		doConvert = false
+		value = genValueNode(c)
+	default:
 		value = genValue(c)
 	}
 
@@ -429,7 +434,11 @@ func convert(n *node) {
 	}
 
 	n.exec = func(f *frame) bltn {
-		dest(f).Set(value(f).Convert(typ))
+		if doConvert {
+			dest(f).Set(value(f).Convert(typ))
+		} else {
+			dest(f).Set(value(f))
+		}
 		return next
 	}
 }