interp: fix redeclarations containing a blank variable

In [short variable declarations](https://go.dev/ref/spec#Short_variable_declarations),
The reuse of existing symbols is possible only if a new variable is defined,
otherwise a new symbol must be created, which was not the case in the issue.

Search for new symbols and correctly ignore blank variables.

Fixes #1434.

_test/assert2.go

@@ -5,8 +5,8 @@ import (
 	"sync"
 )
 
-// Defined an interface of stringBuilder that compatible with
-// strings.Builder(go 1.10) and bytes.Buffer(< go 1.10)
+// Define an interface of stringBuilder that is compatible with
+// strings.Builder(go 1.10) and bytes.Buffer(< go 1.10).
 type stringBuilder interface {
 	WriteRune(r rune) (n int, err error)
 	WriteString(s string) (int, error)

_test/assign17.go

@@ -0,0 +1,21 @@
+package main
+
+func main() {
+	s := make([]map[string]string, 0)
+	m := make(map[string]string)
+	m["m1"] = "m1"
+	m["m2"] = "m2"
+	s = append(s, m)
+	tmpStr := "start"
+	println(tmpStr)
+	for _, v := range s {
+		tmpStr, ok := v["m1"]
+		println(tmpStr, ok)
+	}
+	println(tmpStr)
+}
+
+// Output:
+// start
+// m1 true
+// start

_test/assign18.go

@@ -0,0 +1,21 @@
+package main
+
+func main() {
+	s := make([]map[string]string, 0)
+	m := make(map[string]string)
+	m["m1"] = "m1"
+	m["m2"] = "m2"
+	s = append(s, m)
+	tmpStr := "start"
+	println(tmpStr)
+	for _, v := range s {
+		tmpStr, _ := v["m1"]
+		println(tmpStr)
+	}
+	println(tmpStr)
+}
+
+// Output:
+// start
+// m1
+// start

_test/gen10.go

@@ -0,0 +1,12 @@
+package main
+
+func genFunc() (f func()) {
+	return f
+}
+
+func main() {
+	println(genFunc() == nil)
+}
+
+// Output:
+// true

_test/issue-1425.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"io"
+	"log"
+	"os"
+	"strings"
+)
+
+type WrappedReader struct {
+	reader io.Reader
+}
+
+func (wr WrappedReader) Read(p []byte) (n int, err error) {
+	return wr.reader.Read(p)
+}
+
+// Of course, this implementation is completely stupid because it does not write
+// to the intended writer, as any honest WriteTo implementation should. its
+// implemtion is just to make obvious the divergence of behaviour with yaegi.
+func (wr WrappedReader) WriteTo(w io.Writer) (n int64, err error) {
+	// Ignore w, send to Stdout to prove whether this WriteTo is used.
+	data, err := io.ReadAll(wr)
+	if err != nil {
+		return 0, err
+	}
+	nn, err := os.Stdout.Write(data)
+	return int64(nn), err
+}
+
+func main() {
+	f := strings.NewReader("hello world")
+	wr := WrappedReader{reader: f}
+
+	// behind the scenes, io.Copy is supposed to use wr.WriteTo if the implementation exists.
+	// With Go, it works as expected, i.e. the output is sent to os.Stdout.
+	// With Yaegi, it doesn't, i.e. the output is sent to io.Discard.
+	if _, err := io.Copy(io.Discard, wr); err != nil {
+		log.Fatal(err)
+	}
+}
+
+// Output:
+// hello world

_test/issue-1439.go

@@ -0,0 +1,25 @@
+package main
+
+type Transformer interface {
+	Reset()
+}
+
+type Encoder struct {
+	Transformer
+}
+
+type nop struct{}
+
+func (nop) Reset() { println("Reset") }
+
+func f(e Transformer) {
+	e.Reset()
+}
+
+func main() {
+	e := Encoder{Transformer: nop{}}
+	f(e)
+}
+
+// Output:
+// Reset

_test/issue-1447.go

@@ -0,0 +1,20 @@
+package main
+
+import "fmt"
+
+type I interface {
+	Name() string
+}
+
+type S struct {
+	iMap map[string]I
+}
+
+func main() {
+	s := S{}
+	s.iMap = map[string]I{}
+	fmt.Println(s)
+}
+
+// Output:
+// {map[]}

_test/p4/p4.go

@@ -0,0 +1,3 @@
+package p4
+
+var Value1 = "value1"

_test/p5.go

@@ -0,0 +1,10 @@
+package main
+
+import "github.com/traefik/yaegi/_test/p5"
+
+func main() {
+	println(*p5.Value1)
+}
+
+// Output:
+// value1

_test/p5/p5.go

@@ -0,0 +1,8 @@
+package p5
+
+import "github.com/traefik/yaegi/_test/p4"
+
+var (
+	Value1 = &val1
+	val1   = p4.Value1
+)

_test/switch39.go

@@ -0,0 +1,17 @@
+package main
+
+func f(params ...interface{}) {
+	switch p0 := params[0].(type) {
+	case string:
+		println("string:", p0)
+	default:
+		println("not a string")
+	}
+}
+
+func main() {
+	f("Hello")
+}
+
+// Output:
+// string: Hello

_test/switch40.go

@@ -0,0 +1,17 @@
+package main
+
+func f(params ...interface{}) {
+	switch params[0].(type) {
+	case string:
+		println("a string")
+	default:
+		println("not a string")
+	}
+}
+
+func main() {
+	f("Hello")
+}
+
+// Output:
+// a string

interp/cfg.go

@@ -221,6 +221,14 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 				sc.sym[label] = sym
 				c.sym = sym
 			}
+			// If block is the body of a function, get declared variables in current scope.
+			// This is done in order to add the func signature symbols into sc.sym,
+			// as we will need them in post-processing.
+			if n.anc != nil && n.anc.kind == funcDecl {
+				for k, v := range sc.anc.sym {
+					sc.sym[k] = v
+				}
+			}
 
 		case breakStmt, continueStmt, gotoStmt:
 			if len(n.child) == 0 {
@@ -2051,7 +2059,14 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 			}
 			sbn.start = clauses[0].start
 			n.start = n.child[0].start
+			if n.kind == typeSwitch {
+				// Handle the typeSwitch init (the type assert expression).
+				init := n.child[1].lastChild().child[0]
+				init.tnext = sbn.start
+				n.child[0].tnext = init.start
+			} else {
 				n.child[0].tnext = sbn.start
+			}
 
 		case switchIfStmt: // like an if-else chain
 			sc = sc.pop()
@@ -2281,11 +2296,36 @@ func compDefineX(sc *scope, n *node) error {
 		return n.cfgErrorf("unsupported assign expression")
 	}
 
+	// Handle redeclarations: find out new symbols vs existing ones.
+	symIsNew := map[string]bool{}
+	hasNewSymbol := false
+	for i := range types {
+		id := n.child[i].ident
+		if id == "_" || id == "" {
+			continue
+		}
+		if _, found := symIsNew[id]; found {
+			return n.cfgErrorf("%s repeated on left side of :=", id)
+		}
+		// A new symbol doesn't exist in current scope. Upper scopes are not
+		// taken into accout here, as a new symbol can shadow an existing one.
+		if _, found := sc.sym[id]; found {
+			symIsNew[id] = false
+		} else {
+			symIsNew[id] = true
+			hasNewSymbol = true
+		}
+	}
+
 	for i, t := range types {
 		var index int
 		id := n.child[i].ident
-		if sym, level, ok := sc.lookup(id); ok && level == n.child[i].level && sym.kind == varSym && sym.typ.id() == t.id() {
-			// Reuse symbol in case of a variable redeclaration with the same type.
+		// A variable can be redeclared if at least one other not blank variable is created.
+		// The redeclared variable must be of same type (it is reassigned, not created).
+		// Careful to not reuse a variable which has been shadowed (it must not be a newSym).
+		sym, level, ok := sc.lookup(id)
+		canRedeclare := hasNewSymbol && len(symIsNew) > 1 && !symIsNew[id] && ok
+		if canRedeclare && level == n.child[i].level && sym.kind == varSym && sym.typ.id() == t.id() {
 			index = sym.index
 		} else {
 			index = sc.add(t)
@@ -2420,15 +2460,22 @@ func genGlobalVarDecl(nodes []*node, sc *scope) (*node, error) {
 
 func getVarDependencies(nod *node, sc *scope) (deps []*node) {
 	nod.Walk(func(n *node) bool {
-		if n.kind == identExpr {
-			if sym, _, ok := sc.lookup(n.ident); ok {
+		if n.kind != identExpr {
+			return true
+		}
+		// Process ident nodes, and avoid false dependencies.
+		if n.anc.kind == selectorExpr && childPos(n) == 1 {
+			return false
+		}
+		sym, _, ok := sc.lookup(n.ident)
+		if !ok {
+			return false
+		}
 		if sym.kind != varSym || !sym.global || sym.node == nod {
 			return false
 		}
 		deps = append(deps, sym.node)
-			}
-		}
-		return true
+		return false
 	}, nil)
 	return deps
 }

interp/doc.go

@@ -4,7 +4,7 @@ Package interp provides a complete Go interpreter.
 For the Go language itself, refer to the official Go specification
 https://golang.org/ref/spec.
 
-Importing packages
+# Importing packages
 
 Packages can be imported in source or binary form, using the standard
 Go import statement. In source form, packages are searched first in the
@@ -16,7 +16,7 @@ Binary form packages are compiled and linked with the interpreter
 executable, and exposed to scripts with the Use method. The extract
 subcommand of yaegi can be used to generate package wrappers.
 
-Custom build tags
+# Custom build tags
 
 Custom build tags allow to control which files in imported source
 packages are interpreted, in the same way as the "-tags" option of the

interp/interp.go

@@ -4,16 +4,12 @@ import (
 	"bufio"
 	"context"
 	"errors"
-	"flag"
 	"fmt"
 	"go/build"
-	"go/constant"
 	"go/scanner"
 	"go/token"
 	"io"
 	"io/fs"
-	"log"
-	"math/bits"
 	"os"
 	"os/signal"
 	"path"
@@ -547,62 +543,6 @@ func (interp *Interpreter) EvalTest(path string) error {
 	return err
 }
 
-// Symbols returns a map of interpreter exported symbol values for the given
-// import path. If the argument is the empty string, all known symbols are
-// returned.
-func (interp *Interpreter) Symbols(importPath string) Exports {
-	m := map[string]map[string]reflect.Value{}
-	interp.mutex.RLock()
-	defer interp.mutex.RUnlock()
-
-	for k, v := range interp.srcPkg {
-		if importPath != "" && k != importPath {
-			continue
-		}
-		syms := map[string]reflect.Value{}
-		for n, s := range v {
-			if !canExport(n) {
-				// Skip private non-exported symbols.
-				continue
-			}
-			switch s.kind {
-			case constSym:
-				syms[n] = s.rval
-			case funcSym:
-				syms[n] = genFunctionWrapper(s.node)(interp.frame)
-			case varSym:
-				syms[n] = interp.frame.data[s.index]
-			case typeSym:
-				syms[n] = reflect.New(s.typ.TypeOf())
-			}
-		}
-
-		if len(syms) > 0 {
-			m[k] = syms
-		}
-
-		if importPath != "" {
-			return m
-		}
-	}
-
-	if importPath != "" && len(m) > 0 {
-		return m
-	}
-
-	for k, v := range interp.binPkg {
-		if importPath != "" && k != importPath {
-			continue
-		}
-		m[k] = v
-		if importPath != "" {
-			return m
-		}
-	}
-
-	return m
-}
-
 func isFile(filesystem fs.FS, path string) bool {
 	fi, err := fs.Stat(filesystem, path)
 	return err == nil && fi.Mode().IsRegular()
@@ -664,167 +604,6 @@ func (interp *Interpreter) stop() {
 
 func (interp *Interpreter) runid() uint64 { return atomic.LoadUint64(&interp.id) }
 
-// getWrapper returns the wrapper type of the corresponding interface, or nil if not found.
-func (interp *Interpreter) getWrapper(t reflect.Type) reflect.Type {
-	if p, ok := interp.binPkg[t.PkgPath()]; ok {
-		return p["_"+t.Name()].Type().Elem()
-	}
-	return nil
-}
-
-// Use loads binary runtime symbols in the interpreter context so
-// they can be used in interpreted code.
-func (interp *Interpreter) Use(values Exports) error {
-	for k, v := range values {
-		importPath := path.Dir(k)
-		packageName := path.Base(k)
-
-		if k == "." && v["MapTypes"].IsValid() {
-			// Use mapping for special interface wrappers.
-			for kk, vv := range v["MapTypes"].Interface().(map[reflect.Value][]reflect.Type) {
-				interp.mapTypes[kk] = vv
-			}
-			continue
-		}
-
-		if importPath == "." {
-			return fmt.Errorf("export path %[1]q is missing a package name; did you mean '%[1]s/%[1]s'?", k)
-		}
-
-		if importPath == selfPrefix {
-			interp.hooks.Parse(v)
-			continue
-		}
-
-		if interp.binPkg[importPath] == nil {
-			interp.binPkg[importPath] = make(map[string]reflect.Value)
-			interp.pkgNames[importPath] = packageName
-		}
-
-		for s, sym := range v {
-			interp.binPkg[importPath][s] = sym
-		}
-		if k == selfPath {
-			interp.binPkg[importPath]["Self"] = reflect.ValueOf(interp)
-		}
-	}
-
-	// Checks if input values correspond to stdlib packages by looking for one
-	// well known stdlib package path.
-	if _, ok := values["fmt/fmt"]; ok {
-		fixStdlib(interp)
-	}
-	return nil
-}
-
-// fixStdlib redefines interpreter stdlib symbols to use the standard input,
-// output and errror assigned to the interpreter. The changes are limited to
-// the interpreter only.
-// Note that it is possible to escape the virtualized stdio by
-// read/write directly to file descriptors 0, 1, 2.
-func fixStdlib(interp *Interpreter) {
-	p := interp.binPkg["fmt"]
-	if p == nil {
-		return
-	}
-
-	stdin, stdout, stderr := interp.stdin, interp.stdout, interp.stderr
-
-	p["Print"] = reflect.ValueOf(func(a ...interface{}) (n int, err error) { return fmt.Fprint(stdout, a...) })
-	p["Printf"] = reflect.ValueOf(func(f string, a ...interface{}) (n int, err error) { return fmt.Fprintf(stdout, f, a...) })
-	p["Println"] = reflect.ValueOf(func(a ...interface{}) (n int, err error) { return fmt.Fprintln(stdout, a...) })
-
-	p["Scan"] = reflect.ValueOf(func(a ...interface{}) (n int, err error) { return fmt.Fscan(stdin, a...) })
-	p["Scanf"] = reflect.ValueOf(func(f string, a ...interface{}) (n int, err error) { return fmt.Fscanf(stdin, f, a...) })
-	p["Scanln"] = reflect.ValueOf(func(a ...interface{}) (n int, err error) { return fmt.Fscanln(stdin, a...) })
-
-	// Update mapTypes to virtualized symbols as well.
-	interp.mapTypes[p["Print"]] = interp.mapTypes[reflect.ValueOf(fmt.Print)]
-	interp.mapTypes[p["Printf"]] = interp.mapTypes[reflect.ValueOf(fmt.Printf)]
-	interp.mapTypes[p["Println"]] = interp.mapTypes[reflect.ValueOf(fmt.Println)]
-	interp.mapTypes[p["Scan"]] = interp.mapTypes[reflect.ValueOf(fmt.Scan)]
-	interp.mapTypes[p["Scanf"]] = interp.mapTypes[reflect.ValueOf(fmt.Scanf)]
-	interp.mapTypes[p["Scanln"]] = interp.mapTypes[reflect.ValueOf(fmt.Scanln)]
-
-	if p = interp.binPkg["flag"]; p != nil {
-		c := flag.NewFlagSet(os.Args[0], flag.PanicOnError)
-		c.SetOutput(stderr)
-		p["CommandLine"] = reflect.ValueOf(&c).Elem()
-	}
-
-	if p = interp.binPkg["log"]; p != nil {
-		l := log.New(stderr, "", log.LstdFlags)
-		// Restrict Fatal symbols to panic instead of exit.
-		p["Fatal"] = reflect.ValueOf(l.Panic)
-		p["Fatalf"] = reflect.ValueOf(l.Panicf)
-		p["Fatalln"] = reflect.ValueOf(l.Panicln)
-
-		p["Flags"] = reflect.ValueOf(l.Flags)
-		p["Output"] = reflect.ValueOf(l.Output)
-		p["Panic"] = reflect.ValueOf(l.Panic)
-		p["Panicf"] = reflect.ValueOf(l.Panicf)
-		p["Panicln"] = reflect.ValueOf(l.Panicln)
-		p["Prefix"] = reflect.ValueOf(l.Prefix)
-		p["Print"] = reflect.ValueOf(l.Print)
-		p["Printf"] = reflect.ValueOf(l.Printf)
-		p["Println"] = reflect.ValueOf(l.Println)
-		p["SetFlags"] = reflect.ValueOf(l.SetFlags)
-		p["SetOutput"] = reflect.ValueOf(l.SetOutput)
-		p["SetPrefix"] = reflect.ValueOf(l.SetPrefix)
-		p["Writer"] = reflect.ValueOf(l.Writer)
-
-		// Update mapTypes to virtualized symbols as well.
-		interp.mapTypes[p["Print"]] = interp.mapTypes[reflect.ValueOf(log.Print)]
-		interp.mapTypes[p["Printf"]] = interp.mapTypes[reflect.ValueOf(log.Printf)]
-		interp.mapTypes[p["Println"]] = interp.mapTypes[reflect.ValueOf(log.Println)]
-		interp.mapTypes[p["Panic"]] = interp.mapTypes[reflect.ValueOf(log.Panic)]
-		interp.mapTypes[p["Panicf"]] = interp.mapTypes[reflect.ValueOf(log.Panicf)]
-		interp.mapTypes[p["Panicln"]] = interp.mapTypes[reflect.ValueOf(log.Panicln)]
-	}
-
-	if p = interp.binPkg["os"]; p != nil {
-		p["Args"] = reflect.ValueOf(&interp.args).Elem()
-		if interp.specialStdio {
-			// Inherit streams from interpreter even if they do not have a file descriptor.
-			p["Stdin"] = reflect.ValueOf(&stdin).Elem()
-			p["Stdout"] = reflect.ValueOf(&stdout).Elem()
-			p["Stderr"] = reflect.ValueOf(&stderr).Elem()
-		} else {
-			// Inherits streams from interpreter only if they have a file descriptor and preserve original type.
-			if s, ok := stdin.(*os.File); ok {
-				p["Stdin"] = reflect.ValueOf(&s).Elem()
-			}
-			if s, ok := stdout.(*os.File); ok {
-				p["Stdout"] = reflect.ValueOf(&s).Elem()
-			}
-			if s, ok := stderr.(*os.File); ok {
-				p["Stderr"] = reflect.ValueOf(&s).Elem()
-			}
-		}
-		if !interp.unrestricted {
-			// In restricted mode, scripts can only access to a passed virtualized env, and can not write the real one.
-			getenv := func(key string) string { return interp.env[key] }
-			p["Clearenv"] = reflect.ValueOf(func() { interp.env = map[string]string{} })
-			p["ExpandEnv"] = reflect.ValueOf(func(s string) string { return os.Expand(s, getenv) })
-			p["Getenv"] = reflect.ValueOf(getenv)
-			p["LookupEnv"] = reflect.ValueOf(func(key string) (s string, ok bool) { s, ok = interp.env[key]; return })
-			p["Setenv"] = reflect.ValueOf(func(key, value string) error { interp.env[key] = value; return nil })
-			p["Unsetenv"] = reflect.ValueOf(func(key string) error { delete(interp.env, key); return nil })
-			p["Environ"] = reflect.ValueOf(func() (a []string) {
-				for k, v := range interp.env {
-					a = append(a, k+"="+v)
-				}
-				return
-			})
-		}
-	}
-
-	if p = interp.binPkg["math/bits"]; p != nil {
-		// Do not trust extracted value maybe from another arch.
-		p["UintSize"] = reflect.ValueOf(constant.MakeInt64(bits.UintSize))
-	}
-}
-
 // ignoreScannerError returns true if the error from Go scanner can be safely ignored
 // to let the caller grab one more line before retrying to parse its input.
 func ignoreScannerError(e *scanner.Error, s string) bool {

interp/interp_eval_test.go

@@ -133,6 +133,8 @@ func TestEvalAssign(t *testing.T) {
 		{src: "j := true || _", err: "1:33: cannot use _ as value"},
 		{src: "j := true && _", err: "1:33: cannot use _ as value"},
 		{src: "j := interface{}(int(1)); j.(_)", err: "1:54: cannot use _ as value"},
+		{src: "ff := func() (a, b, c int) {return 1, 2, 3}; x, y, x := ff()", err: "1:73: x repeated on left side of :="},
+		{src: "xx := 1; xx, _ := 2, 3", err: "1:37: no new variables on left side of :="},
 	})
 }
 

interp/interp_export_test.go

@@ -33,7 +33,6 @@ func Hi(h Helloer) {
 //
 // Only the Wrap type definition needs to be exported to the interpreter (not
 // the interfaces and methods definitions).
-//
 type Wrap struct {
 	DoHello func() // related to the Hello() method.
 	// Other interface method wrappers...

interp/run.go

@@ -553,7 +553,7 @@ func convert(n *node) {
 	if c.isNil() { // convert nil to type
 		// TODO(mpl): Try to completely remove, as maybe frameType already does the job for interfaces.
 		if isInterfaceSrc(n.child[0].typ) && !isEmptyInterface(n.child[0].typ) {
-			typ = reflect.TypeOf((*valueInterface)(nil)).Elem()
+			typ = valueInterfaceType
 		}
 		n.exec = func(f *frame) bltn {
 			dest(f).Set(reflect.New(typ).Elem())
@@ -713,7 +713,7 @@ func assign(n *node) {
 		case isFuncSrc(typ):
 			t = reflect.TypeOf((*node)(nil))
 		case isInterfaceSrc(typ):
-			t = reflect.TypeOf((*valueInterface)(nil)).Elem()
+			t = valueInterfaceType
 		default:
 			t = typ.TypeOf()
 		}
@@ -1007,7 +1007,7 @@ func genFunctionWrapper(n *node) func(*frame) reflect.Value {
 				}
 				typ := def.typ.arg[i]
 				switch {
-				case isEmptyInterface(typ):
+				case isEmptyInterface(typ) || typ.TypeOf() == valueInterfaceType:
 					d[i].Set(arg)
 				case isInterfaceSrc(typ):
 					d[i].Set(reflect.ValueOf(valueInterface{value: arg.Elem()}))
@@ -1050,19 +1050,24 @@ func genInterfaceWrapper(n *node, typ reflect.Type) func(*frame) reflect.Value {
 			return value
 		}
 	}
-	mn := typ.NumMethod()
+
+	// Retrieve methods from the interface wrapper, which is a struct where all fields
+	// except the first define the methods to implement.
+	// As the field name was generated with a prefixed first character (in order to avoid
+	// collisions with method names), this first character is ignored in comparisons.
+	wrap := getWrapper(n, typ)
+	mn := wrap.NumField() - 1
 	names := make([]string, mn)
 	methods := make([]*node, mn)
 	indexes := make([][]int, mn)
 	for i := 0; i < mn; i++ {
-		names[i] = typ.Method(i).Name
+		names[i] = wrap.Field(i + 1).Name[1:]
 		methods[i], indexes[i] = n.typ.lookupMethod(names[i])
 		if methods[i] == nil && n.typ.cat != nilT {
 			// interpreted method not found, look for binary method, possibly embedded
 			_, indexes[i], _, _ = n.typ.lookupBinMethod(names[i])
 		}
 	}
-	wrap := n.interp.getWrapper(typ)
 
 	return func(f *frame) reflect.Value {
 		v := value(f)
@@ -1555,12 +1560,9 @@ func callBin(n *node) {
 			case isInterfaceSrc(c.typ):
 				values = append(values, genValueInterfaceValue(c))
 			case c.typ.cat == arrayT || c.typ.cat == variadicT:
-				switch {
-				case isEmptyInterface(c.typ.val):
+				if isEmptyInterface(c.typ.val) {
 					values = append(values, genValueArray(c))
-				case isInterfaceSrc(c.typ.val):
-					values = append(values, genValueInterfaceArray(c))
-				default:
+				} else {
 					values = append(values, genInterfaceWrapper(c, defType))
 				}
 			case isPtrSrc(c.typ):
@@ -1950,10 +1952,12 @@ func getMethodByName(n *node) {
 			}
 			val = v
 		}
+
 		if met := val.value.MethodByName(name); met.IsValid() {
 			getFrame(f, l).data[i] = met
 			return next
 		}
+
 		typ := val.node.typ
 		if typ.node == nil && typ.cat == valueT {
 			// happens with a var of empty interface type, that has value of concrete type
@@ -1963,10 +1967,32 @@ func getMethodByName(n *node) {
 			}
 			return next
 		}
+
 		m, li := typ.lookupMethod(name)
+
+		// Try harder to find a matching embedded valueInterface.
+		// TODO (marc): make sure it works for arbitrary depth and breadth.
+		if m == nil && isStruct(val.node.typ) {
+			v := val.value
+			for v.Type().Kind() == reflect.Ptr {
+				v = v.Elem()
+			}
+			nf := v.NumField()
+			for i := 0; i < nf; i++ {
+				var ok bool
+				if val, ok = v.Field(i).Interface().(valueInterface); !ok {
+					continue
+				}
+				if m, li = val.node.typ.lookupMethod(name); m != nil {
+					break
+				}
+			}
+		}
+
 		if m == nil {
 			panic(n.cfgErrorf("method not found: %s", name))
 		}
+
 		fr := f.clone(!fork)
 		nod := *m
 		nod.val = &nod
@@ -2674,8 +2700,6 @@ func doComposite(n *node, hasType bool, keyed bool) {
 			values[fieldIndex] = func(*frame) reflect.Value { return reflect.New(rft).Elem() }
 		case isFuncSrc(val.typ):
 			values[fieldIndex] = genValueAsFunctionWrapper(val)
-		case isArray(val.typ) && val.typ.val != nil && isInterfaceSrc(val.typ.val) && !isEmptyInterface(val.typ.val):
-			values[fieldIndex] = genValueInterfaceArray(val)
 		case isInterfaceSrc(ft) && (!isEmptyInterface(ft) || len(val.typ.method) > 0):
 			values[fieldIndex] = genValueInterface(val)
 		case isInterface(ft):
@@ -3556,7 +3580,7 @@ func convertLiteralValue(n *node, t reflect.Type) {
 	case n.typ.cat == nilT:
 		// Create a zero value of target type.
 		n.rval = reflect.New(t).Elem()
-	case !(n.kind == basicLit || n.rval.IsValid()) || t == nil || t.Kind() == reflect.Interface || t.Kind() == reflect.Slice && t.Elem().Kind() == reflect.Interface:
+	case !(n.kind == basicLit || n.rval.IsValid()) || t == nil || t.Kind() == reflect.Interface || t == valueInterfaceType || t.Kind() == reflect.Slice && t.Elem().Kind() == reflect.Interface:
 		// Skip non-constant values, undefined target type or interface target type.
 	case n.rval.IsValid():
 		// Convert constant value to target type.
@@ -3917,7 +3941,7 @@ func isNotNil(n *node) {
 	dest := genValue(n)
 
 	if n.fnext == nil {
-		if isInterfaceSrc(c0.typ) {
+		if isInterfaceSrc(c0.typ) && c0.typ.TypeOf() != valueInterfaceType {
 			if isInterface {
 				n.exec = func(f *frame) bltn {
 					dest(f).Set(reflect.ValueOf(!value(f).IsNil()).Convert(typ))
@@ -3962,7 +3986,7 @@ func isNotNil(n *node) {
 
 	fnext := getExec(n.fnext)
 
-	if isInterfaceSrc(c0.typ) {
+	if isInterfaceSrc(c0.typ) && c0.typ.TypeOf() != valueInterfaceType {
 		n.exec = func(f *frame) bltn {
 			if value(f).IsNil() {
 				dest(f).SetBool(false)

interp/scope.go

@@ -72,7 +72,6 @@ type symbol struct {
 //
 // In symbols, the index value corresponds to the index in scope.types, and at
 // execution to the index in frame, created exactly from the types layout.
-//
 type scope struct {
 	anc         *scope             // ancestor upper scope
 	child       []*scope           // included scopes

interp/type.go

@@ -1817,7 +1817,8 @@ func exportName(s string) string {
 
 var (
 	// TODO(mpl): generators.
-	interf   = reflect.TypeOf((*interface{})(nil)).Elem()
+	emptyInterfaceType = reflect.TypeOf((*interface{})(nil)).Elem()
+	valueInterfaceType = reflect.TypeOf((*valueInterface)(nil)).Elem()
 	constVal           = reflect.TypeOf((*constant.Value)(nil)).Elem()
 )
 
@@ -1971,7 +1972,12 @@ func (t *itype) refType(ctx *refTypeContext) reflect.Type {
 		}
 		t.rtype = reflect.FuncOf(in, out, variadic)
 	case interfaceT:
-		t.rtype = interf
+		if len(t.field) == 0 {
+			// empty interface, do not wrap it
+			t.rtype = emptyInterfaceType
+			break
+		}
+		t.rtype = valueInterfaceType
 	case mapT:
 		t.rtype = reflect.MapOf(t.key.refType(ctx), t.val.refType(ctx))
 	case ptrT:
@@ -2056,10 +2062,10 @@ func (t *itype) frameType() (r reflect.Type) {
 	case interfaceT:
 		if len(t.field) == 0 {
 			// empty interface, do not wrap it
-			r = reflect.TypeOf((*interface{})(nil)).Elem()
+			r = emptyInterfaceType
 			break
 		}
-		r = reflect.TypeOf((*valueInterface)(nil)).Elem()
+		r = valueInterfaceType
 	case mapT:
 		r = reflect.MapOf(t.key.frameType(), t.val.frameType())
 	case ptrT:
@@ -2072,6 +2078,14 @@ func (t *itype) frameType() (r reflect.Type) {
 
 func (t *itype) implements(it *itype) bool {
 	if isBin(t) {
+		// Note: in case of a valueInterfaceType, we
+		// miss required data which will be available
+		// later, so we optimistically return true to progress,
+		// and additional checks will be hopefully performed at
+		// runtime.
+		if rt := it.TypeOf(); rt == valueInterfaceType {
+			return true
+		}
 		return t.TypeOf().Implements(it.TypeOf())
 	}
 	return t.methods().contains(it.methods())
@@ -2127,11 +2141,15 @@ func (t *itype) defaultType(v reflect.Value, sc *scope) *itype {
 func (t *itype) isNil() bool { return t.cat == nilT }
 
 func (t *itype) hasNil() bool {
-	switch t.TypeOf().Kind() {
+	switch rt := t.TypeOf(); rt.Kind() {
 	case reflect.UnsafePointer:
 		return true
 	case reflect.Slice, reflect.Ptr, reflect.Func, reflect.Interface, reflect.Map, reflect.Chan:
 		return true
+	case reflect.Struct:
+		if rt == valueInterfaceType {
+			return true
+		}
 	}
 	return false
 }
@@ -2213,7 +2231,7 @@ func isEmptyInterface(t *itype) bool {
 }
 
 func isGeneric(t *itype) bool {
-	return t.cat == funcT && t.node != nil && len(t.node.child[0].child) > 0
+	return t.cat == funcT && t.node != nil && len(t.node.child) > 0 && len(t.node.child[0].child) > 0
 }
 
 func isFuncSrc(t *itype) bool {
@@ -2246,7 +2264,7 @@ func isInterfaceBin(t *itype) bool {
 }
 
 func isInterface(t *itype) bool {
-	return isInterfaceSrc(t) || t.TypeOf() != nil && t.TypeOf().Kind() == reflect.Interface
+	return isInterfaceSrc(t) || t.TypeOf() == valueInterfaceType || t.TypeOf() != nil && t.TypeOf().Kind() == reflect.Interface
 }
 
 func isBin(t *itype) bool {

interp/typecheck.go

@@ -590,7 +590,7 @@ func (check typecheck) typeAssertionExpr(n *node, typ *itype) error {
 	// https://github.com/golang/go/issues/39717 lands. It is currently impractical to
 	// type check Named types as they cannot be asserted.
 
-	if n.typ.TypeOf().Kind() != reflect.Interface {
+	if rt := n.typ.TypeOf(); rt.Kind() != reflect.Interface && rt != valueInterfaceType {
 		return n.cfgErrorf("invalid type assertion: non-interface type %s on left", n.typ.id())
 	}
 	ims := n.typ.methods()

interp/use.go

@@ -0,0 +1,252 @@
+package interp
+
+import (
+	"flag"
+	"fmt"
+	"go/constant"
+	"log"
+	"math/bits"
+	"os"
+	"path"
+	"reflect"
+)
+
+// Symbols returns a map of interpreter exported symbol values for the given
+// import path. If the argument is the empty string, all known symbols are
+// returned.
+func (interp *Interpreter) Symbols(importPath string) Exports {
+	m := map[string]map[string]reflect.Value{}
+	interp.mutex.RLock()
+	defer interp.mutex.RUnlock()
+
+	for k, v := range interp.srcPkg {
+		if importPath != "" && k != importPath {
+			continue
+		}
+		syms := map[string]reflect.Value{}
+		for n, s := range v {
+			if !canExport(n) {
+				// Skip private non-exported symbols.
+				continue
+			}
+			switch s.kind {
+			case constSym:
+				syms[n] = s.rval
+			case funcSym:
+				syms[n] = genFunctionWrapper(s.node)(interp.frame)
+			case varSym:
+				syms[n] = interp.frame.data[s.index]
+			case typeSym:
+				syms[n] = reflect.New(s.typ.TypeOf())
+			}
+		}
+
+		if len(syms) > 0 {
+			m[k] = syms
+		}
+
+		if importPath != "" {
+			return m
+		}
+	}
+
+	if importPath != "" && len(m) > 0 {
+		return m
+	}
+
+	for k, v := range interp.binPkg {
+		if importPath != "" && k != importPath {
+			continue
+		}
+		m[k] = v
+		if importPath != "" {
+			return m
+		}
+	}
+
+	return m
+}
+
+// getWrapper returns the wrapper type of the corresponding interface, trying
+// first the composed ones, or nil if not found.
+func getWrapper(n *node, t reflect.Type) reflect.Type {
+	p, ok := n.interp.binPkg[t.PkgPath()]
+	if !ok {
+		return nil
+	}
+	w := p["_"+t.Name()]
+	lm := n.typ.methods()
+
+	// mapTypes may contain composed interfaces wrappers to test against, from
+	// most complex to simplest (guaranteed by construction of mapTypes). Find the
+	// first for which the interpreter type has all the methods.
+	for _, rt := range n.interp.mapTypes[w] {
+		match := true
+		for i := 1; i < rt.NumField(); i++ {
+			// The interpreter type must have all required wrapper methods.
+			if _, ok := lm[rt.Field(i).Name[1:]]; !ok {
+				match = false
+				break
+			}
+		}
+		if match {
+			return rt
+		}
+	}
+
+	// Otherwise return the direct "non-composed" interface.
+	return w.Type().Elem()
+}
+
+// Use loads binary runtime symbols in the interpreter context so
+// they can be used in interpreted code.
+func (interp *Interpreter) Use(values Exports) error {
+	for k, v := range values {
+		importPath := path.Dir(k)
+		packageName := path.Base(k)
+
+		if k == "." && v["MapTypes"].IsValid() {
+			// Use mapping for special interface wrappers.
+			for kk, vv := range v["MapTypes"].Interface().(map[reflect.Value][]reflect.Type) {
+				interp.mapTypes[kk] = vv
+			}
+			continue
+		}
+
+		if importPath == "." {
+			return fmt.Errorf("export path %[1]q is missing a package name; did you mean '%[1]s/%[1]s'?", k)
+		}
+
+		if importPath == selfPrefix {
+			interp.hooks.Parse(v)
+			continue
+		}
+
+		if interp.binPkg[importPath] == nil {
+			interp.binPkg[importPath] = make(map[string]reflect.Value)
+			interp.pkgNames[importPath] = packageName
+		}
+
+		for s, sym := range v {
+			interp.binPkg[importPath][s] = sym
+		}
+		if k == selfPath {
+			interp.binPkg[importPath]["Self"] = reflect.ValueOf(interp)
+		}
+	}
+
+	// Checks if input values correspond to stdlib packages by looking for one
+	// well known stdlib package path.
+	if _, ok := values["fmt/fmt"]; ok {
+		fixStdlib(interp)
+	}
+	return nil
+}
+
+// fixStdlib redefines interpreter stdlib symbols to use the standard input,
+// output and errror assigned to the interpreter. The changes are limited to
+// the interpreter only.
+// Note that it is possible to escape the virtualized stdio by
+// read/write directly to file descriptors 0, 1, 2.
+func fixStdlib(interp *Interpreter) {
+	p := interp.binPkg["fmt"]
+	if p == nil {
+		return
+	}
+
+	stdin, stdout, stderr := interp.stdin, interp.stdout, interp.stderr
+
+	p["Print"] = reflect.ValueOf(func(a ...interface{}) (n int, err error) { return fmt.Fprint(stdout, a...) })
+	p["Printf"] = reflect.ValueOf(func(f string, a ...interface{}) (n int, err error) { return fmt.Fprintf(stdout, f, a...) })
+	p["Println"] = reflect.ValueOf(func(a ...interface{}) (n int, err error) { return fmt.Fprintln(stdout, a...) })
+
+	p["Scan"] = reflect.ValueOf(func(a ...interface{}) (n int, err error) { return fmt.Fscan(stdin, a...) })
+	p["Scanf"] = reflect.ValueOf(func(f string, a ...interface{}) (n int, err error) { return fmt.Fscanf(stdin, f, a...) })
+	p["Scanln"] = reflect.ValueOf(func(a ...interface{}) (n int, err error) { return fmt.Fscanln(stdin, a...) })
+
+	// Update mapTypes to virtualized symbols as well.
+	interp.mapTypes[p["Print"]] = interp.mapTypes[reflect.ValueOf(fmt.Print)]
+	interp.mapTypes[p["Printf"]] = interp.mapTypes[reflect.ValueOf(fmt.Printf)]
+	interp.mapTypes[p["Println"]] = interp.mapTypes[reflect.ValueOf(fmt.Println)]
+	interp.mapTypes[p["Scan"]] = interp.mapTypes[reflect.ValueOf(fmt.Scan)]
+	interp.mapTypes[p["Scanf"]] = interp.mapTypes[reflect.ValueOf(fmt.Scanf)]
+	interp.mapTypes[p["Scanln"]] = interp.mapTypes[reflect.ValueOf(fmt.Scanln)]
+
+	if p = interp.binPkg["flag"]; p != nil {
+		c := flag.NewFlagSet(os.Args[0], flag.PanicOnError)
+		c.SetOutput(stderr)
+		p["CommandLine"] = reflect.ValueOf(&c).Elem()
+	}
+
+	if p = interp.binPkg["log"]; p != nil {
+		l := log.New(stderr, "", log.LstdFlags)
+		// Restrict Fatal symbols to panic instead of exit.
+		p["Fatal"] = reflect.ValueOf(l.Panic)
+		p["Fatalf"] = reflect.ValueOf(l.Panicf)
+		p["Fatalln"] = reflect.ValueOf(l.Panicln)
+
+		p["Flags"] = reflect.ValueOf(l.Flags)
+		p["Output"] = reflect.ValueOf(l.Output)
+		p["Panic"] = reflect.ValueOf(l.Panic)
+		p["Panicf"] = reflect.ValueOf(l.Panicf)
+		p["Panicln"] = reflect.ValueOf(l.Panicln)
+		p["Prefix"] = reflect.ValueOf(l.Prefix)
+		p["Print"] = reflect.ValueOf(l.Print)
+		p["Printf"] = reflect.ValueOf(l.Printf)
+		p["Println"] = reflect.ValueOf(l.Println)
+		p["SetFlags"] = reflect.ValueOf(l.SetFlags)
+		p["SetOutput"] = reflect.ValueOf(l.SetOutput)
+		p["SetPrefix"] = reflect.ValueOf(l.SetPrefix)
+		p["Writer"] = reflect.ValueOf(l.Writer)
+
+		// Update mapTypes to virtualized symbols as well.
+		interp.mapTypes[p["Print"]] = interp.mapTypes[reflect.ValueOf(log.Print)]
+		interp.mapTypes[p["Printf"]] = interp.mapTypes[reflect.ValueOf(log.Printf)]
+		interp.mapTypes[p["Println"]] = interp.mapTypes[reflect.ValueOf(log.Println)]
+		interp.mapTypes[p["Panic"]] = interp.mapTypes[reflect.ValueOf(log.Panic)]
+		interp.mapTypes[p["Panicf"]] = interp.mapTypes[reflect.ValueOf(log.Panicf)]
+		interp.mapTypes[p["Panicln"]] = interp.mapTypes[reflect.ValueOf(log.Panicln)]
+	}
+
+	if p = interp.binPkg["os"]; p != nil {
+		p["Args"] = reflect.ValueOf(&interp.args).Elem()
+		if interp.specialStdio {
+			// Inherit streams from interpreter even if they do not have a file descriptor.
+			p["Stdin"] = reflect.ValueOf(&stdin).Elem()
+			p["Stdout"] = reflect.ValueOf(&stdout).Elem()
+			p["Stderr"] = reflect.ValueOf(&stderr).Elem()
+		} else {
+			// Inherits streams from interpreter only if they have a file descriptor and preserve original type.
+			if s, ok := stdin.(*os.File); ok {
+				p["Stdin"] = reflect.ValueOf(&s).Elem()
+			}
+			if s, ok := stdout.(*os.File); ok {
+				p["Stdout"] = reflect.ValueOf(&s).Elem()
+			}
+			if s, ok := stderr.(*os.File); ok {
+				p["Stderr"] = reflect.ValueOf(&s).Elem()
+			}
+		}
+		if !interp.unrestricted {
+			// In restricted mode, scripts can only access to a passed virtualized env, and can not write the real one.
+			getenv := func(key string) string { return interp.env[key] }
+			p["Clearenv"] = reflect.ValueOf(func() { interp.env = map[string]string{} })
+			p["ExpandEnv"] = reflect.ValueOf(func(s string) string { return os.Expand(s, getenv) })
+			p["Getenv"] = reflect.ValueOf(getenv)
+			p["LookupEnv"] = reflect.ValueOf(func(key string) (s string, ok bool) { s, ok = interp.env[key]; return })
+			p["Setenv"] = reflect.ValueOf(func(key, value string) error { interp.env[key] = value; return nil })
+			p["Unsetenv"] = reflect.ValueOf(func(key string) error { delete(interp.env, key); return nil })
+			p["Environ"] = reflect.ValueOf(func() (a []string) {
+				for k, v := range interp.env {
+					a = append(a, k+"="+v)
+				}
+				return
+			})
+		}
+	}
+
+	if p = interp.binPkg["math/bits"]; p != nil {
+		// Do not trust extracted value maybe from another arch.
+		p["UintSize"] = reflect.ValueOf(constant.MakeInt64(bits.UintSize))
+	}
+}

interp/value.go

@@ -176,7 +176,7 @@ func genValue(n *node) func(*frame) reflect.Value {
 		convertConstantValue(n)
 		v := n.rval
 		if !v.IsValid() {
-			v = reflect.New(interf).Elem()
+			v = reflect.New(emptyInterfaceType).Elem()
 		}
 		return func(f *frame) reflect.Value { return v }
 	case funcDecl:
@@ -287,19 +287,6 @@ func genValueRangeArray(n *node) func(*frame) reflect.Value {
 	}
 }
 
-func genValueInterfaceArray(n *node) func(*frame) reflect.Value {
-	value := genValue(n)
-	return func(f *frame) reflect.Value {
-		vi := value(f).Interface().([]valueInterface)
-		v := reflect.MakeSlice(reflect.TypeOf([]interface{}{}), len(vi), len(vi))
-		for i, vv := range vi {
-			v.Index(i).Set(vv.value)
-		}
-
-		return v
-	}
-}
-
 func genValueInterface(n *node) func(*frame) reflect.Value {
 	value := genValue(n)
 
@@ -356,7 +343,7 @@ func getConcreteValue(val reflect.Value) reflect.Value {
 
 func zeroInterfaceValue() reflect.Value {
 	n := &node{kind: basicLit, typ: &itype{cat: nilT, untyped: true, str: "nil"}}
-	v := reflect.New(interf).Elem()
+	v := reflect.New(emptyInterfaceType).Elem()
 	return reflect.ValueOf(valueInterface{n, v})
 }
 

stdlib/wrapper-composed.go

@@ -0,0 +1,88 @@
+package stdlib
+
+import (
+	"bufio"
+	"io"
+	"net"
+	"net/http"
+	"reflect"
+)
+
+// Wrappers for composed interfaces which trigger a special behavior in stdlib.
+// Note: it may become useless to pre-compile composed interface wrappers
+// once golang/go#15924 is resolved.
+
+// In net/http, a ResponseWriter may also implement a Hijacker.
+
+type _netHTTPResponseWriterHijacker struct {
+	IValue       interface{}
+	WHeader      func() http.Header
+	WWrite       func(a0 []byte) (int, error)
+	WWriteHeader func(statusCode int)
+
+	WHijack func() (net.Conn, *bufio.ReadWriter, error)
+}
+
+func (w _netHTTPResponseWriterHijacker) Header() http.Header {
+	return w.WHeader()
+}
+
+func (w _netHTTPResponseWriterHijacker) Write(a0 []byte) (int, error) {
+	return w.WWrite(a0)
+}
+
+func (w _netHTTPResponseWriterHijacker) WriteHeader(statusCode int) {
+	w.WWriteHeader(statusCode)
+}
+
+func (w _netHTTPResponseWriterHijacker) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	return w.WHijack()
+}
+
+// In io, a Reader may implement WriteTo, used by io.Copy().
+
+type _ioReaderWriteTo struct {
+	IValue interface{}
+	WRead  func(p []byte) (n int, err error)
+
+	WWriteTo func(w io.Writer) (n int64, err error)
+}
+
+func (w _ioReaderWriteTo) Read(p []byte) (n int, err error) {
+	return w.WRead(p)
+}
+
+func (w _ioReaderWriteTo) WriteTo(wr io.Writer) (n int64, err error) {
+	return w.WWriteTo(wr)
+}
+
+// In io, a Writer may implement ReadFrom, used by io.Copy().
+
+type _ioWriterReadFrom struct {
+	IValue interface{}
+	WWrite func(p []byte) (n int, err error)
+
+	WReadFrom func(r io.Reader) (n int64, err error)
+}
+
+func (w _ioWriterReadFrom) Write(p []byte) (n int, err error) {
+	return w.WWrite(p)
+}
+
+func (w _ioWriterReadFrom) ReadFrom(r io.Reader) (n int64, err error) {
+	return w.WReadFrom(r)
+}
+
+// Each MapType value (each slice) must be sorted by complexity, i.e. by number
+// of interface methods.
+func init() {
+	MapTypes[reflect.ValueOf((*_net_http_ResponseWriter)(nil))] = []reflect.Type{
+		reflect.ValueOf((*_netHTTPResponseWriterHijacker)(nil)).Type().Elem(),
+	}
+	MapTypes[reflect.ValueOf((*_io_Reader)(nil))] = []reflect.Type{
+		reflect.ValueOf((*_ioReaderWriteTo)(nil)).Type().Elem(),
+	}
+	MapTypes[reflect.ValueOf((*_io_Writer)(nil))] = []reflect.Type{
+		reflect.ValueOf((*_ioWriterReadFrom)(nil)).Type().Elem(),
+	}
+}