interp: fix assignable check

The assignable check used to be too strict as it lacked the property that
if an untyped const can be represented as a T, then it is assignable to T.

And we can now use that fixed check to add a missing check: in a return
statement, we now make sure that any of the returned elements are
assignable to what the signature tells us they should be.

_test/fun24.go

@@ -0,0 +1,10 @@
+package main
+
+func f(x int) (int, int) { return x, "foo" }
+
+func main() {
+	print("hello")
+}
+
+// Error:
+// cannot use "foo" (type stringT) as type intT in return argument

_test/fun25.go

@@ -0,0 +1,10 @@
+package main
+
+func f(x string) (a int, b int) { return x, 5 }
+
+func main() {
+	print("hello")
+}
+
+// Error:
+// cannot use x (type stringT) as type intT in return argument

interp/cfg.go

@@ -1302,7 +1302,8 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 				err = n.cfgErrorf("too many arguments to return")
 				break
 			}
-			if mustReturnValue(sc.def.child[2]) {
+			returnSig := sc.def.child[2]
+			if mustReturnValue(returnSig) {
 				nret := len(n.child)
 				if nret == 1 && isCall(n.child[0]) {
 					nret = n.child[0].child[0].typ.numOut()
@@ -1316,13 +1317,19 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 			n.tnext = nil
 			n.val = sc.def
 			for i, c := range n.child {
+				var typ *itype
+				typ, err = nodeType(interp, sc, returnSig.child[1].fieldType(i))
+				if err != nil {
+					return
+				}
+				// TODO(mpl): move any of that code to typecheck?
+				c.typ.node = c
+				if !c.typ.assignableTo(typ) {
+					err = fmt.Errorf("cannot use %v (type %v) as type %v in return argument", c.ident, c.typ.cat, typ.cat)
+					return
+				}
 				if c.typ.cat == nilT {
 					// nil: Set node value to zero of return type
-					f := sc.def
-					var typ *itype
-					if typ, err = nodeType(interp, sc, f.child[2].child[1].fieldType(i)); err != nil {
-						return
-					}
 					if typ.cat == funcT {
 						// Wrap the typed nil value in a node, as per other interpreter functions
 						c.rval = reflect.ValueOf(&node{kind: basicLit, rval: reflect.New(typ.TypeOf()).Elem()})

interp/interp_consistent_test.go

@@ -44,6 +44,8 @@ func TestInterpConsistencyBuild(t *testing.T) {
 			file.Name() == "fun21.go" || // expect error
 			file.Name() == "fun22.go" || // expect error
 			file.Name() == "fun23.go" || // expect error
+			file.Name() == "fun24.go" || // expect error
+			file.Name() == "fun25.go" || // expect error
 			file.Name() == "if2.go" || // expect error
 			file.Name() == "import6.go" || // expect error
 			file.Name() == "init1.go" || // expect error

interp/type.go

@@ -923,7 +923,23 @@ func (t *itype) assignableTo(o *itype) bool {
 	if t.isNil() && o.hasNil() || o.isNil() && t.hasNil() {
 		return true
 	}
-	return t.TypeOf().AssignableTo(o.TypeOf())
+
+	if t.TypeOf().AssignableTo(o.TypeOf()) {
+		return true
+	}
+
+	n := t.node
+	if n == nil || !n.rval.IsValid() {
+		return false
+	}
+	con, ok := n.rval.Interface().(constant.Value)
+	if !ok {
+		return false
+	}
+	if con == nil || !isConstType(o) {
+		return false
+	}
+	return representableConst(con, o.TypeOf())
 }
 
 // convertibleTo returns true if t is convertible to o.
@@ -932,7 +948,7 @@ func (t *itype) convertibleTo(o *itype) bool {
 		return true
 	}
 
-	// unsafe checkes
+	// unsafe checks
 	tt, ot := t.TypeOf(), o.TypeOf()
 	if (tt.Kind() == reflect.Ptr || tt.Kind() == reflect.Uintptr) && ot.Kind() == reflect.UnsafePointer {
 		return true