interp: fix handling interface in operators

In case of interface values, make sure that the concrete type is preserved during type inference.

Fixes #1466.

_test/issue-1466.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"fmt"
+)
+
+func SomeFunc(defaultValue interface{}) interface{} {
+	switch v := defaultValue.(type) {
+	case string:
+		return v + " abc"
+	case int:
+		return v - 234
+	}
+	panic("whoops")
+}
+
+func main() {
+	fmt.Println(SomeFunc(1234))
+	fmt.Println(SomeFunc("test"))
+}
+
+// Output:
+// 1000
+// test abc

interp/cfg.go

@@ -679,7 +679,7 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 					n.gen = nop
 					src.findex = dest.findex
 					src.level = level
-				case len(n.child) < 4 && isArithmeticAction(src):
+				case len(n.child) < 4 && isArithmeticAction(src) && !isInterface(dest.typ):
 					// Optimize single assignments from some arithmetic operations.
 					src.typ = dest.typ
 					src.findex = dest.findex
@@ -828,9 +828,7 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 			case n.anc.kind == returnStmt:
 				// To avoid a copy in frame, if the result is to be returned, store it directly
 				// at the frame location reserved for output arguments.
-				pos := childPos(n)
-				n.typ = sc.def.typ.ret[pos]
-				n.findex = pos
+				n.findex = childPos(n)
 			default:
 				// Allocate a new location in frame, and store the result here.
 				n.findex = sc.add(n.typ)

interp/type.go

@@ -598,7 +598,10 @@ func nodeType2(interp *Interpreter, sc *scope, n *node, seen []*node) (t *itype,
 		}
 
 		if isInterfaceSrc(dt) {
-			dt.val = t
+			// Set a new interface type preserving the concrete type (.val field).
+			t2 := *dt
+			t2.val = t
+			dt = &t2
 		}
 		t = dt