interp: error instead of panic when assigning to a constant

Add early detection of assigning to a constant during compiling instead of panicking at runtime.

interp/cfg.go

@@ -579,6 +579,10 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 				var sym *symbol
 				var level int
 
+				if dest.rval.IsValid() && isConstType(dest.typ) {
+					err = n.cfgErrorf("cannot assign to %s (%s constant)", dest.rval, dest.typ.str)
+					break
+				}
 				if isBlank(src) {
 					err = n.cfgErrorf("cannot use _ as value")
 					break

interp/interp_eval_test.go

@@ -135,6 +135,7 @@ func TestEvalAssign(t *testing.T) {
 		{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 :="},
+		{src: "1 = 2", err: "1:28: cannot assign to 1 (untyped int constant)"},
 	})
 }