interp: fix variables scope in functions

Variables declared as exported or readonly in functions
without assignment are not correctly propagated to the
global scope, this snippet should return a `bar: readonly variable`
error:

```
_foo() {
    bar=foo
    readonly bar
}

_foo
bar=bar
```

This snippet should also export `bar` variable correctly:

```
_foo() {
    export bar
}

_foo
bar=foo
env | grep ^bar=
```

interp/interp_test.go

@@ -2303,6 +2303,10 @@ set +o pipefail
 	{"export foo=(1 2); $ENV_PROG | grep '^foo='", "exit status 1"},
 	{"declare -A foo=([a]=b); export foo; $ENV_PROG | grep '^foo='", "exit status 1"},
 	{"export foo=(b c); foo=x; $ENV_PROG | grep '^foo='", "exit status 1"},
+	{"foo() { bar=foo; export bar; }; foo; $ENV_PROG | grep ^bar=", "bar=foo\n"},
+	{"foo() { export bar; }; bar=foo; foo; $ENV_PROG | grep ^bar=", "bar=foo\n"},
+	{"foo() { export bar; }; foo; bar=foo; $ENV_PROG | grep ^bar=", "bar=foo\n"},
+	{"foo() { export bar=foo; }; foo; readonly bar; $ENV_PROG | grep ^bar=", "bar=foo\n"},
 
 	// local
 	{
@@ -2430,6 +2434,18 @@ set +o pipefail
 		"readonly foo=bar; foo=etc",
 		"foo: readonly variable\nexit status 1 #JUSTERR",
 	},
+	{
+		"foo() { bar=foo; readonly bar; }; foo; bar=bar",
+		"bar: readonly variable\nexit status 1 #JUSTERR",
+	},
+	{
+		"foo() { readonly bar; }; foo; bar=foo",
+		"bar: readonly variable\nexit status 1 #JUSTERR",
+	},
+	{
+		"foo() { readonly bar=foo; }; foo; export bar; $ENV_PROG | grep '^bar='",
+		"bar=foo\n",
+	},
 
 	// multiple var modes at once
 	{

interp/vars.go

@@ -33,7 +33,14 @@ func (o *overlayEnviron) Get(name string) expand.Variable {
 func (o *overlayEnviron) Set(name string, vr expand.Variable) error {
 	// "foo=bar" in a function updates the global scope, unless the variable
 	// has been declared as local.
-	if o.funcScope && vr.IsSet() && !vr.Local && !o.values[name].Local {
+	if o.funcScope && !vr.Local && !o.values[name].Local {
+		// "foo=bar" followed by "export foo" or "readonly foo"
+		if !vr.IsSet() {
+			prev := o.Get(name)
+			prev.Exported = prev.Exported || vr.Exported
+			prev.ReadOnly = prev.ReadOnly || vr.ReadOnly
+			vr = prev
+		}
 		return o.parent.(expand.WriteEnviron).Set(name, vr)
 	}
 
@@ -64,6 +71,9 @@ func (o *overlayEnviron) Set(name string, vr expand.Variable) error {
 			writeEnv.Set(name, vr)
 			return nil
 		}
+	} else if prev.Exported {
+		// variable is set and was marked as exported
+		vr.Exported = true
 	}
 	// modifying the entire variable
 	vr.Local = prev.Local || vr.Local