I've run into an unusual problem. In my real code, the result is inconsistent:

If I run my code once, it succeeds, but the second run, with no manual changes, produces the error:

$ gopherjs run ./foo/foo.go
gopherjs: Source maps disabled. Install source-map-support module for nice stack traces. See https://github.com/gopherjs/gopherjs#gopherjs-run-gopherjs-test.
2020-04-10 13:42:11.405 +0200 Central European Summer Time m=+0.083000001
2020-04-10 13:42:11
{13806968577655557952 83000001 0x1}
jonhall@hyaline:~/go/src/gitlab.com/FlashbackSRS/priv/fbrender$ gopherjs run ./foo/foo.go
foo/foo.go:14:14: cannot convert x (variable of type time.Time) to fb.Due

Running go mod vendor somehow "resets" the state, so that the next run will work again, but after one success, it's broken again.

Strange.

I have been able to reproduce a minimal test case, but it consistently exhibits the failure, so I'm not sure what's different. Full code is here: https://github.com/flimzy/timeconvertiontest

Summary:

Define a custom type Time in an imported package:

package foo

import "time"

type Time time.Time

Then try to convert from time.Time to the custom type:

package main

import (
    "time"
    "foo"
)

func main() {
    _ = foo.Time(time.Now())
}

When I build the code in the linked repository, using both Go (1.14) and GopherJS (1.12):

$ go build main.go
$ gopherjs build main.go
main.go:10:17: cannot convert time.Now() (value of type time.Time) to types.Time