Can you consistently reproduce this error? Does the folder with spaces always show errors or only occasionally?

Yes, I also made sure this reproduces on Windows 10 (I encountered it first on Ubuntu 19.04), and it always shows this error on any file that is not of package main when the project is found within a folder with spaces.

Update: I also encountered it today on the same project under folder VSCode (the presumable temporary workaround), and the error showed again, until I restarted Visual Studio Code, after which it didn't show again.
I don't really understand the behavior of this error so I'll edit the issue to remove the Temporary workaround section.

Thanks for sharing. We will investigate this issue.

As an additional data point, I am seeing the same error. The difference is that I don't have any folder with spaces in it. I am seeing it when the file is part of a vscode workspace.

What I noticed is the following: the error is reported by gopls when I add a comment to to the first line of the file, and the file is part of a Go package split in multiple files. If I keep the comment and just restart vscode, the error goes away :-)

It only happens to me when I am opening one of the files in the VSCode workspace.
When the directory is open VSCode is not reporting on any error in the problems pan.
The issue I am seeing was changed into:

{
	"resource": "/home/eliezer/Scripts/golang-http-proxy/config.go",
	"owner": "_generated_diagnostic_collection_name_#0",
	"severity": 8,
	"message": "undeclared name: CertificateCache",
	"source": "LSP",
	"startLineNumber": 60,
	"startColumn": 19,
	"endLineNumber": 60,
	"endColumn": 35
}

When it's declared as:

package main
....
type CertificateCache struct {
	lock        sync.RWMutex
	cache       map[certCacheKey]certCacheEntry
	TTL         time.Duration
	lastCleaned time.Time
}

In a tls.go file in the same folder.
I do not know what might even causing this issue.

@marco-m, @elico: Can you please open new issues? It seems like the problems you are seeing are not related to the spaces in the path names, so it will be easier for me to help you troubleshoot on separate issues. Please include the information listed here.

@omrishtam: Can you clarify your edit (the strikethrough on the workaround)? Do you still encounter errors when your code is in a folder with no spaces?

To follow up, I tried reproducing the issue when using a folder with spaces and was unable to. It seems like something else is going on here.

I think the vscode-go plugin should take the responsibility.

It seems vscode-go plugin assumes that there should be a go.mod file in the work-space (or the directory you chose at "Open Directory" in vscode) or it's parent directories.

If the go.mod file is in a child directory the vscode-go plugin can not find the right module that the .go file belongs to.

I've tested all the cases that I can imagine. The best practice is to place only one go.mod file in the "project path" that contains all the necessary files, then open that project path from inside vscode. In this case everything would be ok, no errors, no warinings.

@terender: Your comment sounds more related to #32394. You are correct, gopls requires the user to open the workspace to the module root.

It seems like the majority of comments on this issue are not actually related to the folder having spaces, and as I was unable to reproduce that issue, I will mark this as closed. If others have different issues with gopls, please open new issues.