text/template: weird template block parameters precedence

[sylr]

What version of Go are you using (go version)?

$ go version
go1.12.7. 

Does this issue reproduce with the latest release?

Yes

What did you do?

https://play.golang.org/p/zPTLk2G-iAP

I'd like to gzip and base64 the output of the template so I've written this:

{{- define "pwet" }}
My string
{{- end}}
{{ template "pwet" . | stringToGzipedBase64 }}

This seems to be syntactically right but the way it's interpreted is not what I expected

{{ template "pwet" . | stringToGzipedBase64 }} // What I wrote
{{ template "pwet" (. | stringToGzipedBase64) }} // How it's interpreted by the parser
{{ (template "pwet" .) | stringToGzipedBase64 }} // How I think it should be interpreted 

[beoran]

The syntax for the template statement is specified in https://golang.org/pkg/text/template:

{{template "name"}}
	The template with the specified name is executed with nil data.

{{template "name" pipeline}}
	The template with the specified name is executed with dot set
	to the value of the pipeline.

This means that {{template "pwet" . | stringToGzipedBase64}} is indeed interpreted as {{ template "pwet" (. | stringToGzipedBase64) }}, because the template key word cannot send it's output to a pipeline, only take it's input from one.

So the behavior we are seeing is as specified. Unfortunately, what you are trying to do, that is, capture the result of a template statement is not supported yet. Maybe it should be, but for the moment your example is working as expected.