text/template, html/template: nil argument to function yields wrong number of args error

[sethvargo]

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

go version go1.7.4 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/sethvargo/Development/go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.7.4_2/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.7.4_2/libexec/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/d0/g1brm3hj0431461p8cxqtjth0000gn/T/go-build267389745=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

https://play.golang.org/p/R3CMVl_nmx

What did you expect to see?

Passing a nested field in a struct that is nil should pass in the zero value for the accepting function. I either expected a different error, or I expected text/template to coerce the value into the signature of the receiving function (string in this case), and use the string zero value.

What did you see instead?

An error is returned indicating an incorrect number of arguments was provided. This is especially confusing because, even if the zero value is provided, the result of the function call should be nil. At worst you're passing nil to a function that accepts a string, but that's not "incorrect number of args", it's a TypeError.

Related Code

From my tracing, the code appears to be here.

[sethvargo]

Even if I initialize the map and/or the map key, the error message is still the same if the provided value is nil: https://play.golang.org/p/lpFcNJYNdu

[sethvargo]

Further playing has narrowed it down to

go/src/text/template/exec.go

Lines 622 to 624 in 6593d86

	if final.IsValid() {
	numIn++
	}

. In most cases, it looks like final is treated as an argument (input) to the function. In the case that the value is nil, it's not "valid" and therefore does not increment the number of valid args.

[paranoiacblack]

I updated the description since this is not specific to maps, per se. Passing nil in any way to template-evaluated function causes it to exit early rather than type-checking the argument and giving the error you expect. Consider the following example: https://play.golang.org/p/PkAsQyklDW.

It seems like it might be a good idea to try to allow nil specifically to be used as an argument although reflect.ValueOf(nil) is !reflect.Value.IsValid.

[bradfitz]

/cc @robpike

[robpike]

This is unchanged since 1.4 and probably the beginning, so no urgency but it is annoying. Not a regression, in other words.

Looks like a bug to me, might be hard to fix because nil is poorly handled by reflect and the template code doesn't do much to improve the situation.

[odeke-em]

Same problem for html/template, by modifying @paranoiacblack's test program in #18716 (comment) to import "html/template" instead i.e https://play.golang.org/p/Kf4gtQWnOn

[rsc]

Clearly this is the wrong error, but it should be some error, so this can wait until Go 1.10.

[odeke-em]

Ooh unfortunately we didn't get a CL in for Go1.10, @sethvargo might you be interested in solving this for Go1.11? Or @rsc is this still open game for Go1.10?

[sethvargo]

Hey @odeke-em

I tried to solve it myself, but unfortunately I got lost in the reflection bits 😦

[odeke-em]

Not a problem @sethvargo, I had asked because, since you had been investigating it, it'd be nice to carry on the ownership/credit and dibs :) We can take a look at it for Go1.11. Thank you!

[odeke-em]

@rsc am punting it to Go1.11, please feel free to rollback.

[gopherbot]

Change https://golang.org/cl/95215 mentions this issue: text/template: differentiate nil from missing arg

[gopherbot]

Change https://golang.org/cl/121815 mentions this issue: html/template: ignore untyped nil arguments to default escapers