text/template: unintuitive behavior of parenthesized arguments in text/template and html/template

[julianbrost]

What did you do?

Execute a template like {{ (1) 2 }} with text/template and html/template. Full example on Go Playground: https://play.golang.org/p/nDdB_6tzx8G

What did you expect to see?

I'd expect {{ 1 2 }} and {{ (1) 2 }} to behave the same.The last item at https://golang.org/pkg/text/template/#hdr-Arguments reads like parenthesizing a single argument should have no effect.

What did you see instead?

{{ 1 2 }} yields an error (as I'd expect) but {{ (1) 2 }} outputs 1 without an error.

System details

go version go1.12.4 linux/amd64
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/julian/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/julian/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/lib/go"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
GOROOT/bin/go version: go version go1.12.4 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.12.4
uname -sr: Linux 5.0.9-arch1-1-ARCH
LSB Version:	1.4
Distributor ID:	Arch
Description:	Arch Linux
Release:	rolling
Codename:	n/a
/usr/lib/libc.so.6: GNU C Library (GNU libc) stable release version 2.29.
gdb --version: GNU gdb (GDB) 8.2.1

[katiehockman]

/cc @mvdan @robpike

[gopherbot]

Change https://golang.org/cl/206124 mentions this issue: text/template: add error check for parenthesized first argument in pipeline

[gopherbot]

Change https://golang.org/cl/217978 mentions this issue: doc/go1.14: mention better error checking in text/template