proposal: text/template: Template block function that receives what is inside the block as input

[lucasew]

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

$ go version
go version go1.15.7 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
NixOS flake github:lucasew/nixcfg/a3c60d9cc069a7274f17b1a8b95b323d14bfaa14#acer-nix
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/lucasew/.cache/go-build"
GOENV="/home/lucasew/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/lucasew/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/lucasew/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/nix/store/zymy4dammwvlw8v01h23iycx6v8sdp9m-go-1.15.7/share/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/nix/store/zymy4dammwvlw8v01h23iycx6v8sdp9m-go-1.15.7/share/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/run/user/1000/go-build733009741=/tmp/go-build -gno-record-gcc-swi
tches"

What did you do?

https://play.golang.org/p/SYFeODZTB4G

What did you expect to see?

*Sample text*

What did you see instead?

panic: template: test:2: function "blockapply" not defined

goroutine 1 [running]:
text/template.Must(...)
	/usr/local/go-faketime/src/text/template/helper.go:23
main.main()
	/tmp/sandbox276814028/prog.go:16 +0x28b

Program exited: status 2.

[seankhliao]

previously proposed (and declined) as #33273