context: Breaking change in Go 1.15 not called out in the release notes

[dylan-bourque]

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

$ go version
go version go1.15 linux/amd64

Does this issue reproduce with the latest release?

Yes. This is a new issue introduced in Go 1.15.

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

go env Output (using the `golang:1.15` Docker image)

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/root/sandbox/go.mod"
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=/tmp/go-build266629529=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Run pre-existing code that was passing nil as the parent context to context.WithValue(). Minimal example: https://play.golang.org/p/J0Bxux7Bfs6

What did you expect to see?

The string "didn't panic" written to standard out

What did you see instead?

panic: cannot create context from nil parent

goroutine 1 [running]:
context.WithValue(0x0, 0x0, 0x4afc80, 0xc0000961f0, 0x4afa40, 0x4e7ae0, 0x0, 0x0)
	/usr/local/go-faketime/src/context/context.go:521 +0x187
main.main()
	/tmp/sandbox506034916/prog.go:14 +0x77

Program exited: status 2.

This change in behavior was introduced by 0205790. While I agree with the change in principle, I feel like this should have been specifically called out in the release notes for Go 1.15.

Updating context.WithCancel(), context.WithDeadline() and context.WithValue() to panic when they did not in Go 1.14 and prior will likely be a breaking change for users and, I feel, should be a highlighted change for visibility.

[icholy]

It would be nice if passing a nil parent behaved the same as passing a context.Background()

[gopherbot]

Change https://golang.org/cl/248329 mentions this issue: doc/go1.15: include behavior updates to the context package

[gopherbot]

Change https://golang.org/cl/248331 mentions this issue: [release-branch.go1.15] doc/go1.15: include behavior updates to the context package