go: internal/syscall/windows: GetACP returns wrong codepage

1. What version of Go are you using (go version)? 1.7

2. What operating system and processor architecture are you using (go env)? Windows, arm64 go env:

   set GOARCH=amd64
   set GOBIN=
   set GOEXE=.exe
   set GOHOSTARCH=amd64
   set GOHOSTOS=windows
   set GOOS=windows
   set GOPATH=C:\dev\golang
   set GORACE=
   set GOROOT=C:\Go
   set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
   set CC=gcc
   set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\Tobias\AppData\Local\Temp\go-build473402786=/tmp/go-build -gno-record-gcc-switches
   set CXX=g++
   set CGO_ENABLED=1
   

3. What did you do? I wrote a program which just reads bytes from os.Stdin and prints their values as Hex to Stdout. I started the program, typed a ä to the console and pressed enter.

4. What did you expect to see? I expected to see the correct UTF-8 sequence for a ä which is C3 A4

5. What did you see instead? I see the Hex-sequence E2 80 9E

I found that internal/syscall/windows GetACP() returns 1252 even though I can verify that the ä is encoded in CP850. Because of this wrongly returned codepage, Stdin.readConsole tries to decode the character from 1252 to UTF-8 instead of from 850 to UTF-8. As you see in my Post on Stackoverflow, when I read from stdin through CGO I get the byte 0x84 which is the value for a ä in CP850, it should’ve been 0xE4 if it were in 1252. The value 0x84 decoded from golang.org/x/text/encoding/charmap.CodePage850 decodes to the correct UTF-8 character.

chcp tells me that the active codepage is 850.