cmd/go: reports errors for a correct preamble when it contains a builtin C type and a `C.*` name can't be resolved

[qmuntal]

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

go version go1.17.2 windows/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\qmuntaldiaz\AppData\Local\go-build
set GOENV=C:\Users\qmuntaldiaz\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\qmuntaldiaz\go\pkg\mod
set GONOPROXY=github.com/microsoft/*
set GONOSUMDB=github.com/microsoft/*
set GOOS=windows
set GOPATH=C:\Users\qmuntaldiaz\go
set GOPRIVATE=github.com/microsoft/*
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.17.2
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=C:\Users\qmuntaldiaz\code\gotest\go.mod
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\QMUNTA~1\AppData\Local\Temp\go-build2404042358=/tmp/go-build -gno-record-gcc-switches

What did you do?

==foo.go==

package foo

// int StrLen(_GoString_ s) {
//	return _GoStringLen(s);
// }
import "C"
import "fmt"

func Test() {
	fmt.Println(C.StrLen1("goo"))
}

go tool cgo foo.go

What did you expect to see?

Only this error message:

foo.go:10:14: could not determine kind of name for C.StrLen1

What did you see instead?

I saw the previous error and a preamble error, when the preamble is perfectly fine.

foo.go:10:14: could not determine kind of name for C.StrLen1
cgo: 
gcc errors for preamble:
foo.go:3:13: error: unknown type name '_GoString_'
    3 | // int StrLen(_GoString_ s) {
      |             ^~~~~~~~~~

_GoString_ is defined in the buildin prolog but we are not adding the prolog to the preamble when checking if the preamble has any error:

go/src/cmd/cgo/gcc.go

Lines 492 to 495 in e7d5857

	preambleErrors := p.gccErrors([]byte(f.Preamble))
	if len(preambleErrors) > 0 {
	error_(token.NoPos, "\n%s errors for preamble:\n%s", gccBaseCmd[0], preambleErrors)
	}

[gopherbot]

Change https://golang.org/cl/379754 mentions this issue: cmd/go: prepend builtin prolog when checking for preamble errors

[heschi]

cc @ianlancetaylor