go: x/tools/gopls: single file erroneously full of "undeclared name: X" errors (and more!)

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

$ go version
go version go1.12.6 darwin/amd64

$ gopls version
golang.org/x/tools/gopls v0.1.2
    golang.org/x/tools/gopls@v0.1.3 h1:CB5ECiPysqZrwxcyRjN+exyZpY0gODTZvNiqQi3lpeo=

Does this issue reproduce with the latest release?

I believe so.

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

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/peter/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/peter"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/Cellar/go/1.12.6/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.12.6/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/peter/mod/plasma/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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/8s/kvkb2dzx37n057vk54y9b_ym0000gn/T/go-build417011332=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I was working in a modules-based project, and reviewing an existing file, pkg/api/public.go. I created a new test file in the same package, contracts_test.go. I opened the file and wrote

package api

func TestContract_foo_BarBaz(t *testing.T) {

}

And saved the file. At some point between creating the new file and saving it, the public.go file I had been reading became full of red squiggles, each under a variety of identifiers for things defined in the same package, but another file, util.go: things like ErrFoo, or unexportedFunctions. Each of the errors suggested the identifier was not defined.

I ran gopls ... check on public.go at this point and got the following complete output: gist.

I opened the util.go file in the same package and confirmed the identifiers existed. When I went back to public.go, the red squiggles under the identifiers declared in the util.go file were gone, and now the only red squiggle was under an identifier for a type in another package, with the same error. Command-clicking on the identifier, however, brought me to the proper declaration in the other package.

Confused, I clicked back into public.go, and discovered: (a) the red squiggle under the identifier I had just clicked on was gone; (b) several other files in the same package which were not previously red (errored) had become red; and © there were several brand new red squiggles under all sorts of identifiers in public.go that had no common theme. For example, some code:

r := mux.NewRouter() // this is github.comgorilla/mux
// ...
s.Handler = r // s.Handler being of type http.Handler

had a red squiggle under r, the right hand side of the assignment, claiming “cannot use r (variable of type *mux.Router) as http.Handler value in assignment: wrong type for method ServeHTTP.”

At this point I ran gopls ... check on public.go again and got the following complete output: gist. (I think it is identical to before.)

I then captured the following output from gopls debug log in VS Code: gist.

I then decided I spent enough time whacking moles and ran pkill gopls. Within a second, all red files returned to their normal color, and all red squiggles went away.