diff --git a/.github/workflows/go.yml b/.github/workflows/go.yml
index 3f0e45e..26242e6 100644
--- a/.github/workflows/go.yml
+++ b/.github/workflows/go.yml
@@ -16,7 +16,7 @@ jobs:
           fetch-depth: 0
 
       - name: Set up Go
-        uses: actions/setup-go@v2
+        uses: actions/setup-go@v4
         with:
           go-version: 1.19
 
diff --git a/.github/workflows/golangci-lint.yml b/.github/workflows/golangci-lint.yml
new file mode 100644
index 0000000..dd6f4a7
--- /dev/null
+++ b/.github/workflows/golangci-lint.yml
@@ -0,0 +1,53 @@
+name: golangci-lint
+on:
+  push:
+    tags:
+      - v*
+    branches:
+      - master
+      - main
+  pull_request:
+
+permissions:
+  contents: read
+  # Optional: allow read access to pull request. Use with `only-new-issues` option.
+  # pull-requests: read
+
+jobs:
+  golangci:
+    name: lint
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up Go
+        uses: actions/setup-go@v4
+        with:
+          go-version: '1.19'
+          cache: false
+
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: golangci-lint
+        uses: golangci/golangci-lint-action@v3
+        with:
+          # Optional: version of golangci-lint to use in form of v1.2 or v1.2.3 or `latest` to use the latest version
+          version: v1.52.2
+
+          # Optional: working directory, useful for monorepos
+          # working-directory: somedir
+
+          # Optional: golangci-lint command line arguments.
+          # args: --issues-exit-code=0
+
+          # Optional: show only new issues if it's a pull request. The default value is `false`.
+          # only-new-issues: true
+
+          # Optional: if set to true then the all caching functionality will be complete disabled,
+          #           takes precedence over all other caching options.
+          # skip-cache: true
+
+          # Optional: if set to true then the action don't cache or restore ~/go/pkg.
+          # skip-pkg-cache: true
+
+          # Optional: if set to true then the action don't cache or restore ~/.cache/go-build.
+          # skip-build-cache: true
diff --git a/_testdata/foo.go b/_testdata/foo.go
index 92e5fb9..e549b66 100644
--- a/_testdata/foo.go
+++ b/_testdata/foo.go
@@ -200,3 +200,30 @@ func F27(r *os.File) (data []byte, err error) {
 func F28(r *os.File) map[int]io.Reader {
 	return map[int]io.Reader{7: r}
 }
+
+func F29(r *os.File) ([]byte, error) {
+	s := struct {
+		rd io.Reader
+	}{rd: r}
+	return io.ReadAll(s.rd)
+}
+
+func F30(f *os.File) ([]byte, error) {
+	s := struct {
+		file *os.File
+	}{file: f}
+	return io.ReadAll(s.file)
+}
+
+func F31(r *os.File) ([]byte, error) {
+	fn := func(readers ...io.Reader) ([]byte, error) {
+		for _, reader := range readers {
+			if reader != nil {
+				return io.ReadAll(reader)
+			}
+		}
+		return nil, nil
+	}
+
+	return fn(nil, r)
+}
diff --git a/debug.go b/debug.go
index 9841b57..f3d48da 100644
--- a/debug.go
+++ b/debug.go
@@ -11,9 +11,9 @@ func (a *analyzer) debugf(format string, args ...any) {
 		return
 	}
 	s := fmt.Sprintf(format, args...)
-	strings.TrimRight(s, "\r\n")
+	s = strings.TrimRight(s, "\r\n")
 	if a.level > 0 {
-		fmt.Fprintf(os.Stderr, strings.Repeat("  ", a.level))
+		fmt.Fprint(os.Stderr, strings.Repeat("  ", a.level))
 	}
 	fmt.Fprintln(os.Stderr, s)
 }
diff --git a/decouple.go b/decouple.go
index 5a49090..de9abfe 100644
--- a/decouple.go
+++ b/decouple.go
@@ -349,21 +349,27 @@ func (a *analyzer) stmt(stmt ast.Stmt) (ok bool) {
 		}
 		for i, rhs := range stmt.Rhs {
 			// xxx do a recursive analysis of how this var is used!
-			if a.isObj(rhs) && stmt.Tok != token.DEFINE {
-				if stmt.Tok != token.ASSIGN {
-					// Reject OP=
-					return false
-				}
-				tv, ok := a.pkg.TypesInfo.Types[stmt.Lhs[i]]
-				if !ok {
-					panic(errf("no type info for lvalue %d in assignment at %s", i, a.pos(stmt)))
-				}
-				intf := getInterface(tv.Type)
-				if intf == nil {
+			if a.isObj(rhs) {
+				switch stmt.Tok {
+				case token.DEFINE:
+					continue
+
+				case token.ASSIGN:
+					tv, ok := a.pkg.TypesInfo.Types[stmt.Lhs[i]]
+					if !ok {
+						panic(errf("no type info for lvalue %d in assignment at %s", i, a.pos(stmt)))
+					}
+					intf := getInterface(tv.Type)
+					if intf == nil {
+						return false
+					}
+					a.addMethods(intf)
+					continue
+
+				default:
+					// Reject OP= (e.g., foo += obj)
 					return false
 				}
-				a.addMethods(intf)
-				continue
 			}
 			if !a.expr(rhs) {
 				return false
diff --git a/decouple_test.go b/decouple_test.go
index fbc8164..53591e4 100644
--- a/decouple_test.go
+++ b/decouple_test.go
@@ -13,7 +13,7 @@ import (
 	// "github.com/davecgh/go-spew/spew"
 )
 
-func TestAnalyze(t *testing.T) {
+func TestCheckParam(t *testing.T) {
 	ctx := context.Background()
 
 	conf := &packages.Config{
diff --git a/fab.d/fab.go b/fab.d/fab.go
new file mode 100644
index 0000000..68c4640
--- /dev/null
+++ b/fab.d/fab.go
@@ -0,0 +1,30 @@
+package fab
+
+import (
+	"context"
+	"os"
+
+	"github.com/bobg/fab"
+	"github.com/bobg/fab/deps"
+	"github.com/pkg/errors"
+)
+
+var (
+	Cover = fab.Deps(showCoverage, computeCoverage)
+	Clean = fab.Clean("cover.out")
+
+	showCoverage = fab.Command("go tool cover -html cover.out")
+)
+
+var computeCoverage = fab.Register("computeCoverage", "compute coverage profile", fab.F(func(ctx context.Context) error {
+	in, err := deps.Go(".", true)
+	if err != nil {
+		return errors.Wrap(err, "in deps.Go")
+	}
+	filesTarget := fab.Files{
+		Target: fab.Command("go test -coverprofile cover.out ./...", fab.CmdStdout(os.Stdout)),
+		In:     in,
+		Out:    []string{"cover.out"},
+	}
+	return filesTarget.Run(ctx)
+}))