shfmt: state -ln=auto on posix parse errors

explicitly state language dialect on `syntax.LangError`s when ln
is set to `auto`

for example, incompatible bash constructs will cause parse errors
due to the parser using posix as language dialect

fixes #803

cmd/shfmt/main.go

@@ -426,18 +426,21 @@ func formatPath(path string, checkShebang bool) error {
 	return formatBytes(readBuf.Bytes(), path, fileLang)
 }
 
-func formatBytes(src []byte, path string, lang syntax.LangVariant) error {
+func formatBytes(src []byte, path string, fileLang syntax.LangVariant) error {
 	if useEditorConfig {
 		props, err := ecQuery.Find(path)
 		if err != nil {
 			return err
 		}
-		propsOptions(lang, props)
+		propsOptions(fileLang, props)
 	} else {
-		syntax.Variant(lang)(parser)
+		syntax.Variant(fileLang)(parser)
 	}
 	prog, err := parser.Parse(bytes.NewReader(src), path)
 	if err != nil {
+		if s, ok := err.(syntax.LangError); ok && lang.val == syntax.LangAuto {
+			return fmt.Errorf("%w (parsed as %s via -%s=%s)", s, fileLang, lang.short, lang.val)
+		}
 		return err
 	}
 	if simplify.val {

cmd/shfmt/testdata/scripts/flags.txt

@@ -92,6 +92,15 @@ stdin input-mksh
 shfmt -ln=auto -filename=input.mksh
 stdout 'coprocess'
 
+# Explicitly state language on parse errors
+stdin input-bash-arrays
+! shfmt -ln=auto -filename=input.sh
+stderr 'parsed as posix via -ln=auto'
+
+stdin input-bash-extglobs
+! shfmt -ln=auto -filename=input.sh
+stderr 'parsed as posix via -ln=auto'
+
 stdin flags-input
 shfmt -i 2
 cmp stdout flags-output.indent-golden
@@ -151,6 +160,12 @@ coprocess |&
 -- input-mksh-shebang --
 #!/bin/mksh
 coprocess |&
+-- input-bash-extglobs --
+#!/bin/sh
+echo !(a)
+-- input-bash-arrays --
+#!/bin/sh
+foo=(bar)
 
 -- flags-input --
 foo() {