Reject option tokens as arg values

lib/bash/arg/README.md

@@ -39,7 +39,9 @@ Spec entries use `name|kind|token[|token...]`:
 - each `token` is an exact option token, such as `--verbose` or `-v`.
 
 The parser supports `--option value`, `--option=value`, repeated options where
-the last value wins, and `--` to stop option parsing. Unknown options,
+the last value wins, and `--` to stop option parsing. A value option followed by
+another registered option token is treated as missing its value; use
+`--option=value` when a value is intentionally option-like. Unknown options,
 malformed specs, and missing values return status `2`.
 
 ## Tests

lib/bash/arg/lib_arg.sh

@@ -162,6 +162,10 @@ arg_parse() {
                 log_error "arg_parse: option '$__arg_option_token' requires a value."
                 return 2
             fi
+            if [[ -n "${__arg_token_kind[$1]+set}" ]]; then
+                log_error "arg_parse: option '$__arg_option_token' requires a value before option '$1'."
+                return 2
+            fi
 
             __arg_option_value="$1"
             shift

lib/bash/arg/tests/lib_arg.bats

@@ -93,6 +93,22 @@ create_script() {
     [ "$parse_status" -eq 2 ]
 }
 
+@test "arg_parse rejects registered options as missing option values" {
+    local -a specs=(
+        "verbose|flag|--verbose|-v"
+        "output|value|--output|-o"
+    )
+    local -A options=()
+    local -a positionals=()
+    local parse_status=0
+
+    arg_parse options positionals specs -- --output --verbose || parse_status=$?
+
+    [ "$parse_status" -eq 2 ]
+    [ -z "${options[output]+set}" ]
+    [ -z "${options[verbose]+set}" ]
+}
+
 @test "arg_parse rejects invalid variable names without echoing values" {
     local script="$TEST_TMPDIR/arg-invalid-vars.sh"