argparse: Allow usage without optspecs

It's still useful without, for instance to implement a command that
takes no options, or to check min-args or max-args.

(technically no optspecs, no min/max args and --ignore-unknown does
nothing, but that's a very specific error that we don't need to forbid)

Fixes #9006

doc_src/cmds/argparse.rst

@@ -64,10 +64,11 @@ If ``$argv`` is empty then there is nothing to parse and ``argparse`` returns ze
 
 The ``or return`` means that the function returns ``argparse``'s status if it failed, so if it goes on ``argparse`` succeeded.
 
-The ``--`` argument is required. You do not have to include any arguments after the ``--`` but you must include the ``--``. For example, this is acceptable::
+The ``--`` argument is required. You do not have to include any option specifications or arguments after the ``--`` but you must include the ``--``. For example, this is acceptable::
 
-    set -l argv
+    set -l argv foo
     argparse 'h/help' 'n/name' -- $argv
+    argparse --min-args=1 -- $argv
 
 But this is not::
 

src/builtins/argparse.cpp

@@ -353,11 +353,6 @@ static int collect_option_specs(argparse_cmd_opts_t &opts, int *optind, int argc
         return STATUS_INVALID_ARGS;
     }
 
-    if (opts.options.empty()) {
-        streams.err.append_format(_(L"%ls: No option specs were provided\n"), cmd);
-        return STATUS_INVALID_ARGS;
-    }
-
     return STATUS_CMD_OK;
 }
 
@@ -428,9 +423,13 @@ static int parse_cmd_opts(argparse_cmd_opts_t &opts, int *optind,  //!OCLINT(hig
 
     if (opts.print_help) return STATUS_CMD_OK;
 
-    if (argc == w.woptind || std::wcscmp(L"--", argv[w.woptind - 1]) == 0) {
+    if (std::wcscmp(L"--", argv[w.woptind - 1]) == 0) {
+        --w.woptind;
+    }
+
+    if (argc == w.woptind) {
         // The user didn't specify any option specs.
-        streams.err.append_format(_(L"%ls: No option specs were provided\n"), cmd);
+        streams.err.append_format(_(L"%ls: Missing -- separator\n"), cmd);
         return STATUS_INVALID_ARGS;
     }
 

tests/checks/argparse.fish

@@ -1,15 +1,16 @@
 #RUN: %fish %s
 ##########
 
-set -g LANG C
+# NOTE: This uses argparse, which touches the local variables.
+# Any call that isn't an error should be enclosed in a begin/end block!
 
 # Start by verifying a bunch of error conditions.
 # These are *argparse* errors, and therefore bugs in the script,
 # so they print a stack trace.
 
-# No args is an error
+# No args (not even --) is an error
 argparse
-#CHECKERR: argparse: No option specs were provided
+#CHECKERR: argparse: Missing -- separator
 #CHECKERR: checks/argparse.fish (line {{\d+}}):
 #CHECKERR: argparse
 #CHECKERR: ^
@@ -23,13 +24,14 @@ argparse h/help
 #CHECKERR: ^
 #CHECKERR: (Type 'help argparse' for related documentation)
 
-# Flags but no option specs is an error
-argparse -s -- hello
-#CHECKERR: argparse: No option specs were provided
-#CHECKERR: checks/argparse.fish (line {{\d+}}):
-#CHECKERR: argparse -s -- hello
-#CHECKERR: ^
-#CHECKERR: (Type 'help argparse' for related documentation)
+# Flags but no option specs is not an error
+begin
+    argparse -s -- hello
+    echo $status
+    # CHECK: 0
+    set -l
+    # CHECK: argv hello
+end
 
 # Invalid option specs
 argparse h-
@@ -67,7 +69,10 @@ argparse h-help=x
 begin
     argparse --name min-max --min-args 1 h/help --
     #CHECKERR: min-max: expected >= 1 arguments; got 0
+    argparse --name min-max --min-args 1 --
+    #CHECKERR: min-max: expected >= 1 arguments; got 0
     argparse --name min-max --min-args 1 --max-args 3 h/help -- arg1
+    argparse --name min-max --min-args 1 --max-args 3 -- arg1
     argparse --name min-max --min-args 1 --max-args 3 h/help -- arg1 arg2
     argparse --name min-max --min-args 1 --max-args 3 h/help -- --help arg1 arg2 arg3
     argparse --name min-max --min-args 1 --max-args 3 h/help -- arg1 arg2 -h arg3 arg4
@@ -76,6 +81,8 @@ begin
     argparse --name min-max --max-args 1 h/help -- arg1
     argparse --name min-max --max-args 1 h/help -- arg1 arg2
     #CHECKERR: min-max: expected <= 1 arguments; got 2
+    argparse --name min-max --max-args 1 -- arg1 arg2
+    #CHECKERR: min-max: expected <= 1 arguments; got 2
 end
 
 # Invalid \"#-val\" spec