py/compile: Reject *arg after keyword argument.

Document this in cpydiff and add a test with expected output
for coverage testing.

As discussed in #11441, the code growth from handling this
case seems to outweigh the benefit of implementing it
properly.

Closes: #11439

Signed-off-by: Jeff Epler <jepler@unpythonic.net>

py/compile.c

@@ -2421,6 +2421,14 @@ static void compile_trailer_paren_helper(compiler_t *comp, mp_parse_node_t pn_ar
                     compile_syntax_error(comp, (mp_parse_node_t)pns_arg, MP_ERROR_TEXT("* arg after **"));
                     return;
                 }
+                if (n_keyword) {
+                    // Support for *arg after kwarg is a CPython feature omitted
+                    // from MicroPython in order to reduce code size. See
+                    // https://github.com/micropython/micropython/issues/11439 for
+                    // more info.
+                    compile_syntax_error(comp, (mp_parse_node_t)pns_arg, MP_ERROR_TEXT("* arg after kwarg"));
+                    return;
+                }
                 #if MICROPY_DYNAMIC_COMPILER
                 if (i >= (size_t)mp_dynamic_compiler.small_int_bits - 1)
                 #else

tests/basics/fun_callstar_kwarg.py

@@ -0,0 +1,8 @@
+# A CPython syntax feature not supported by MicroPython. For more information
+# see `this issue <https://github.com/micropython/micropython/issues/11439>`_.
+# and tests/cpydiff/core_function_star.py
+
+try:
+    exec("f(y=1, *(3,))")
+except SyntaxError as e:
+    print("SyntaxError")

tests/basics/fun_callstar_kwarg.py.exp

@@ -0,0 +1 @@
+SyntaxError

tests/cpydiff/core_function_star.py

@@ -0,0 +1,16 @@
+"""
+categories: Core,Functions
+description: ``*args`` cannot follow a keyword argument
+cause: MicroPython is optimised for code space. For more information see `this issue <https://github.com/micropython/micropython/issues/11439>`_.
+workaround: Re-order the arguments
+"""
+
+
+def f(x, y):
+    return x + y
+
+
+try:
+    print(f(y=1, *(3,)))
+except Exception as e:
+    print(e)