Make creating dictionary with mutable key fail

An immediate dictionary defined with a mutable key was skipping the
invalid element of the dictionary, but going ahead and returning a
dictionary with the remaining valid elements. Make that an error
instead.

Signed-off-by: Keith Packard <keithp@keithp.com>

snek-list.c

@@ -280,7 +280,7 @@ snek_list_imm(snek_offset_t size, snek_list_type_t type)
 			s -= 2;
 			snek_poly_t key = snek_stack_pick(s+1);
 			snek_poly_t value = snek_stack_pick(s);
-			snek_poly_t *ref = _snek_list_ref(list, key, false, true);
+			snek_poly_t *ref = _snek_list_ref(list, key, true, true);
 			if (ref)
 				*ref = value;
 		}

test/Makefile

@@ -72,6 +72,7 @@ FAIL_TESTS = \
 	fail-interpolate-missing.py \
 	fail-interpolate-extra.py \
 	fail-interpolate-badformat.py \
+	fail-dictionary-mutable.py \
 	$(SYNTAX_TESTS)
 
 check:

test/fail-dictionary-mutable.py

@@ -0,0 +1,14 @@
+#
+# Copyright © 2020 Keith Packard <keithp@keithp.com>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# General Public License for more details.
+#
+a = {{"hello": "world"}: "foo"}