Fix walrus in subscripts

Resolves   #820.

coconut/compiler/grammar.py

@@ -649,8 +649,8 @@ class Grammar(object):
         unsafe_fat_arrow = Literal("=>") | fixto(Literal("\u21d2"), "=>")
         colon_eq = Literal(":=")
         unsafe_dubcolon = Literal("::")
-        unsafe_colon = Literal(":")
         colon = disambiguate_literal(":", ["::", ":="])
+        indexing_colon = disambiguate_literal(":", [":="])  # same as : but :: is allowed
         lt_colon = Literal("<:")
         semicolon = Literal(";") | invalid_syntax("\u037e", "invalid Greek question mark instead of semicolon", greedy=True)
         multisemicolon = combine(OneOrMore(semicolon))
@@ -1199,21 +1199,23 @@ class Grammar(object):
             | op_item
         )
 
+        # for .[]
         subscript_star = Forward()
         subscript_star_ref = star
         slicetest = Optional(test_no_chain)
-        sliceop = condense(unsafe_colon + slicetest)
+        sliceop = condense(indexing_colon + slicetest)
         subscript = condense(
             slicetest + sliceop + Optional(sliceop)
-            | Optional(subscript_star) + test
+            | Optional(subscript_star) + new_namedexpr_test
         )
-        subscriptlist = itemlist(subscript, comma, suppress_trailing=False) | new_namedexpr_test
+        subscriptlist = itemlist(subscript, comma, suppress_trailing=False)
 
+        # for .$[]
         slicetestgroup = Optional(test_no_chain, default="")
-        sliceopgroup = unsafe_colon.suppress() + slicetestgroup
+        sliceopgroup = indexing_colon.suppress() + slicetestgroup
         subscriptgroup = attach(
             slicetestgroup + sliceopgroup + Optional(sliceopgroup)
-            | test,
+            | new_namedexpr_test,
             subscriptgroup_handle,
         )
         subscriptgrouplist = itemlist(subscriptgroup, comma)

coconut/root.py

@@ -26,7 +26,7 @@
 VERSION = "3.0.4"
 VERSION_NAME = None
 # False for release, int >= 1 for develop
-DEVELOP = 14
+DEVELOP = 15
 ALPHA = False  # for pre releases rather than post releases
 
 assert DEVELOP is False or DEVELOP >= 1, "DEVELOP must be False or an int >= 1"

coconut/tests/src/cocotest/target_311/py311_test.coco

@@ -7,4 +7,6 @@ def py311_test() -> bool:
     except* ValueError as err:
         got_err = err
     assert repr(got_err) == repr(multi_err), (got_err, multi_err)
+    assert [1, 2, 3][x := 1] == 2
+    assert x == 1
     return True