gh-123539: Improve SyntaxError msg for import as with not a name

Author: sobolevn

Grammar/python.gram

@@ -227,15 +227,17 @@ import_from_targets[asdl_alias_seq*]:
 import_from_as_names[asdl_alias_seq*]:
     | a[asdl_alias_seq*]=','.import_from_as_name+ { a }
 import_from_as_name[alias_ty]:
-    | a=NAME b=['as' z=NAME { z }] { _PyAST_alias(a->v.Name.id,
-                                               (b) ? ((expr_ty) b)->v.Name.id : NULL,
-                                               EXTRA) }
+    | invalid_import_from_as_name
+    | a=NAME b=['as' z=NAME { z }] { _PyAST_alias(
+        a->v.Name.id, (b) ? ((expr_ty) b)->v.Name.id : NULL, EXTRA) }
+
 dotted_as_names[asdl_alias_seq*]:
     | a[asdl_alias_seq*]=','.dotted_as_name+ { a }
 dotted_as_name[alias_ty]:
-    | a=dotted_name b=['as' z=NAME { z }] { _PyAST_alias(a->v.Name.id,
-                                                      (b) ? ((expr_ty) b)->v.Name.id : NULL,
-                                                      EXTRA) }
+    | invalid_dotted_as_name
+    | a=dotted_name b=['as' z=NAME { z }] { _PyAST_alias(
+        a->v.Name.id, (b) ? ((expr_ty) b)->v.Name.id : NULL, EXTRA) }
+
 dotted_name[expr_ty]:
     | a=dotted_name '.' b=NAME { _PyPegen_join_names_with_dot(p, a, b) }
     | NAME
@@ -1331,6 +1333,14 @@ invalid_import:
         RAISE_SYNTAX_ERROR_STARTING_FROM(a, "Did you mean to use 'from ... import ...' instead?") }
     | 'import' token=NEWLINE { 
         RAISE_SYNTAX_ERROR_STARTING_FROM(token, "Expected one or more names after 'import'") }
+invalid_dotted_as_name:
+    | dotted_name 'as' !(NAME (',' | ')' | NEWLINE)) a=expression {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, 
+            "cannot use %s as import target", _PyPegen_get_expr_name(a)) }
+invalid_import_from_as_name:
+    | NAME 'as' !(NAME (',' | ')' | NEWLINE)) a=expression {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, 
+            "cannot use %s as import target", _PyPegen_get_expr_name(a)) }
 
 invalid_import_from_targets:
     | import_from_as_names ',' NEWLINE {

Lib/test/test_syntax.py

@@ -1831,6 +1831,56 @@
 Traceback (most recent call last):
 SyntaxError: cannot assign to __debug__
 
+>>> import a as b.c
+Traceback (most recent call last):
+SyntaxError: cannot use attribute as import target
+
+>>> import a.b as (a, b)
+Traceback (most recent call last):
+SyntaxError: cannot use tuple as import target
+
+>>> import a, a.b as 1
+Traceback (most recent call last):
+SyntaxError: cannot use literal as import target
+
+>>> import a.b as 'a', a
+Traceback (most recent call last):
+SyntaxError: cannot use literal as import target
+
+>>> from a import (b as c.d)
+Traceback (most recent call last):
+SyntaxError: cannot use attribute as import target
+
+>>> from a import b as 1
+Traceback (most recent call last):
+SyntaxError: cannot use literal as import target
+
+>>> from a import (
+...   b as f())
+Traceback (most recent call last):
+SyntaxError: cannot use function call as import target
+
+>>> from a import (
+...   b as [],
+... )
+Traceback (most recent call last):
+SyntaxError: cannot use list as import target
+
+>>> from a import (
+...   b,
+...   c as ()
+... )
+Traceback (most recent call last):
+SyntaxError: cannot use tuple as import target
+
+>>> from a import b, с as d[e]
+Traceback (most recent call last):
+SyntaxError: cannot use subscript as import target
+
+>>> from a import с as d[e], b
+Traceback (most recent call last):
+SyntaxError: cannot use subscript as import target
+
 # Check that we dont raise the "trailing comma" error if there is more
 # input to the left of the valid part that we parsed.