[3.9] bpo-41659: Disallow curly brace directly after primary (GH-22996)

Grammar/python.gram

@@ -460,6 +460,7 @@ await_primary[expr_ty] (memo):
     | AWAIT a=primary { CHECK_VERSION(5, "Await expressions are", _Py_Await(a, EXTRA)) }
     | primary
 primary[expr_ty]:
+    | invalid_primary  # must be before 'primay genexp' because of invalid_genexp
     | a=primary '.' b=NAME { _Py_Attribute(a, b->v.Name.id, Load, EXTRA) }
     | a=primary b=genexp { _Py_Call(a, CHECK(_PyPegen_singleton_seq(p, b)), NULL, EXTRA) }
     | a=primary '(' b=[arguments] ')' {
@@ -664,6 +665,8 @@ invalid_del_stmt:
         RAISE_SYNTAX_ERROR_INVALID_TARGET(DEL_TARGETS, a) }
 invalid_block:
     | NEWLINE !INDENT { RAISE_INDENTATION_ERROR("expected an indented block") }
+invalid_primary:
+    | primary a='{' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "invalid syntax") }
 invalid_comprehension:
     | ('[' | '(' | '{') a=starred_expression for_if_clauses {
         RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "iterable unpacking cannot be used in comprehension") }

Lib/test/test_exceptions.py

@@ -205,6 +205,7 @@ def testSyntaxErrorOffset(self):
         check(b'Python = "\xcf\xb3\xf2\xee\xed" +', 1, 18)
         check('x = "a', 1, 7)
         check('lambda x: x = 2', 1, 1)
+        check('f{a + b + c}', 1, 2)
 
         # Errors thrown by compile.c
         check('class foo:return 1', 1, 11)

Lib/test/test_syntax.py

@@ -802,6 +802,9 @@ def _check_error(self, code, errtext,
         else:
             self.fail("compile() did not raise SyntaxError")
 
+    def test_curly_brace_after_primary_raises_immediately(self):
+        self._check_error("f{", "invalid syntax", mode="single")
+
     def test_assign_call(self):
         self._check_error("f() = 1", "assign")
 

Misc/NEWS.d/next/Core and Builtins/2020-10-27-18-32-49.bpo-41659.d4a-8o.rst

@@ -0,0 +1,3 @@
+Fix a bug in the parser, where a curly brace following a `primary` didn't fail immediately.
+This led to invalid expressions like `a {b}` to throw a :exc:`SyntaxError` with a wrong offset,
+or invalid expressions ending with a curly brace like `a {` to not fail immediately in the REPL.