Update error messages; add an assert(0)

Author: davepeck

Grammar/python.gram

@@ -1558,9 +1558,9 @@ invalid_tstring_conversion_character:
 
 invalid_string_tstring_concat:
     | a=(fstring|string)+ b[expr_ty]=tstring {
-        RAISE_SYNTAX_ERROR_KNOWN_RANGE(PyPegen_last_item(a, expr_ty), b, "cannot mix t-strings with strings or f-strings") }
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(PyPegen_last_item(a, expr_ty), b, "cannot mix t-string literals with string or bytes literals") }
     | a=tstring+ b[expr_ty]=(fstring|string) {
-        RAISE_SYNTAX_ERROR_KNOWN_RANGE(PyPegen_last_item(a, expr_ty), b, "cannot mix t-strings with strings or f-strings") }
+        RAISE_SYNTAX_ERROR_KNOWN_RANGE(PyPegen_last_item(a, expr_ty), b, "cannot mix t-string literals with string or bytes literals") }
 
 invalid_arithmetic:
     | sum ('+'|'-'|'*'|'/'|'%'|'//'|'@') a='not' b=inversion { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "'not' after an operator must be parenthesized") }

Lib/test/test_tstring.py

@@ -242,7 +242,7 @@ def test_literal_concatenation(self):
 
         # Test disallowed mix of t-string and string/f-string (incl. bytes)
         what = 't'
-        expected_msg = 'cannot mix t-strings with strings or f-strings'
+        expected_msg = 'cannot mix t-string literals with string or bytes literals'
         for case in (
             "t'{what}-string literal' 'str literal'",
             "t'{what}-string literal' u'unicode literal'",

Parser/action_helpers.c

@@ -1871,6 +1871,10 @@ _PyPegen_concatenate_strings(Parser *p, asdl_expr_seq *strings,
             case JoinedStr_kind:
                 f_string_found = 1;
                 break;
+            case TemplateStr_kind:
+                // python.gram handles this; we should never get here
+                assert(0);
+                break;
             default:
                 f_string_found = 1;
                 break;