pp: Move errors about stray backslashes

they were emitted too early, in particular also in macro
substitution which might turn out to not be stray in case it's
further stringified.  Check in next() instead.  Add two testcases
that an error is still emitted for obvious top-level baskslashes,
and that stringifying such a thing works.

tccpp.c

@@ -3511,8 +3511,6 @@ static void macro_subst(
             if (tok_str->len)
                 spc = is_space(t = tok_str->str[tok_str->lastlen]);
         } else {
-            if (t == '\\' && !(parse_flags & PARSE_FLAG_ACCEPT_STRAYS))
-                tcc_error("stray '\\' in program");
 no_subst:
             if (!check_space(t, &spc))
                 tok_str_add2(tok_str, t, &cval);
@@ -3569,7 +3567,8 @@ ST_FUNC void next(void)
     } else if (tok == TOK_PPSTR) {
         if (parse_flags & PARSE_FLAG_TOK_STR)
             parse_string((char *)tokc.str.data, tokc.str.size - 1);
-    }
+    } else if (tok == '\\' && !(parse_flags & PARSE_FLAG_ACCEPT_STRAYS))
+        tcc_error("stray '\\' in program");
 }
 
 /* push back current token and set current token to 'last_tok'. Only

tests/tcctest.c

@@ -420,6 +420,9 @@ comment
     have_included_42test_h = 1;
     have_included_42test_h_second = 1;
     have_included_42test_h_third = 1;
+
+    /* Check that we don't complain about stray \ here */
+    printf("print a backslash: %s\n", stringify(\\));
 }
 
 

tests/tests2/60_errors_and_warnings.c

@@ -318,5 +318,8 @@ int main()
     int n = _Generic(*a, double:0, long double:1);
 }
 
+#elif defined test_stray_backslash
+#define x \a
+x
 /******************************************************************/
 #endif

tests/tests2/60_errors_and_warnings.expect

@@ -150,3 +150,6 @@ bar  : 3 ; 3
 
 [test_long_double_type_for_win32]
 60_errors_and_warnings.c:317: warning: assignment from incompatible pointer type
+
+[test_stray_backslash]
+60_errors_and_warnings.c:323: error: stray '\' in program