For each of the attributes currently included in C2x, it has a
constraint that the attribute shall appear at most once in each
attribute list (attribute-list being what appear between a single [[
and ]]).
This patch implements that check. As the corresponding check in the
C++ front end (cp_parser_check_std_attribute) makes violations into
errors, I made them into errors, with the same wording, for C as well.
There is an existing check in the case of the fallthrough attribute,
with a warning rather than an error, in attribute_fallthrough_p. That
is more general, as it also covers __attribute__ ((fallthrough)) and
the case of [[fallthrough]] [[fallthrough]] (multiple attribute-lists
in a single attribute-specifier-sequence), which is not a constraint
violation. To avoid some [[fallthrough, fallthrough]] being diagnosed
twice, the check I added avoids adding duplicate attributes to the
list.
Bootstrapped with no regressions on x86_64-pc-linux-gnu. Applied to
mainline.
gcc/c:
2019-11-15 Joseph Myers <jos...@codesourcery.com>
* c-parser.c (c_parser_std_attribute_specifier): Diagnose
duplicate standard attributes.
gcc/testsuite:
2019-11-15 Joseph Myers <jos...@codesourcery.com>
* gcc.dg/c2x-attr-deprecated-4.c, gcc.dg/c2x-attr-fallthrough-4.c,
gcc.dg/c2x-attr-maybe_unused-4.c: New tests.
Index: gcc/c/c-parser.c
===================================================================
--- gcc/c/c-parser.c (revision 278310)
+++ gcc/c/c-parser.c (working copy)
@@ -4873,6 +4873,9 @@ c_parser_std_attribute (c_parser *parser)
static tree
c_parser_std_attribute_specifier (c_parser *parser, bool for_tm)
{
+ bool seen_deprecated = false;
+ bool seen_fallthrough = false;
+ bool seen_maybe_unused = false;
location_t loc = c_parser_peek_token (parser)->location;
if (!c_parser_require (parser, CPP_OPEN_SQUARE, "expected %<[%>"))
return NULL_TREE;
@@ -4898,8 +4901,55 @@ c_parser_std_attribute_specifier (c_parser *parser
tree attribute = c_parser_std_attribute (parser);
if (attribute != error_mark_node)
{
- TREE_CHAIN (attribute) = attributes;
- attributes = attribute;
+ bool duplicate = false;
+ tree name = get_attribute_name (attribute);
+ tree ns = get_attribute_namespace (attribute);
+ if (ns == NULL_TREE)
+ {
+ /* Some standard attributes may appear at most once in
+ each attribute list. Diagnose duplicates and remove
+ them from the list to avoid subsequent diagnostics
+ such as the more general one for multiple
+ "fallthrough" attributes in the same place (including
+ in separate attribute lists in the same attribute
+ specifier sequence, which is not a constraint
+ violation). */
+ if (is_attribute_p ("deprecated", name))
+ {
+ if (seen_deprecated)
+ {
+ error ("attribute %<deprecated%> can appear at most "
+ "once in an attribute-list");
+ duplicate = true;
+ }
+ seen_deprecated = true;
+ }
+ else if (is_attribute_p ("fallthrough", name))
+ {
+ if (seen_fallthrough)
+ {
+ error ("attribute %<fallthrough%> can appear at most "
+ "once in an attribute-list");
+ duplicate = true;
+ }
+ seen_fallthrough = true;
+ }
+ else if (is_attribute_p ("maybe_unused", name))
+ {
+ if (seen_maybe_unused)
+ {
+ error ("attribute %<maybe_unused%> can appear at most "
+ "once in an attribute-list");
+ duplicate = true;
+ }
+ seen_maybe_unused = true;
+ }
+ }
+ if (!duplicate)
+ {
+ TREE_CHAIN (attribute) = attributes;
+ attributes = attribute;
+ }
}
if (c_parser_next_token_is_not (parser, CPP_COMMA))
break;
Index: gcc/testsuite/gcc.dg/c2x-attr-deprecated-4.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-attr-deprecated-4.c (nonexistent)
+++ gcc/testsuite/gcc.dg/c2x-attr-deprecated-4.c (working copy)
@@ -0,0 +1,13 @@
+/* Test C2x deprecated attribute: duplicates. */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+[[deprecated, __deprecated__]] int a; /* { dg-error "can appear at most once"
} */
+[[__deprecated__, deprecated("message")]] int b; /* { dg-error "can appear at
most once" } */
+int c [[deprecated("message"), deprecated]]; /* { dg-error "can appear at most
once" } */
+[[deprecated, deprecated]]; /* { dg-error "can appear at most once" } */
+/* { dg-warning "ignored" "ignored" { target *-*-* } .-1 } */
+
+/* Separate attribute lists in the same attribute specifier sequence,
+ with the same attribute in them, are OK. */
+[[deprecated]] [[deprecated]] int d [[deprecated]] [[deprecated]];
Index: gcc/testsuite/gcc.dg/c2x-attr-fallthrough-4.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-attr-fallthrough-4.c (nonexistent)
+++ gcc/testsuite/gcc.dg/c2x-attr-fallthrough-4.c (working copy)
@@ -0,0 +1,23 @@
+/* Test C2x fallthrough attribute: duplicates. */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+int
+f (int a)
+{
+ switch (a)
+ {
+ case 1:
+ a++;
+ [[fallthrough, __fallthrough__]]; /* { dg-error "can appear at most
once" } */
+ case 2:
+ a++;
+ /* Separate attribute lists in the same attribute specifier
+ sequence, with the same attribute in them, are OK (but
+ receive a warning). */
+ [[fallthrough]] [[fallthrough]]; /* { dg-warning "specified multiple
times" } */
+ case 3:
+ a++;
+ }
+ return a;
+}
Index: gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-4.c
===================================================================
--- gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-4.c (nonexistent)
+++ gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-4.c (working copy)
@@ -0,0 +1,13 @@
+/* Test C2x maybe_unused attribute: duplicates. */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+[[maybe_unused, __maybe_unused__]] int a; /* { dg-error "can appear at most
once" } */
+[[__maybe_unused__, maybe_unused]] int b; /* { dg-error "can appear at most
once" } */
+int c [[maybe_unused, maybe_unused]]; /* { dg-error "can appear at most once"
} */
+[[maybe_unused, maybe_unused]]; /* { dg-error "can appear at most once" } */
+/* { dg-warning "ignored" "ignored" { target *-*-* } .-1 } */
+
+/* Separate attribute lists in the same attribute specifier sequence,
+ with the same attribute in them, are OK. */
+[[maybe_unused]] [[maybe_unused]] int d [[maybe_unused]] [[maybe_unused]];
--
Joseph S. Myers
jos...@codesourcery.com
