Support C2x [[maybe_unused]] attribute.

This patch adds support for the C2x [[maybe_unused]] attribute, using
the same handler as for GNU __attribute__ ((unused)).

As with other such attribute support, I think turning certain warnings
into pedwarns for usage in cases where that is a constraint violation
can be addressed later as a bug fix, as can the C2x constraint for
various standard attributes that they do not appear more than once
inside a single [[]].

However, the warnings that appear in c2x-attr-maybe_unused-1.c (that
the attribute is ignored on member declarations) need to remain as
warnings not pedwarns, since C2x does permit the attribute there.  (Or
they could be silenced, on the basis that GCC doesn't have warnings
for unused struct and union members so it's completely harmless that
it's ignoring an attribute that might do something useful with another
compiler that does have such warnings.)

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/c:
	* c-decl.c (std_attribute_table): Add maybe_unused.

gcc/testsuite:
	* gcc.dg/c2x-attr-maybe_unused-1.c,
	gcc.dg/c2x-attr-maybe_unused-2.c,
	gcc.dg/c2x-attr-maybe_unused-3.c: New tests.

From-SVN: r278310

6 files changed, 79 insertions, 0 deletions

diff --git a/gcc/c/ChangeLog b/gcc/c/ChangeLog
index c2cab57..75a44e2 100644
--- a/gcc/c/ChangeLog
+++ b/gcc/c/ChangeLog
@@ -1,5 +1,9 @@
 2019-11-15  Joseph Myers  <joseph@codesourcery.com>
 
+	* c-decl.c (std_attribute_table): Add maybe_unused.
+
+2019-11-15  Joseph Myers  <joseph@codesourcery.com>
+
 	* c-decl.c (std_attribute_table): Add fallthrough.
 	* c-parser.c (c_parser_declaration_or_fndef): Diagnose fallthrough
 	attribute at top level.
diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index 98b71ea..cb6169c 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -4345,6 +4345,8 @@ const struct attribute_spec std_attribute_table[] =
     handle_deprecated_attribute, NULL },
   { "fallthrough", 0, 0, false, false, false, false,
     handle_fallthrough_attribute, NULL },
+  { "maybe_unused", 0, 0, false, false, false, false,
+    handle_unused_attribute, NULL },
   { NULL, 0, 0, false, false, false, false, NULL, NULL }
 };
 
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index b835d67..8473e7a 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2019-11-15  Joseph Myers  <joseph@codesourcery.com>
+
+	* gcc.dg/c2x-attr-maybe_unused-1.c,
+	gcc.dg/c2x-attr-maybe_unused-2.c,
+	gcc.dg/c2x-attr-maybe_unused-3.c: New tests.
+
 2019-11-15  Matthew Malcomson  <matthew.malcomson@arm.com>
 
 	* gcc.dg/rtl/aarch64/rtl-handle-column-numbers.c: New test.
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-1.c b/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-1.c
new file mode 100644
index 0000000..221ebdd
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-1.c
@@ -0,0 +1,31 @@
+/* Test C2x maybe_unused attribute: valid uses.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors -Wall -Wextra -Wunused" } */
+
+[[maybe_unused]] static void f1 (void) {}
+
+[[__maybe_unused__]] const int c1 = 1;
+static int c2 [[maybe_unused]] = 2;
+
+int
+g ([[maybe_unused]] int x, int y)
+{
+  [[maybe_unused]] typedef float F;
+  [[maybe_unused]] int a;
+  int b [[__maybe_unused__]];
+  int c [[maybe_unused]];
+  c = y;
+  return y;
+}
+
+struct [[maybe_unused]] s { double d; };
+
+struct s2 { [[__maybe_unused__]] int a; int b [[maybe_unused]]; } x; /* { dg-warning "attribute ignored" } */
+
+enum e { E1 [[maybe_unused]] };
+
+union [[maybe_unused]] u { int x; };
+
+enum [[maybe_unused]] eu { E2 };
+
+union u2 { [[maybe_unused]] int a; int b [[maybe_unused]]; } y; /* { dg-warning "attribute ignored" } */
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-2.c b/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-2.c
new file mode 100644
index 0000000..9b5055b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-2.c
@@ -0,0 +1,25 @@
+/* Test C2x maybe_unused attribute: invalid contexts.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+/* This attribute is not valid in most cases on types other than their
+   definitions, or on statements, or as an attribute-declaration.  */
+
+[[maybe_unused]]; /* { dg-warning "ignored" } */
+
+int [[maybe_unused]] var; /* { dg-warning "ignored" } */
+/* { dg-message "that appertains to a type-specifier" "appertains" { target *-*-* } .-1 } */
+
+int array_with_dep_type[2] [[maybe_unused]]; /* { dg-warning "ignored" } */
+/* { dg-message "that appertains to a type-specifier" "appertains" { target *-*-* } .-1 } */
+
+void fn_with_dep_type () [[maybe_unused]]; /* { dg-warning "ignored" } */
+/* { dg-message "that appertains to a type-specifier" "appertains" { target *-*-* } .-1 } */
+
+void
+f (void)
+{
+  int a;
+  [[maybe_unused]]; /* { dg-warning "ignored" } */
+  [[maybe_unused]] a = 1; /* { dg-warning "ignored" } */
+}
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-3.c b/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-3.c
new file mode 100644
index 0000000..4d4da08
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-attr-maybe_unused-3.c
@@ -0,0 +1,11 @@
+/* Test C2x maybe_unused attribute: invalid syntax.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+[[maybe_unused()]] int a; /* { dg-error "does not take any arguments" } */
+
+[[maybe_unused(0)]] int b; /* { dg-error "does not take any arguments|expected" } */
+
+[[maybe_unused("", 123)]] int c; /* { dg-error "does not take any arguments|expected" } */
+
+[[maybe_unused("")]] int d; /* { dg-error "does not take any arguments|expected" } */