c++: Fix up pedantic handling of alignas [PR110345]

The following patch on top of the PR110345 P2552R3 series
emits pedantic pedwarns for alignas appertaining to incorrect entities.
As the middle-end and attribute exclusions look for "aligned" attribute,
the patch transforms alignas into "internal "::aligned attribute (didn't
use [[aligned (x)]] so that people can't type it that way).

2024-12-18  Jakub Jelinek  <jakub@redhat.com>

	PR c++/110345
gcc/c-family/
	* c-common.h (attr_aligned_exclusions): Declare.
	(handle_aligned_attribute): Likewise.
	* c-attribs.cc (handle_aligned_attribute): No longer
	static.
	(attr_aligned_exclusions): Use extern instead of static.
gcc/cp/
	* cp-tree.h (enum cp_tree_index): Add CPTI_INTERNAL_IDENTIFIER.
	(internal_identifier): Define.
	(internal_attribute_table): Declare.
	* parser.cc (cp_parser_exception_declaration): Error on alignas
	on exception declaration.
	(cp_parser_std_attribute_spec): Turn alignas into internal
	ns aligned attribute rather than gnu.
	* decl.cc (initialize_predefined_identifiers): Initialize
	internal_identifier.
	* tree.cc (handle_alignas_attribute): New function.
	(internal_attributes): New variable.
	(internal_attribute_table): Likewise.
	* cp-objcp-common.h (cp_objcp_attribute_table): Add
	internal_attribute_table entry.
gcc/testsuite/
	* g++.dg/cpp0x/alignas1.C: Add dg-options "".
	* g++.dg/cpp0x/alignas2.C: Likewise.
	* g++.dg/cpp0x/alignas7.C: Likewise.
	* g++.dg/cpp0x/alignas21.C: New test.
	* g++.dg/ext/bitfield9.C: Expect a warning.
	* g++.dg/cpp2a/is-layout-compatible3.C: Add dg-options -pedantic.
	Expect a warning.

4 files changed, 159 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp0x/alignas1.C b/gcc/testsuite/g++.dg/cpp0x/alignas1.C
index d73c875..7138095 100644
--- a/gcc/testsuite/g++.dg/cpp0x/alignas1.C
+++ b/gcc/testsuite/g++.dg/cpp0x/alignas1.C
@@ -1,5 +1,6 @@
 // PR c++/65734
 // { dg-do compile { target c++11 } }
+// { dg-options "" }
 
 template <class T> struct A
 {
diff --git a/gcc/testsuite/g++.dg/cpp0x/alignas2.C b/gcc/testsuite/g++.dg/cpp0x/alignas2.C
index 2e7d051..b359154bf 100644
--- a/gcc/testsuite/g++.dg/cpp0x/alignas2.C
+++ b/gcc/testsuite/g++.dg/cpp0x/alignas2.C
@@ -1,5 +1,6 @@
 // PR c++/65734
 // { dg-do compile { target c++11 } }
+// { dg-options "" }
 
 template <typename T>
 struct BVector
diff --git a/gcc/testsuite/g++.dg/cpp0x/alignas21.C b/gcc/testsuite/g++.dg/cpp0x/alignas21.C
new file mode 100644
index 0000000..b7be7c5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alignas21.C
@@ -0,0 +1,156 @@
+// C++ 26 P2552R3 - On the ignorability of standard attributes
+// { dg-do compile { target c++11 } }
+
+int arr[2];
+struct S { int a, b; };
+S arr2[2];
+
+void
+foo (int n)
+{
+  alignas (int) int x1;
+  alignas ("foobar") int x2;			// { dg-error "'alignas' argument has non-integral type 'const char \\\[7\\\]'" }
+  alignas (0) int x3;				// { dg-warning "requested alignment '0' is not a positive power of 2" }
+  alignas ("foo", "bar", "baz") int x4;		// { dg-error "'alignas' argument has non-integral type 'const char \\\[4\\\]'" }
+						// { dg-error "expected '\\\)' before ',' token" "" { target *-*-* } .-1 }
+						// { dg-error "expected declaration before ',' token" "" { target *-*-* } .-2 }
+						// { dg-error "expected primary-expression before ',' token" "" { target *-*-* } .-3 }
+  alignas (0, 1, 2) int x5;			// { dg-error "expected '\\\)' before ',' token" }
+						// { dg-error "expected declaration before ',' token" "" { target *-*-* } .-1 }
+						// { dg-error "expected primary-expression before ',' token" "" { target *-*-* } .-2 }
+
+  auto a = [] alignas (int) () {};		// { dg-error "'alignas' on function declaration" }
+  auto b = [] constexpr alignas (int) {};	// { dg-error "'alignas' on a type other than class" }
+						// { dg-error "parameter declaration before lambda declaration specifiers only optional with" "" { target c++20_down } .-1 }
+						// { dg-error "'constexpr' lambda only available with" "" { target c++14_down } .-2 }
+  auto c = [] noexcept alignas (int) {};	// { dg-error "'alignas' on a type other than class" }
+						// { dg-error "parameter declaration before lambda exception specification only optional with" "" { target c++20_down } .-1 }
+  auto d = [] () alignas (int) {};		// { dg-error "'alignas' on a type other than class" }
+  auto e = new int [n] alignas (int);		// { dg-warning "attributes ignored on outermost array type in new expression" }
+  auto e2 = new int [n] alignas (int) [42];	// { dg-warning "attributes ignored on outermost array type in new expression" }
+  auto f = new int [n][42] alignas (int);	// { dg-error "'alignas' on a type other than class" }
+  alignas (int);				// { dg-warning "attributes at the beginning of statement are ignored" }
+  alignas (int) {}				// { dg-warning "attributes at the beginning of statement are ignored" }
+  alignas (int) if (true) {}			// { dg-warning "attributes at the beginning of statement are ignored" }
+  alignas (int) while (false) {}		// { dg-warning "attributes at the beginning of statement are ignored" }
+  alignas (int) goto lab;			// { dg-warning "attributes at the beginning of statement are ignored" }
+  alignas (int) lab:;				// { dg-error "alignment may not be specified for 'lab'" }
+  alignas (int) try {} catch (int) {}		// { dg-warning "attributes at the beginning of statement are ignored" }
+  if (alignas (int) int x = 0) {}
+  switch (n)
+    {
+    alignas (int) case 1:			// { dg-error "alignment may not be specified for" }
+    alignas (int) break;			// { dg-warning "attributes at the beginning of statement are ignored" }
+    alignas (int) default:			// { dg-error "alignment may not be specified for" }
+	 break;
+    }
+  for (alignas (int) auto a : arr) {}
+  for (alignas (int) auto [a, b] : arr2) {}	// { dg-error "structured bindings only available with" "" { target c++14_down } }
+  alignas (int) asm ("");			// { dg-warning "attributes ignored on 'asm' declaration" }
+  try {} catch (alignas (int) int x) {}		// { dg-error "'alignas' on exception declaration" }
+  try {} catch (alignas (int) int) {}		// { dg-error "'alignas' on exception declaration" }
+  try {} catch (int alignas (int) x) {}		// { dg-warning "attribute ignored" }
+  try {} catch (int alignas (int)) {}		// { dg-warning "attribute ignored" }
+  try {} catch (int x alignas (int)) {}		// { dg-error "'alignas' on exception declaration" }
+}
+
+alignas (int) int bar ();			// { dg-error "'alignas' on function declaration" }
+using foobar alignas (int) = int;		// { dg-error "'alignas' on a type alias" }
+alignas (int) int a;
+alignas (int) auto [b, c] = arr;		// { dg-error "structured bindings only available with" "" { target c++14_down } }
+alignas (int);					// { dg-warning "attribute ignored" }
+inline alignas (int) void baz () {}		// { dg-warning "attribute ignored" }
+						// { dg-error "standard attributes in middle of decl-specifiers" "" { target *-*-* } .-1 }
+constexpr alignas (int) int qux () { return 0; }	// { dg-warning "attribute ignored" }
+						// { dg-error "standard attributes in middle of decl-specifiers" "" { target *-*-* } .-1 }
+int alignas (int) d;				// { dg-warning "attribute ignored" }
+int const alignas (int) e = 1;			// { dg-warning "attribute ignored" }
+struct A {} alignas (int);			// { dg-warning "attribute ignored in declaration of 'struct A'" }
+struct A alignas (int);				// { dg-warning "attribute ignored" }
+struct A alignas (int) a1;			// { dg-warning "attribute ignored" }
+A alignas (int) a2;				// { dg-warning "attribute ignored" }
+enum B { B0 } alignas (int);			// { dg-warning "attribute ignored in declaration of 'enum B'" }
+enum B alignas (int);				// { dg-warning "attribute ignored" }
+enum B alignas (int) b1;			// { dg-warning "attribute ignored" }
+B alignas (int) b2;				// { dg-warning "attribute ignored" }
+struct alignas (int) C {};
+int f alignas (int);
+struct alignas (int) Y;
+int g[2] alignas (int);				// { dg-error "'alignas' on a type other than class" }
+int g2 alignas (int) [2];
+int corge () alignas (int);			// { dg-error "'alignas' on a type other than class" }
+int *alignas (int) h;				// { dg-error "'alignas' on a type other than class" }
+int & alignas (int) i = f;			// { dg-error "'alignas' on a type other than class" }
+int S::* alignas (int) k;			// { dg-error "'alignas' on a type other than class" }
+auto l = sizeof (int [2] alignas (int));	// { dg-error "'alignas' on a type other than class" }
+int freddy (alignas (int) int a,		// { dg-error "alignment may not be specified for 'a'" }
+	    alignas (int) int,			// { dg-error "alignment may not be specified for '<anonymous>'" }
+	    alignas (int) int c = 0,		// { dg-error "alignment may not be specified for 'c'" }
+	    alignas (int) int = 0);		// { dg-error "alignment may not be specified for '<anonymous>'" }
+void
+corge (alignas (int) int a,			// { dg-error "alignment may not be specified for 'a'" }
+       alignas (int) int,			// { dg-error "alignment may not be specified for '<anonymous>'" }
+       alignas (int) int c = 0,			// { dg-error "alignment may not be specified for 'c'" }
+       alignas (int) int = 0)			// { dg-error "alignment may not be specified for '<anonymous>'" }
+{
+}
+alignas (int) void
+garply ()					// { dg-error "'alignas' on function declaration" }
+{
+}
+int grault (int alignas (int) a,		// { dg-warning "attribute ignored" }
+	    int alignas (int),			// { dg-warning "attribute ignored" }
+	    int alignas (int) c = 0,		// { dg-warning "attribute ignored" }
+	    int alignas (int) = 0);		// { dg-warning "attribute ignored" }
+void
+waldo (int alignas (int) a,			// { dg-warning "attribute ignored" }
+       int alignas (int),			// { dg-warning "attribute ignored" }
+       int alignas (int) c = 0,			// { dg-warning "attribute ignored" }
+       int alignas (int) = 0)			// { dg-warning "attribute ignored" }
+{
+}
+int plugh (int a alignas (int),			// { dg-error "alignment may not be specified for 'a'" }
+	    int b alignas (int) = 0);		// { dg-error "alignment may not be specified for 'b'" }
+void
+thud (int a alignas (int),			// { dg-error "alignment may not be specified for 'a'" }
+      int b alignas (int) = 0)			// { dg-error "alignment may not be specified for 'b'" }
+{
+}
+enum alignas (int) D { D0 };			// { dg-error "'alignas' on enumerated type" }
+enum class alignas (int) E { E0 };		// { dg-error "'alignas' on enumerated type" }
+enum F {};
+enum alignas (int) F;				// { dg-warning "type attributes ignored after type is already defined" }
+enum G {
+  G0 alignas (int),				// { dg-error "alignment may not be specified for 'G0'" }
+  G1 alignas (int) = 2				// { dg-error "alignment may not be specified for 'G1'" }
+};
+namespace alignas (int) H { using H0 = int; }	// { dg-error "expected identifier before 'alignas'" }
+						// { dg-error "H' does not name a type" "" { target *-*-* } .-1 }
+namespace alignas (int) {}			// { dg-error "expected identifier before 'alignas'" }
+						// { dg-error "expected unqualified-id before '\\\{' token" "" { target *-*-* } .-1 }
+alignas (int) using namespace H;
+						// { dg-error "'H' is not a namespace-name" "" { target *-*-* } .-1 }
+struct alignas (int) I
+{
+  alignas (int);				// { dg-error "declaration does not declare anything" }
+  alignas (int) int i;
+  alignas (int) int foo ();			// { dg-error "'alignas' on function declaration" }
+  alignas (int) int bar () { return 1; }	// { dg-error "'alignas' on function declaration" }
+  alignas (int) int : 0;			// { dg-error "'alignas' on bit-field" }
+  alignas (int) int i2 : 5;			// { dg-error "'alignas' on bit-field" }
+  alignas (int) static int i3;
+  static int i4;
+};
+alignas (int) int I::i4 = 0;
+struct J : alignas (int) C {};			// { dg-warning "attributes on base specifiers are ignored" }
+#if __cpp_concepts >= 201907L
+template <typename T>
+concept K alignas (int) = requires { true; };	// { dg-error "alignment may not be specified for 'K'" "" { target c++20 } }
+#endif
+typedef int L alignas (int);			// { dg-error "'alignas' on a type alias" }
+template <typename T>
+struct M {};
+template <>
+struct alignas (int) M<int> { int m; };
+typedef int N[2] alignas (int);			// { dg-error "'alignas' on a type other than class" }
+typedef int O alignas (int) [2];		// { dg-error "'alignas' on a type alias" }
diff --git a/gcc/testsuite/g++.dg/cpp0x/alignas7.C b/gcc/testsuite/g++.dg/cpp0x/alignas7.C
index a209250..90cac3f 100644
--- a/gcc/testsuite/g++.dg/cpp0x/alignas7.C
+++ b/gcc/testsuite/g++.dg/cpp0x/alignas7.C
@@ -1,5 +1,6 @@
 // PR c++/71513
 // { dg-do compile { target c++11 } }
+// { dg-options "" }
 
 template < int N, typename T >
 struct A