c++: Reject pack expansion of assume attribute [PR109756]

http://eel.is/c++draft/dcl.attr#grammar-4 says
"In an attribute-list, an ellipsis may appear only if that attribute's
specification permits it."
and doesn't explicitly permit it on any standard attribute.
The https://wg21.link/p1774r8 paper which introduced assume attribute says
"We could therefore hypothetically permit the assume attribute to directly
support pack expansion:
template <int... args>
void f() {
[[assume(args >= 0)...]];
}
However, we do not propose this. It would require substantial additional work
for a very rare use case. Note that this can instead be expressed with a fold
expression, which is equivalent to the above and works out of the box without
any extra effort:
template <int... args>
void f() {
[[assume(((args >= 0) && ...))]];
}
", but as the testcase shows, GCC 13+ ICEs on assume attribute followed by
... if it contains packs.
The following patch rejects those instead of ICE and for C++17 or later
suggests using fold expressions instead (it doesn't make sense to suggest
it for C++14 and earlier when we'd error on the fold expressions).

2023-05-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/109756
	* cp-gimplify.cc (process_stmt_assume_attribute): Diagnose pack
	expansion of assume attribute.

	* g++.dg/cpp23/attr-assume11.C: New test.

gcc/cp/cp-gimplify.cc

@@ -3267,6 +3267,16 @@ process_stmt_assume_attribute (tree std_attrs, tree statement,
   for (; attr; attr = lookup_attribute ("gnu", "assume", TREE_CHAIN (attr)))
     {
       tree args = TREE_VALUE (attr);
+      if (args && PACK_EXPANSION_P (args))
+	{
+	  auto_diagnostic_group d;
+	  error_at (attrs_loc, "pack expansion of %qE attribute",
+		    get_attribute_name (attr));
+	  if (cxx_dialect >= cxx17)
+	    inform (attrs_loc, "use fold expression in the attribute "
+			       "argument instead");
+	  continue;
+	}
       int nargs = list_length (args);
       if (nargs != 1)
 	{

gcc/testsuite/g++.dg/cpp23/attr-assume11.C

+// PR c++/109756
+// { dg-do compile { target c++11 } }
+
+template <int ...args>
+void
+foo ()
+{
+  [[assume (1 > 0)...]];		// { dg-error "expansion pattern '\\\(1 > 0\\\)' contains no parameter packs" }
+					// { dg-warning "attributes at the beginning of statement are ignored" "" { target *-*-* } .-1 }
+  [[assume (args > 0)...]];		// { dg-error "pack expansion of 'assume' attribute" }
+					// { dg-message "use fold expression in the attribute argument instead" "" { target c++17 } .-1 }
+#if __cpp_fold_expressions >= 201603L
+  [[assume (((args > 0) && ...))]];
+#endif
+  [[gnu::assume (1 > 0)...]];		// { dg-error "expansion pattern '\\\(1 > 0\\\)' contains no parameter packs" }
+					// { dg-warning "attributes at the beginning of statement are ignored" "" { target *-*-* } .-1 }
+  [[gnu::assume (args > 0)...]];	// { dg-error "pack expansion of 'assume' attribute" }
+					// { dg-message "use fold expression in the attribute argument instead" "" { target c++17 } .-1  }
+#if __cpp_fold_expressions >= 201603L
+  [[gnu::assume (((args > 0) && ...))]];
+#endif
+}