[PATCH] c++: Fix ICE with parameter pack of decltype(auto) [PR103497]

[Nathaniel Shead <nathanieloshead@gmail.com>]

Bootstrapped and tested on x86_64-pc-linux-gnu.

-- 8< --

This patch raises an error early when the decltype(auto) specifier is
used as a parameter of a function. This prevents any issues with an
unexpected tree type later on when performing the call.

	PR 103497

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_simple_type_specifier): Add check for
	decltype(auto) as function parameter.

gcc/testsuite/ChangeLog:

	* g++.dg/pr103497.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
 gcc/cp/parser.cc                | 10 ++++++++++
 gcc/testsuite/g++.dg/pr103497.C |  7 +++++++
 2 files changed, 17 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/pr103497.C

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index e5f032f2330..1415e07e152 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -19884,6 +19884,16 @@ cp_parser_simple_type_specifier (cp_parser* parser,
       && cp_lexer_peek_nth_token (parser->lexer, 2)->type != CPP_SCOPE)
     {
       type = saved_checks_value (token->u.tree_check_value);
+      /* Within a function parameter declaration, decltype(auto) is always an
+	 error.  */
+      if (parser->auto_is_implicit_function_template_parm_p
+	  && TREE_CODE (type) == TEMPLATE_TYPE_PARM
+	  && AUTO_IS_DECLTYPE (type))
+	{
+	  error_at (token->location,
+		    "cannot declare a parameter with %<decltype(auto)%>");
+	  type = error_mark_node;
+	}
       if (decl_specs)
 	{
 	  cp_parser_set_decl_spec_type (decl_specs, type,
diff --git a/gcc/testsuite/g++.dg/pr103497.C b/gcc/testsuite/g++.dg/pr103497.C
new file mode 100644
index 00000000000..bcd421c2907
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr103497.C
@@ -0,0 +1,7 @@
+// { dg-do compile { target c++14 } }
+
+void foo(decltype(auto)... args);  // { dg-error "parameter with .decltype.auto..|no parameter packs" }
+
+int main() {
+  foo();
+}
-- 
2.34.1