c++: Diagnose cv-qualified decltype(auto) [PR79815]

"If the placeholder is the decltype(auto) type-specifier, T shall be the
placeholder alone." but we weren't detecting "const decltype(auto)".

I've just expanded the existing diagnostic detecting "decltype(auto) &"
and similar.

gcc/cp/ChangeLog:

	PR c++/79815
	* decl.c (grokdeclarator): Detect cv-qual decltype(auto).
	* pt.c (do_auto_deduction): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/79815
	* g++.dg/cpp1y/auto-fn59.C: New test.

3 files changed, 27 insertions, 4 deletions

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 6e4d546..db91b50 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -12250,11 +12250,20 @@ grokdeclarator (const cp_declarator *declarator,
 	    /* Only plain decltype(auto) is allowed.  */
 	    if (tree a = type_uses_auto (type))
 	      {
-		if (AUTO_IS_DECLTYPE (a) && a != type)
+		if (AUTO_IS_DECLTYPE (a))
 		  {
-		    error_at (typespec_loc, "%qT as type rather than "
-			      "plain %<decltype(auto)%>", type);
-		    return error_mark_node;
+		    if (a != type)
+		      {
+			error_at (typespec_loc, "%qT as type rather than "
+				  "plain %<decltype(auto)%>", type);
+			return error_mark_node;
+		      }
+		    else if (TYPE_QUALS (type) != TYPE_UNQUALIFIED)
+		      {
+			error_at (typespec_loc, "%<decltype(auto)%> cannot be "
+				  "cv-qualified");
+			return error_mark_node;
+		      }
 		  }
 	      }
 
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 4e1c77a..defc2a9 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28993,6 +28993,12 @@ do_auto_deduction (tree type, tree init, tree auto_node,
 	    error ("%qT as type rather than plain %<decltype(auto)%>", type);
 	  return error_mark_node;
 	}
+      else if (TYPE_QUALS (type) != TYPE_UNQUALIFIED)
+	{
+	  if (complain & tf_error)
+	    error ("%<decltype(auto)%> cannot be cv-qualified");
+	  return error_mark_node;
+	}
     }
   else
     {
diff --git a/gcc/testsuite/g++.dg/cpp1y/auto-fn59.C b/gcc/testsuite/g++.dg/cpp1y/auto-fn59.C
new file mode 100644
index 0000000..8f6ec9b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/auto-fn59.C
@@ -0,0 +1,8 @@
+// PR c++/79815
+// { dg-do compile { target c++14 } }
+
+decltype(auto) const x = 1; // { dg-error "cannot be cv-qualified" }
+volatile decltype(auto) x2 = 1; // { dg-error "cannot be cv-qualified" }
+const volatile decltype(auto) x3 = 1; // { dg-error "cannot be cv-qualified" }
+const decltype(auto) fn() { return 42; } // { dg-error "cannot be cv-qualified" }
+const decltype(auto) fn2(); // { dg-error "cannot be cv-qualified" }