c++: Allow constexpr decltype(auto) [PR102229]

My r11-2202 was trying to enforce [dcl.type.auto.deduct]/4, which says
"If the placeholder-type-specifier is of the form type-constraint[opt]
decltype(auto), T shall be the placeholder alone."  But this made us
reject 'constexpr decltype(auto)', which, after clarification from CWG,
should be valid.  [dcl.type.auto.deduct]/4 is supposed to be a syntactic
constraint, not semantic, so it's OK that the constexpr marks the object
as const.

As a consequence, checking TYPE_QUALS in do_auto_deduction is too late,
and we have a FIXME there anyway.  So in this patch I'm attempting to
detect 'const decltype(auto)' earlier.  If I'm going to use TYPE_QUALS,
it needs to happen before we mark the object as const due to constexpr,
that is, before grokdeclarator's

  /* A `constexpr' specifier used in an object declaration declares
     the object as `const'.  */
  if (constexpr_p && innermost_code != cdk_function)
    ...

Constrained decltype(auto) was a little problem, hence the TYPENAME
check.  But in a typename context you can't use decltype(auto) anyway,
I think.

	PR c++/102229

gcc/cp/ChangeLog:

	* decl.c (check_decltype_auto): New.
	(grokdeclarator): Call it.
	* pt.c (do_auto_deduction): Don't check decltype(auto) here.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/decltype-auto5.C: New test.

gcc/cp/decl.c

@@ -11354,6 +11354,33 @@ name_unnamed_type (tree type, tree decl)
   gcc_assert (!TYPE_UNNAMED_P (type));
 }
 
+/* Check that decltype(auto) was well-formed: only plain decltype(auto)
+   is allowed.  TYPE might contain a decltype(auto).  Returns true if
+   there was a problem, false otherwise.  */
+
+static bool
+check_decltype_auto (location_t loc, tree type)
+{
+  if (tree a = type_uses_auto (type))
+    {
+      if (AUTO_IS_DECLTYPE (a))
+	{
+	  if (a != type)
+	    {
+	      error_at (loc, "%qT as type rather than plain "
+			"%<decltype(auto)%>", type);
+	      return true;
+	    }
+	  else if (TYPE_QUALS (type) != TYPE_UNQUALIFIED)
+	    {
+	      error_at (loc, "%<decltype(auto)%> cannot be cv-qualified");
+	      return true;
+	    }
+	}
+    }
+  return false;
+}
+
 /* Given declspecs and a declarator (abstract or otherwise), determine
    the name and type of the object declared and construct a DECL node
    for it.
@@ -12702,25 +12729,9 @@ grokdeclarator (const cp_declarator *declarator,
 			  "allowed");
 		return error_mark_node;
 	      }
-	    /* Only plain decltype(auto) is allowed.  */
-	    if (tree a = type_uses_auto (type))
-	      {
-		if (AUTO_IS_DECLTYPE (a))
-		  {
-		    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;
-		      }
-		  }
-	      }
+
+	    if (check_decltype_auto (typespec_loc, type))
+	      return error_mark_node;
 
 	    if (ctype == NULL_TREE
 		&& decl_context == FIELD
@@ -13080,6 +13091,15 @@ grokdeclarator (const cp_declarator *declarator,
 
   id_loc = declarator ? declarator->id_loc : input_location;
 
+  if (innermost_code != cdk_function
+    /* Don't check this if it can be the artifical decltype(auto)
+       we created when building a constraint in a compound-requirement:
+       that the type-constraint is plain is going to be checked in
+       cp_parser_compound_requirement.  */
+      && decl_context != TYPENAME
+      && check_decltype_auto (id_loc, type))
+    return error_mark_node;
+
   /* A `constexpr' specifier used in an object declaration declares
      the object as `const'.  */
   if (constexpr_p && innermost_code != cdk_function)

gcc/cp/pt.c

@@ -29894,19 +29894,6 @@ do_auto_deduction (tree type, tree init, tree auto_node,
 	return error_mark_node;
       targs = make_tree_vec (1);
       TREE_VEC_ELT (targs, 0) = deduced;
-      /* FIXME: These errors ought to be diagnosed at parse time. */
-      if (type != auto_node)
-	{
-          if (complain & tf_error)
-	    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
     {

gcc/testsuite/g++.dg/cpp1y/decltype-auto5.C

@@ -0,0 +1,35 @@
+// PR c++/102229
+// { dg-do compile { target c++14 } }
+
+struct S {
+  constexpr static decltype(auto) x = 42;
+  const constexpr static decltype(auto) y = 42; // { dg-error "cannot be cv-qualified" }
+
+  constexpr decltype(auto) mfn1 () { return 0; }
+  const constexpr decltype(auto) mfn2 () { return 0; } // { dg-error "cannot be cv-qualified" }
+};
+
+constexpr decltype(auto) i = 42;
+const constexpr decltype(auto) j = 42; // { dg-error "cannot be cv-qualified" }
+
+constexpr decltype(auto) fn() { return 42; }
+const decltype(auto) fn2() { return 42; } // { dg-error "cannot be cv-qualified" }
+
+auto constexpr foo() -> const decltype(auto) // { dg-error "cannot be cv-qualified" }
+{
+  return 0;
+}
+
+#if __cpp_concepts
+template<typename>
+concept C = true;
+
+constexpr C decltype(auto) x1 = 0;
+const constexpr C decltype(auto) x2 = 0; // { dg-error "cannot be cv-qualified" "" { target c++20 } }
+
+constexpr C decltype(auto) fn3() { return 0; }
+const constexpr C decltype(auto) fn4() { return 0; } // { dg-error "cannot be cv-qualified" "" { target c++20 } }
+#endif
+
+template<const decltype(auto) = 42> // { dg-error "cannot be cv-qualified" }
+void g ();