https://gcc.gnu.org/PR94929

From f6965321b1c00bfb2b9c8407df56bcf38f096088 Mon Sep 17 00:00:00 2001
From: Marek Polacek <polacek@redhat.com>
Date: Mon, 4 May 2020 13:25:42 -0400
Subject: [PATCH] c++: Bogus error with alignof [PR90736]

	PR c++/90736 - bogus error with alignof.
	* constexpr.c (adjust_temp_type): Use cv_unqualified type.
---
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1255,7 +1255,9 @@ adjust_temp_type (tree type, tree temp)
   if (TREE_CODE (temp) == EMPTY_CLASS_EXPR)
     return build0 (EMPTY_CLASS_EXPR, type);
   gcc_assert (scalarish_type_p (type));
-  return cp_fold_convert (type, temp);
+  /* Now we know we're dealing with a scalar, and a prvalue of non-class
+     type is cv-unqualified.  */
+  return cp_fold_convert (cv_unqualified (type), temp);
 }
 
 /* Callback for walk_tree used by unshare_constructor.  */
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/alignof5.C
@@ -0,0 +1,6 @@
+// PR c++/90736 - bogus error with alignof.
+// { dg-do compile { target c++11 } }
+
+constexpr int fn(const int b) { return b; }
+constexpr int c = fn(alignof(int));
+alignas(c) char d;
-- 
2.26.2