[gcc r11-8222] c++: Fix empty base stores in cxx_eval_store_expression [PR100111]

[gcc r11-8222] c++: Fix empty base stores in cxx_eval_store_expression [PR100111]

[Jakub Jelinek]

https://gcc.gnu.org/g:35e8b38a91d9fb49a4759649576f15e76c129d99

commit r11-8222-g35e8b38a91d9fb49a4759649576f15e76c129d99
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Apr 16 17:37:07 2021 +0200

    c++: Fix empty base stores in cxx_eval_store_expression [PR100111]
    
    In r11-6895 handling of empty bases has been fixed such that non-lval
    stores of empty classes are not added when the type of *valp doesn't
    match the type of the initializer, but as this testcase shows it is
    done only when *valp is non-NULL.  If it is NULL, we still shouldn't
    add empty class constructors if the type of the constructor elt *valp
    points to doesn't match.
    
    2021-04-16  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/100111
            * constexpr.c (cxx_eval_store_expression): Don't add CONSTRUCTORs
            for empty classes into *valp when types don't match even when *valp
            is NULL.
    
            * g++.dg/cpp0x/constexpr-100111.C: New test.

Diff:
---
 gcc/cp/constexpr.c                            | 8 ++++++++
 gcc/testsuite/g++.dg/cpp0x/constexpr-100111.C | 7 +++++++
 2 files changed, 15 insertions(+)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index b74bbac3cd2..0fb0ab44b39 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5538,6 +5538,14 @@ cxx_eval_store_expression (const constexpr_ctx *ctx, tree t,
       CONSTRUCTOR_NO_CLEARING (*valp)
 	= CONSTRUCTOR_NO_CLEARING (init);
     }
+  else if (TREE_CODE (init) == CONSTRUCTOR
+	   && !same_type_ignoring_top_level_qualifiers_p (TREE_TYPE (init),
+							  type))
+    {
+      /* See above on initialization of empty bases.  */
+      gcc_assert (is_empty_class (TREE_TYPE (init)) && !lval);
+      return init;
+    }
   else
     *valp = init;
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-100111.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-100111.C
new file mode 100644
index 00000000000..446d21d03c5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-100111.C
@@ -0,0 +1,7 @@
+// PR c++/100111
+// { dg-do compile { target c++11 } }
+// { dg-options "-fno-elide-constructors" }
+
+struct A {};
+struct B : A { int b; constexpr B (A x) : A(x), b() {} };
+struct C { B c; constexpr C () : c({}) {} } d;