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C++ PATCH for c++/67364 (constexpr vs. empty class)

Message ID 56DA06A2.7060200@redhat.com
State New
Headers show

Commit Message

Jason Merrill March 4, 2016, 10:05 p.m. UTC 
On 03/03/2016 05:37 PM, Jason Merrill wrote:
> On 02/25/2016 09:08 AM, Jason Merrill wrote:
>> We don't bother evaluating a store to an empty class member, and we
>> shouldn't complain about accesses either.
>
> This needs to use really_empty_class, since that's what
> expand_aggr_init_1 uses.

And even calling build_value_init is wrong for these classes, as it 
might not be well-formed.  Just build a value directly.

Tested x86_64-pc-linux-gnu, applying to trunk and 5.
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Patch

commit 6207dfd6bce685275831f468e9954bec71d39430
Author: Jason Merrill <jason@redhat.com>
Date:   Fri Mar 4 15:40:16 2016 -0500

    	PR c++/67364
    
    	* constexpr.c (cxx_eval_component_reference): Further tweak.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 4fadc0f..d308175 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1990,13 +1990,16 @@  cxx_eval_component_reference (const constexpr_ctx *ctx, tree t,
       return t;
     }
 
-  if (CONSTRUCTOR_NO_IMPLICIT_ZERO (whole)
-      && !is_really_empty_class (TREE_TYPE (t)))
+  /* We only create a CONSTRUCTOR for a subobject when we modify it, so empty
+     classes never get represented; throw together a value now.  */
+  if (is_really_empty_class (TREE_TYPE (t)))
+    return build_constructor (TREE_TYPE (t), NULL);
+
+  if (CONSTRUCTOR_NO_IMPLICIT_ZERO (whole))
     {
       /* 'whole' is part of the aggregate initializer we're currently
 	 building; if there's no initializer for this member yet, that's an
-	 error.  But expand_aggr_init_1 doesn't bother to initialize really
-	 empty classes, so ignore them here, too.  */
+	 error.  */
       if (!ctx->quiet)
 	error ("accessing uninitialized member %qD", part);
       *non_constant_p = true;
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-empty2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-empty2.C
new file mode 100644
index 0000000..2acfa98
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-empty2.C
@@ -0,0 +1,27 @@ 
+// { dg-do compile { target c++14 } }
+
+struct A
+{
+  constexpr A(int) { }
+};
+
+struct B: A {
+  constexpr B(int i): A(i) { }
+  constexpr B(const B& b): A(b) { }
+};
+
+struct C {
+  B b;
+  constexpr C(int i): b(i) { }
+  constexpr C(const C&c): b(c.b) {}
+};
+
+constexpr int f()
+{
+  C b1{42};
+  C b2{b1};
+  b2.b;
+  return 42;
+}
+
+constexpr int i = f();