[C++ Patch] PR 70808 ("Spurious -Wzero-as-null-pointer-constant for nullptr_t")

[Paolo Carlini]

Hi,

this isn't a regression, but deciding what we want to do should be easy 
and quick enough. The issue is that "mysteriously" we warn 
-Wzero-as-null-pointer-constant for:

decltype( nullptr ) warn = {};

and we don't for:

int* no_warn = {};

That's easily explained given the code we have in build_zero_init_1 
which handles types satisfying TYPE_PTR_OR_PTRMEM_P separately from all 
the other scalar types, std::nullptr_t included. I think we should 
resolve the inconsistency - the below does that without regressions - 
but frankly, at first, given the letter of the standard under 11.6/6 
about zero-initialization I expected that we weren't handling types 
satisfying TYPE_PTR_OR_PTRMEM_P in a different way, thus something like 
a sheer integer_zero_node consistently for all the scalars (which would 
mean resolving the inconsistency precisely the other way round). 
Probably it's a tricky detail of our internal representations...

Thanks, Paolo.

///////////////////

/cp
2018-04-10  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/70808
        * init.c (build_zero_init_1): Handle NULLPTR_TYPE_P being true of
        the type like TYPE_PTR_OR_PTRMEM_P.

/testsuite
2018-04-10  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/70808
        * g++.dg/warn/Wzero-as-null-pointer-constant-7.C: New.
Index: cp/init.c
===================================================================
--- cp/init.c   (revision 259287)
+++ cp/init.c   (working copy)
@@ -180,7 +180,7 @@ build_zero_init_1 (tree type, tree nelts, bool sta
        items with static storage duration that are not otherwise
        initialized are initialized to zero.  */
     ;
-  else if (TYPE_PTR_OR_PTRMEM_P (type))
+  else if (TYPE_PTR_OR_PTRMEM_P (type) || NULLPTR_TYPE_P (type))
     init = fold (convert (type, nullptr_node));
   else if (SCALAR_TYPE_P (type))
     init = fold (convert (type, integer_zero_node));
Index: testsuite/g++.dg/warn/Wzero-as-null-pointer-constant-7.C
===================================================================
--- testsuite/g++.dg/warn/Wzero-as-null-pointer-constant-7.C    (nonexistent)
+++ testsuite/g++.dg/warn/Wzero-as-null-pointer-constant-7.C    (working copy)
@@ -0,0 +1,5 @@
+// PR c++/70808
+// { dg-options "-Wzero-as-null-pointer-constant" }
+
+int* no_warn = {};
+decltype( nullptr ) warn = {};