[PATCH] c++: Allow new char[4]{"foo"} [PR77841]

[Jason Merrill]

On 9/1/20 3:41 PM, Marek Polacek wrote:
> On Tue, Sep 01, 2020 at 03:27:36PM -0400, Jason Merrill via Gcc-patches wrote:
>> On 9/1/20 12:10 PM, Marek Polacek wrote:
>>> Currently, we allow new char[]{"foo"}, but not new char[4]{"foo"}.
>>> We should accept the latter too: [dcl.init.list]p3.3 says to treat
>>> this as [dcl.init.string].
>>>
>>> We were rejecting this code because we never called reshape_init before
>>> the digest_init in build_new_1.  reshape_init handles [dcl.init.string]
>>> by unwrapping the STRING_CST from its enclosing { }, and digest_init
>>> assumes that reshape_init has been called for aggregates anyway, and an
>>> array is an aggregate.
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	PR c++/77841
>>> 	* init.c (build_new_1): Call reshape_init.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	PR c++/77841
>>> 	* g++.dg/cpp0x/initlist-new4.C: New test.
>>> ---
>>>    gcc/cp/init.c                              | 6 ++++++
>>>    gcc/testsuite/g++.dg/cpp0x/initlist-new4.C | 6 ++++++
>>>    2 files changed, 12 insertions(+)
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/initlist-new4.C
>>>
>>> diff --git a/gcc/cp/init.c b/gcc/cp/init.c
>>> index 360ab8c0b52..d4540db3605 100644
>>> --- a/gcc/cp/init.c
>>> +++ b/gcc/cp/init.c
>>> @@ -3575,6 +3575,12 @@ build_new_1 (vec<tree, va_gc> **placement, tree type, tree nelts,
>>>    		    /* We'll check the length at runtime.  */
>>>    		    domain = NULL_TREE;
>>>    		  arraytype = build_cplus_array_type (type, domain);
>>> +		  /* If we have new char[4]{"foo"}, we have to reshape
>>> +		     so that the STRING_CST isn't wrapped in { }.  */
>>> +		  vecinit = reshape_init (arraytype, vecinit, complain);
>>> +		  /* The middle end doesn't cope with the location wrapper
>>> +		     around a STRING_CST.  */
>>> +		  STRIP_ANY_LOCATION_WRAPPER (vecinit);
>>>    		  vecinit = digest_init (arraytype, vecinit, complain);
>>>    		}
>>
>> This is OK, but now I wonder why your earlier addition,
>>
>>>            /* This handles code like new char[]{"foo"}.  */
>>>            else if (len == 1
>>>                     && char_type_p (TYPE_MAIN_VARIANT (type))
>>>                     && TREE_CODE (tree_strip_any_location_wrapper ((**init)[0]))
>>>                        == STRING_CST)
>>>              {
>>>                vecinit = (**init)[0];
>>>                STRIP_ANY_LOCATION_WRAPPER (vecinit);
>>>              }
>>
>> isn't handled by the block you're changing in this patch.  Why isn't
>> DIRECT_LIST_INIT_P true for new char[]{"foo"}?
> 
> Yes, I was hoping this hunk would handle the new char[4]{"foo"} case too,
> but for new char[]{"foo"} DIRECT_LIST_INIT_P is false because earlier in
> build_new we called reshape_init:
> 
> 4011       /* Otherwise we should have 'new T[]{e_0, ..., e_k}'.  */
> 4012       if (BRACE_ENCLOSED_INITIALIZER_P (elt))
> 4013         elt = reshape_init (type, elt, complain);
> 
> which unwraps the { } from {"foo"}, so it's no longer a list init.

Ah, I see.

> We
> won't get there with new char[4]{"foo"} because TREE_CODE (type) will
> not be ARRAY_TYPE; instead, nelts is set to INTEGER_CST 4 when we know
> the array bound.
> 
> I could make it so that we call reshape_init in build_new for the [4]
> case too, but it was uglier than this fix.
> 
> Should I go ahead with this patch as-is or would you prefer any changes?

Go ahead with this for now, but I notice that we also still don't support

new char[4](1,2,3,4);

because the handling of parenthesized-init is limited to the deduced 
array size case.

It would be nice to find a way to combine the two places that we're 
messing with array initializers.

Jason