c++: prvalue array decay [PR94264]

My change for PR53220 made array to pointer decay for prvalue arrays
ill-formed to catch well-defined C code that produces a dangling pointer in
C++ due to the shorter lifetime of compound literals.  This wasn't really
correct, but wasn't a problem until C++17 added prvalue arrays, at which
point it started rejecting valid C++ code.

I wanted to make sure that we still diagnose the problematic code;
-Wdangling-pointer covers the array-lit.c case, but I needed to extend
-Wreturn-local-addr to handle the return case.

	PR c++/94264
	PR c++/53220

gcc/c/ChangeLog:

	* c-typeck.cc (array_to_pointer_conversion): Adjust -Wc++-compat
	diagnostic.

gcc/cp/ChangeLog:

	* call.cc (convert_like_internal): Remove obsolete comment.
	* typeck.cc (decay_conversion): Allow array prvalue.
	(maybe_warn_about_returning_address_of_local): Check
	for returning pointer to temporary.

gcc/testsuite/ChangeLog:

	* c-c++-common/array-lit.c: Adjust.
	* g++.dg/cpp1z/array-prvalue1.C: New test.
	* g++.dg/ext/complit17.C: New test.
This commit is contained in:
Jason Merrill 2023-11-28 13:54:47 -05:00
parent 3d104d93a7
commit 305a2686c9
6 changed files with 17 additions and 13 deletions

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@ -1748,7 +1748,7 @@ array_to_pointer_conversion (location_t loc, tree exp)
if (!TREE_READONLY (decl) && !TREE_STATIC (decl))
warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wc___compat,
"converting an array compound literal to a pointer "
"is ill-formed in C++");
"leads to a dangling pointer in C++");
}
adr = build_unary_op (loc, ADDR_EXPR, exp, true);

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@ -8578,8 +8578,6 @@ convert_like_internal (conversion *convs, tree expr, tree fn, int argnum,
array = finish_compound_literal (array, new_ctor, complain);
/* This is dubious now, should be blessed by P2752. */
DECL_MERGEABLE (TARGET_EXPR_SLOT (array)) = true;
/* Take the address explicitly rather than via decay_conversion
to avoid the error about taking the address of a temporary. */
array = cp_build_addr_expr (array, complain);
}
else

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@ -2534,15 +2534,6 @@ decay_conversion (tree exp,
return error_mark_node;
}
/* Don't let an array compound literal decay to a pointer. It can
still be used to initialize an array or bind to a reference. */
if (TREE_CODE (exp) == TARGET_EXPR)
{
if (complain & tf_error)
error_at (loc, "taking address of temporary array");
return error_mark_node;
}
ptrtype = build_pointer_type (TREE_TYPE (type));
if (VAR_P (exp))
@ -10535,6 +10526,9 @@ maybe_warn_about_returning_address_of_local (tree retval, location_t loc)
if (TYPE_REF_P (valtype))
warning_at (loc, OPT_Wreturn_local_addr,
"returning reference to temporary");
else if (TYPE_PTR_P (valtype))
warning_at (loc, OPT_Wreturn_local_addr,
"returning pointer to temporary");
else if (is_std_init_list (valtype))
warning_at (loc, OPT_Winit_list_lifetime,
"returning temporary %<initializer_list%> does not extend "

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@ -1,10 +1,11 @@
/* { dg-options "-std=c99 -Wc++-compat -Werror" { target c } } */
/* { dg-options "-Werror=dangling-pointer=1" { target c++ } } */
/* { dg-prune-output "treated as errors" } */
#include <stdio.h>
int main()
{
for (int *p = (int[]){ 1, 2, 3, 0 }; /* { dg-error "array" } */
for (int *p = (int[]){ 1, 2, 3, 0 }; /* { dg-error "array|temporary" } */
*p; ++p) {
printf("%d\n", *p);
}

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@ -0,0 +1,7 @@
// PR c++/94264
// { dg-do compile { target c++17 } }
int main() {
using T = int[];
T{1, 2} == nullptr;
}

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@ -0,0 +1,4 @@
// PR c++/53220
// { dg-options "" }
int* f() { return (int[]){42}; } // { dg-warning "pointer to temporary" }