[gcc r14-5941] c++: prvalue array decay [PR94264]

[gcc r14-5941] c++: prvalue array decay [PR94264]

[Jason Merrill]

https://gcc.gnu.org/g:305a2686c99bf9b57490419f25c79f6fb3ae0feb

commit r14-5941-g305a2686c99bf9b57490419f25c79f6fb3ae0feb
Author: Jason Merrill <jason@redhat.com>
Date:   Tue Nov 28 13:54:47 2023 -0500

    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.

Diff:
---
 gcc/c/c-typeck.cc                           |  2 +-
 gcc/cp/call.cc                              |  2 --
 gcc/cp/typeck.cc                            | 12 +++---------
 gcc/testsuite/c-c++-common/array-lit.c      |  3 ++-
 gcc/testsuite/g++.dg/cpp1z/array-prvalue1.C |  7 +++++++
 gcc/testsuite/g++.dg/ext/complit17.C        |  4 ++++
 6 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/gcc/c/c-typeck.cc b/gcc/c/c-typeck.cc
index 1dbb4471a88..17fdc9789b4 100644
--- a/gcc/c/c-typeck.cc
+++ b/gcc/c/c-typeck.cc
@@ -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);
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 81b104f4b40..ae0decd87f1 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -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
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index e995fb6ddd7..0839d0a4167 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -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 "
diff --git a/gcc/testsuite/c-c++-common/array-lit.c b/gcc/testsuite/c-c++-common/array-lit.c
index 6505c2091b4..a6b3adf7cc8 100644
--- a/gcc/testsuite/c-c++-common/array-lit.c
+++ b/gcc/testsuite/c-c++-common/array-lit.c
@@ -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);
   }
diff --git a/gcc/testsuite/g++.dg/cpp1z/array-prvalue1.C b/gcc/testsuite/g++.dg/cpp1z/array-prvalue1.C
new file mode 100644
index 00000000000..e837d3253a1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/array-prvalue1.C
@@ -0,0 +1,7 @@
+// PR c++/94264
+// { dg-do compile { target c++17 } }
+
+int main() {
+  using T = int[];
+  T{1, 2} == nullptr;
+}
diff --git a/gcc/testsuite/g++.dg/ext/complit17.C b/gcc/testsuite/g++.dg/ext/complit17.C
new file mode 100644
index 00000000000..acc80847be3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/complit17.C
@@ -0,0 +1,4 @@
+// PR c++/53220
+// { dg-options "" }
+
+int* f() { return (int[]){42}; } // { dg-warning "pointer to temporary" }