c++: DR2397 - auto specifier for * and & to arrays [PR100975]

This patch implements DR2397, which removes the restriction in
[dcl.array]p4 that the array element type may not be a placeholder
type.  We don't need to worry about decltype(auto) here, so this
allows code like

  int a[3];
  auto (*p)[3] = &a;
  auto (&r)[3] = a;

However, note that

  auto (&&r)[2] = { 1, 2 };
  auto arr[2] = { 1, 2 };

still doesn't work (although one day it might) and neither does

  int arr[5];
  auto x[5] = arr;

given that auto deduction is performed in terms of function template
argument deduction, so the array decays to *.

	PR c++/100975
	DR 2397

gcc/cp/ChangeLog:

	* decl.c (create_array_type_for_decl): Allow array of auto.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/auto24.C: Remove dg-error.
	* g++.dg/cpp0x/auto3.C: Adjust dg-error.
	* g++.dg/cpp0x/auto42.C: Likewise.
	* g++.dg/cpp0x/initlist75.C: Likewise.
	* g++.dg/cpp0x/initlist80.C: Likewise.
	* g++.dg/diagnostic/auto1.C: Remove dg-error.
	* g++.dg/cpp23/auto-array.C: New test.

8 files changed, 45 insertions, 18 deletions

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index fa6af6f..7672947 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -10969,17 +10969,6 @@ create_array_type_for_decl (tree name, tree type, tree size, location_t loc)
   if (type == error_mark_node || size == error_mark_node)
     return error_mark_node;
 
-  /* 8.3.4/1: If the type of the identifier of D contains the auto
-     type-specifier, the program is ill-formed.  */
-  if (type_uses_auto (type))
-    {
-      if (name)
-	error_at (loc, "%qD declared as array of %qT", name, type);
-      else
-	error ("creating array of %qT", type);
-      return error_mark_node;
-    }
-
   /* If there are some types which cannot be array elements,
      issue an error-message and return.  */
   switch (TREE_CODE (type))
diff --git a/gcc/testsuite/g++.dg/cpp0x/auto24.C b/gcc/testsuite/g++.dg/cpp0x/auto24.C
index 193f92e..ac1ba24 100644
--- a/gcc/testsuite/g++.dg/cpp0x/auto24.C
+++ b/gcc/testsuite/g++.dg/cpp0x/auto24.C
@@ -1,5 +1,6 @@
 // PR c++/48599
 // { dg-do compile { target c++11 } }
+// Allowed since DR2397.
 
 int v[1];
-auto (*p)[1] = &v;		// { dg-error "8:.p. declared as array of .auto" }
+auto (*p)[1] = &v;
diff --git a/gcc/testsuite/g++.dg/cpp0x/auto3.C b/gcc/testsuite/g++.dg/cpp0x/auto3.C
index 2cd0520..7cde745 100644
--- a/gcc/testsuite/g++.dg/cpp0x/auto3.C
+++ b/gcc/testsuite/g++.dg/cpp0x/auto3.C
@@ -9,8 +9,8 @@ auto x;				// { dg-error "auto" }
 // deduction, the program is ill-formed.
 auto i = 42, j = 42.0;		// { dg-error "auto" }
 
-// New CWG issue
-auto a[2] = { 1, 2 };		// { dg-error "6:.a. declared as array of .auto" }
+// CWG issue 2397: [dcl.type.auto.deduct]/2: "T shall not be an array type".
+auto a[2] = { 1, 2 };		// { dg-error "20:unable to deduce" }
 
 template<class T>
 struct A { };
diff --git a/gcc/testsuite/g++.dg/cpp0x/auto42.C b/gcc/testsuite/g++.dg/cpp0x/auto42.C
index 8d15fc9..5b2f677 100644
--- a/gcc/testsuite/g++.dg/cpp0x/auto42.C
+++ b/gcc/testsuite/g++.dg/cpp0x/auto42.C
@@ -5,5 +5,5 @@
 
 void foo(int i)
 {
-  auto x[1] = { 0 };		// { dg-error "8:.x. declared as array of .auto" }
+  auto x[1] = { 0 };		// { dg-error "19:unable to deduce" }
 }
diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist75.C b/gcc/testsuite/g++.dg/cpp0x/initlist75.C
index 9a45087..f572f51 100644
--- a/gcc/testsuite/g++.dg/cpp0x/initlist75.C
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist75.C
@@ -3,4 +3,4 @@
 
 #include <initializer_list>
 
-auto foo[] = {};    // { dg-error "6:.foo. declared as array of .auto" }
+auto foo[] = {};    // { dg-error "15:unable to deduce" }
diff --git a/gcc/testsuite/g++.dg/cpp0x/initlist80.C b/gcc/testsuite/g++.dg/cpp0x/initlist80.C
index 15723be..a6ab40c 100644
--- a/gcc/testsuite/g++.dg/cpp0x/initlist80.C
+++ b/gcc/testsuite/g++.dg/cpp0x/initlist80.C
@@ -3,4 +3,4 @@
 
 #include <initializer_list>
 
-auto x[2] = {};			// { dg-error "6:.x. declared as array of .auto" }
+auto x[2] = {};			// { dg-error "14:unable to deduce" }
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-array.C b/gcc/testsuite/g++.dg/cpp23/auto-array.C
new file mode 100644
index 0000000..42f2b0c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-array.C
@@ -0,0 +1,36 @@
+// PR c++/100975
+// DR 2397 - auto specifier for pointers and references to arrays 
+// { dg-do compile { target c++11 } }
+
+struct false_type { static constexpr bool value = false; };
+struct true_type { static constexpr bool value = true; };
+template<class T, class U>
+struct is_same : false_type {}; 
+template<class T>
+struct is_same<T, T> : true_type {};
+
+using U = int[3];
+
+void
+g ()
+{
+  int a[3];
+  auto (*p)[3] = &a;
+  auto (&r)[3] = a;
+  int aa[3][3];
+  auto (*pp)[3][3] = &aa;
+  auto (&rr)[3][3] = aa;
+
+  auto (&&rv)[3] = U{};
+
+  static_assert (is_same<decltype (p), int(*)[3]>::value, "");
+  static_assert (is_same<decltype (pp), int(*)[3][3]>::value, "");
+  static_assert (is_same<decltype (r), int(&)[3]>::value, "");
+  static_assert (is_same<decltype (rv), int(&&)[3]>::value, "");
+  static_assert (is_same<decltype (rr), int(&)[3][3]>::value, "");
+
+#if __cplusplus >= 201402L
+  // In a generic lambda parameter this was OK even before.
+  auto l = [](auto (&arr)[5]) { return arr[0]; };
+#endif
+}
diff --git a/gcc/testsuite/g++.dg/diagnostic/auto1.C b/gcc/testsuite/g++.dg/diagnostic/auto1.C
index ee2eefd..9d9979e 100644
--- a/gcc/testsuite/g++.dg/diagnostic/auto1.C
+++ b/gcc/testsuite/g++.dg/diagnostic/auto1.C
@@ -1,4 +1,5 @@
 // PR c++/86915
 // { dg-do compile { target c++17 } }
+// Allowed since DR2397.
 
-template<auto [1]> struct S; // { dg-error "creating array of .auto." }
+template<auto [1]> struct S;