Re: [PATCH] c++: Fix for decltype and bit-fields [PR95009]

[Jason Merrill via Gcc-patches]

On 12/4/21 15:26, Marek Polacek wrote:

Here, decltype deduces the wrong type for certain expressions involving
bit-fields.  Unlike in C, in C++ bit-field width is explicitly not part
of the type, so I think decltype should never deduce to 'int:N'.  The
problem isn't that we're not calling unlowered_expr_type--we are--it's
that is_bitfield_expr_with_lowered_type only handles certain codes, but
not others.  For example, += works fine but ++ does not.

This also fixes decltype-bitfield2.C where we were crashing (!), but
unfortunately it does not fix 84516 or 70733 where the problem is likely
a missing call to unlowered_expr_type.  It occurs to me now that typeof
likely has had the same issue, but this patch should fix that too.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/11?

OK.

        PR c++/95009

gcc/cp/ChangeLog:

        * typeck.c (is_bitfield_expr_with_lowered_type) <case MODIFY_EXPR>:
        Handle UNARY_PLUS_EXPR, NEGATE_EXPR, NON_LVALUE_EXPR, BIT_NOT_EXPR,
        *CREMENT_EXPR too.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp0x/decltype-bitfield1.C: New test.
        * g++.dg/cpp0x/decltype-bitfield2.C: New test.
---
  gcc/cp/typeck.c                               | 14 +++-
  .../g++.dg/cpp0x/decltype-bitfield1.C         | 65 +++++++++++++++++++
  .../g++.dg/cpp0x/decltype-bitfield2.C         | 18 +++++
  3 files changed, 94 insertions(+), 3 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp0x/decltype-bitfield1.C
  create mode 100644 gcc/testsuite/g++.dg/cpp0x/decltype-bitfield2.C

diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index 5ed9a5ab9ee..4e60db40c76 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -2209,9 +2209,9 @@ invalid_nonstatic_memfn_p (location_t loc, tree expr, 
tsubst_flags_t complain)
    return false;
  }

-/* If EXP is a reference to a bitfield, and the type of EXP does not

-   match the declared type of the bitfield, return the declared type
-   of the bitfield.  Otherwise, return NULL_TREE.  */
+/* If EXP is a reference to a bit-field, and the type of EXP does not
+   match the declared type of the bit-field, return the declared type
+   of the bit-field.  Otherwise, return NULL_TREE.  */

tree

  is_bitfield_expr_with_lowered_type (const_tree exp)
@@ -2230,6 +2230,14 @@ is_bitfield_expr_with_lowered_type (const_tree exp)

case MODIFY_EXPR:

      case SAVE_EXPR:
+    case UNARY_PLUS_EXPR:
+    case PREDECREMENT_EXPR:
+    case PREINCREMENT_EXPR:
+    case POSTDECREMENT_EXPR:
+    case POSTINCREMENT_EXPR:
+    case NEGATE_EXPR:
+    case NON_LVALUE_EXPR:
+    case BIT_NOT_EXPR:
        return is_bitfield_expr_with_lowered_type (TREE_OPERAND (exp, 0));

case COMPONENT_REF:

diff --git a/gcc/testsuite/g++.dg/cpp0x/decltype-bitfield1.C 
b/gcc/testsuite/g++.dg/cpp0x/decltype-bitfield1.C
new file mode 100644
index 00000000000..2d8d8e81bff
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/decltype-bitfield1.C
@@ -0,0 +1,65 @@
+// PR c++/95009
+// { 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 {};
+
+struct A {
+  int i : 31;
+  unsigned long l : 37;
+} a;
+
+void
+g ()
+{
+  // Careful: pre{in,de}crements are lvalues -> deduce T&.  */
+  static_assert (is_same<decltype(a.i), int>::value, "");
+  static_assert (is_same<decltype((a.i)), int&>::value, "");
+  static_assert (is_same<decltype(++a.i), int&>::value, "");
+  static_assert (is_same<decltype((++a.i)), int&>::value, "");
+  static_assert (is_same<decltype(a.i++), int>::value, "");
+  static_assert (is_same<decltype((a.i++)), int>::value, "");
+  static_assert (is_same<decltype(--a.i), int&>::value, "");
+  static_assert (is_same<decltype((--a.i)), int&>::value, "");
+  static_assert (is_same<decltype(a.i--), int>::value, "");
+  static_assert (is_same<decltype((a.i--)), int>::value, "");
+  static_assert (is_same<decltype(a.i += 1), int&>::value, "");
+  static_assert (is_same<decltype((a.i += 1)), int&>::value, "");
+  static_assert (is_same<decltype(a.i -= 1), int&>::value, "");
+  static_assert (is_same<decltype((a.i -= 1)), int&>::value, "");
+  static_assert (is_same<decltype(a.i *= 1), int&>::value, "");
+  static_assert (is_same<decltype((a.i *= 1)), int&>::value, "");
+  static_assert (is_same<decltype(+a.i), int>::value, "");
+  static_assert (is_same<decltype((+a.i)), int>::value, "");
+  static_assert (is_same<decltype(-a.i), int>::value, "");
+  static_assert (is_same<decltype((-a.i)), int>::value, "");
+  static_assert (is_same<decltype(~a.i), int>::value, "");
+  static_assert (is_same<decltype((~a.i)), int>::value, "");
+
+  static_assert (is_same<decltype(a.l), unsigned long>::value, "");
+  static_assert (is_same<decltype((a.l)), unsigned long&>::value, "");
+  static_assert (is_same<decltype(++a.l), unsigned long&>::value, "");
+  static_assert (is_same<decltype((++a.l)), unsigned long&>::value, "");
+  static_assert (is_same<decltype(a.l++), unsigned long>::value, "");
+  static_assert (is_same<decltype((a.l++)), unsigned long>::value, "");
+  static_assert (is_same<decltype(--a.l), unsigned long&>::value, "");
+  static_assert (is_same<decltype((--a.l)), unsigned long&>::value, "");
+  static_assert (is_same<decltype(a.l--), unsigned long>::value, "");
+  static_assert (is_same<decltype((a.l--)), unsigned long>::value, "");
+  static_assert (is_same<decltype(a.l += 1), unsigned long&>::value, "");
+  static_assert (is_same<decltype((a.l += 1)), unsigned long&>::value, "");
+  static_assert (is_same<decltype(a.l -= 1), unsigned long&>::value, "");
+  static_assert (is_same<decltype((a.l -= 1)), unsigned long&>::value, "");
+  static_assert (is_same<decltype(a.l *= 1), unsigned long&>::value, "");
+  static_assert (is_same<decltype((a.l *= 1)), unsigned long&>::value, "");
+  static_assert (is_same<decltype(+a.l), unsigned long>::value, "");
+  static_assert (is_same<decltype((+a.l)), unsigned long>::value, "");
+  static_assert (is_same<decltype(-a.l), unsigned long>::value, "");
+  static_assert (is_same<decltype((-a.l)), unsigned long>::value, "");
+  static_assert (is_same<decltype(~a.l), unsigned long>::value, "");
+  static_assert (is_same<decltype((~a.l)), unsigned long>::value, "");
+}
diff --git a/gcc/testsuite/g++.dg/cpp0x/decltype-bitfield2.C 
b/gcc/testsuite/g++.dg/cpp0x/decltype-bitfield2.C
new file mode 100644
index 00000000000..4bf9c7682ff
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/decltype-bitfield2.C
@@ -0,0 +1,18 @@
+// PR c++/95009
+// { dg-do compile { target c++11 } }
+
+struct A {
+  int i:31;
+};
+
+template<typename>
+void f ()
+{
+}
+
+int main ()
+{
+  A a;
+  f<decltype(a.i += 1)>();
+  f<decltype(++a.i)>();
+}

base-commit: 689407ef916503b2f5a3c8c07fe7d5ab1913f956