c++: 'mutable' subobject of constexpr variable [PR109745]

r13-2701-g7107ea6fb933f1 made us correctly accept during constexpr
evaluation 'mutable' member accesses of objects constructed during
that evaluation, while continuing to reject such accesses for constexpr
objects constructed outside of that evaluation, by considering the
CONSTRUCTOR_MUTABLE_POISON flag during cxx_eval_component_reference.

However, this flag is set only for the outermost CONSTRUCTOR of a
constexpr variable initializer, so if we're accessing a 'mutable' member
of a nested CONSTRUCTOR, the flag won't be set and we won't reject the
access.  This can lead to us accepting invalid code, as in the first
testcase, or even wrong code generation due to our speculative constexpr
evaluation, as in the second and third testcase.

This patch fixes this by setting CONSTRUCTOR_MUTABLE_POISON recursively
rather than only on the outermost CONSTRUCTOR.

	PR c++/109745

gcc/cp/ChangeLog:

	* typeck2.cc (poison_mutable_constructors): Define.
	(store_init_value): Use it instead of setting
	CONSTRUCTOR_MUTABLE_POISON directly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-mutable4.C: New test.
	* g++.dg/cpp0x/constexpr-mutable5.C: New test.
	* g++.dg/cpp1y/constexpr-mutable2.C: New test.

(cherry picked from commit 02777f20be4f40160f1b4ed34fa59ba75245b5b7)

diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index bf03967a71f07bcff1ef39d7352770dc15b62392..610fa081e7ceb73a1dcf74ea0b51b6188adecd3d 100644 (file)
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -776,6 +776,27 @@ split_nonconstant_init (tree dest, tree init)
   return code;
 }
 
+/* T is the initializer of a constexpr variable.  Set CONSTRUCTOR_MUTABLE_POISON
+   for any CONSTRUCTOR within T that contains (directly or indirectly) a mutable
+   member, thereby poisoning it so it can't be copied to another a constexpr
+   variable or read during constexpr evaluation.  */
+
+static void
+poison_mutable_constructors (tree t)
+{
+  if (TREE_CODE (t) != CONSTRUCTOR)
+    return;
+
+  if (cp_has_mutable_p (TREE_TYPE (t)))
+    {
+      CONSTRUCTOR_MUTABLE_POISON (t) = true;
+
+      if (vec<constructor_elt, va_gc> *elts = CONSTRUCTOR_ELTS (t))
+       for (const constructor_elt &ce : *elts)
+         poison_mutable_constructors (ce.value);
+    }
+}
+
 /* Perform appropriate conversions on the initial value of a variable,
    store it in the declaration DECL,
    and print any error messages that are appropriate.
@@ -864,10 +885,7 @@ store_init_value (tree decl, tree init, vec<tree, va_gc>** cleanups, int flags)
       else
        value = fold_non_dependent_init (value, tf_warning_or_error,
                                         /*manifestly_const_eval=*/true, decl);
-      if (TREE_CODE (value) == CONSTRUCTOR && cp_has_mutable_p (type))
-       /* Poison this CONSTRUCTOR so it can't be copied to another
-          constexpr variable.  */
-       CONSTRUCTOR_MUTABLE_POISON (value) = true;
+      poison_mutable_constructors (value);
       const_init = (reduced_constant_expression_p (value)
                    || error_operand_p (value));
       DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl) = const_init;

diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable4.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable4.C
new file mode 100644 (file)
index 0000000..fd982bc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable4.C
@@ -0,0 +1,15 @@
+// PR c++/109745
+// Similar to constexpr-mutable1.C, but with nested 'mutable' accesses.
+// { dg-do compile { target c++11 } }
+
+struct A { mutable int m = 0; };
+
+struct B { A a; };
+
+struct C { B b; };
+
+constexpr B b;
+constexpr int bam = b.a.m;    // { dg-error "mutable" }
+
+constexpr C c;
+constexpr int cbam = c.b.a.m; // { dg-error "mutable" }

diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable5.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable5.C
new file mode 100644 (file)
index 0000000..6a530e2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-mutable5.C
@@ -0,0 +1,39 @@
+// PR c++/109745
+// { dg-do run { target c++11 } }
+// { dg-additional-options "-O" }
+
+struct A {
+  mutable int m = 0;
+  void f() const { ++m; };
+  constexpr int get_m() const { return m; }
+};
+
+struct B { A a; };
+
+struct C { B b; };
+
+int main() {
+  constexpr A a;
+  a.m++;
+  if (a.get_m() != 1 || a.m != 1)
+    __builtin_abort();
+  a.m++;
+  if (a.get_m() != 2 || a.m != 2)
+    __builtin_abort();
+
+  constexpr B b;
+  b.a.m++;
+  if (b.a.get_m() != 1 || b.a.m != 1)
+    __builtin_abort();
+  b.a.m++;
+  if (b.a.get_m() != 2 || b.a.m != 2)
+    __builtin_abort();
+
+  constexpr C c;
+  c.b.a.m++;
+  if (c.b.a.get_m() != 1 || c.b.a.m != 1)
+    __builtin_abort();
+  c.b.a.m++;
+  if (c.b.a.get_m() != 2 || c.b.a.m != 2)
+    __builtin_abort();
+}

diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-mutable2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-mutable2.C
new file mode 100644 (file)
index 0000000..4898aa0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-mutable2.C
@@ -0,0 +1,20 @@
+// PR c++/109745
+// { dg-do run { target c++14 } }
+// { dg-additional-options "-O" }
+
+template<class T>
+struct Foo { T val; };
+
+struct Bar {
+  constexpr Bar() = default;
+  constexpr Bar(Bar const& other) { other.val_ = 42; }
+  constexpr int val() const { return val_; }
+  mutable int val_{};
+};
+
+int main() {
+  constexpr Foo<Bar> x{};
+  Foo<Bar> y{x};
+  if (x.val.val() != 42 || x.val.val_ != 42)
+    __builtin_abort();
+}