[PATCH] c++: Fix wrong-code with non-constexpr constructor [PR93169]

[Marek Polacek]

In order to detect modifying constant objects in constexpr evaluation,
which is UB, in r10-2655 I added code that sets TREE_READONLY on
CONSTRUCTORs of const-qualified objects after they have been fully
constructed.  But I never made sure that what we're setting the flag
on actually is a CONSTRUCTOR.  Consequently, as this test case shows,
we could set TREE_READONLY on a VAR_DECL that in fact wasn't constant,
causing problems later.  Fixed by setting the flag on CONSTRUCTORs
only, and only when the evaluation produced something constant.

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

2020-02-19  Marek Polacek  <pola...@redhat.com>

        PR c++/93169 - wrong-code with a non-constexpr constructor.
        * constexpr.c (cxx_eval_call_expression): Only set TREE_READONLY
        on constant CONSTRUCTORs.

        * g++.dg/cpp0x/constexpr-93169.C: New test.
---
 gcc/cp/constexpr.c                           |  3 ++-
 gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C | 21 ++++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index bf7a2643003..128f760778b 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2474,7 +2474,8 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree 
t,
                                                     /*lval*/false,
                                                     non_constant_p,
                                                     overflow_p);
-             TREE_READONLY (e) = true;
+             if (TREE_CODE (e) == CONSTRUCTOR && !*non_constant_p)
+               TREE_READONLY (e) = true;
            }
 
          /* Forget the saved values of the callee's SAVE_EXPRs and
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C 
b/gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C
new file mode 100644
index 00000000000..79fd352c0c5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C
@@ -0,0 +1,21 @@
+// PR c++/93169 - Wrong-code with a non-constexpr constructor.
+// { dg-do run { target c++11 } }
+// { dg-options "-O2" }
+
+template <typename T> class B {
+  struct C {
+    T h;
+    constexpr C() {}
+    ~C() {}
+  } c;
+};
+struct S {
+  int g;
+  S() { g = 2; }
+};
+
+int
+main()
+{
+  static const B<S> f;
+}

base-commit: 77f5310f0205714538668b88fbf1de44f1f435be
-- 
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA