[gcc/devel/ranger] c++: Fix wrong-code with non-constexpr constructor [PR93169]

[Aldy Hernandez]

https://gcc.gnu.org/g:8f9dd1b0bdd935592ba151e9d843fddf6193afbc

commit 8f9dd1b0bdd935592ba151e9d843fddf6193afbc
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Feb 19 16:36:38 2020 -0500

    c++: Fix wrong-code with non-constexpr constructor [PR93169]
    
    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.
    
    2020-02-19  Marek Polacek  <polacek@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.

Diff:
---
 gcc/cp/ChangeLog                             |  6 ++++++
 gcc/cp/constexpr.c                           |  3 ++-
 gcc/testsuite/ChangeLog                      |  5 +++++
 gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C | 21 +++++++++++++++++++++
 4 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index f6b5517d5e4..e1ac5b0f29e 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+2020-02-19  Marek Polacek  <polacek@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.
+
 2020-02-15  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/93710 - poor diagnostic for array initializer.
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/ChangeLog b/gcc/testsuite/ChangeLog
index 049c343fe43..a54505ed73e 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-02-19  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93169 - wrong-code with a non-constexpr constructor.
+	* g++.dg/cpp0x/constexpr-93169.C: New test.
+
 2020-02-19  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/92128
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;
+}