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.

gcc/cp/ChangeLog

+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.

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

gcc/testsuite/ChangeLog

+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

gcc/testsuite/g++.dg/cpp0x/constexpr-93169.C

+// 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;
+}