c++: Fix up constexpr CLEANUP_POINT_EXPR and TRY_FINALLY_EXPR handling [PR97790]

As the testcase shows, CLEANUP_POINT_EXPR (and I think TRY_FINALLY_EXPR too)
suffer from the same problem that I was trying to fix in
r10-3597-g1006c9d4395a939820df76f37c7b085a4a1a003f
for CLEANUP_STMT, namely that if in the middle of the body expression of
those stmts is e.g. return stmt, goto, break or continue (something that
changes *jump_target and makes it start skipping stmts), we then skip the
cleanups too, which is not appropriate - the cleanups were either queued up
during the non-skipping execution of the body (for CLEANUP_POINT_EXPR), or
for TRY_FINALLY_EXPR are relevant already after entering the body block.

> Would it make sense to always use a NULL jump_target when evaluating
> cleanups?

I was afraid of that, especially for TRY_FINALLY_EXPR, but it seems that
during constexpr evaluation the cleanups will most often be just very simple
destructor calls (or calls to cleanup attribute functions).
Furthermore, for neither of these 3 tree codes we'll reach that code if
jump_target && *jump_target initially (there is a return NULL_TREE much
earlier for those except for trees that could embed labels etc. in it and
clearly these 3 don't count in that).

2020-11-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/97790
	* constexpr.c (cxx_eval_constant_expression) <case CLEANUP_POINT_EXPR,
	case TRY_FINALLY_EXPR, case CLEANUP_STMT>: Don't pass jump_target to
	cxx_eval_constant_expression when evaluating the cleanups.

	* g++.dg/cpp2a/constexpr-dtor9.C: New test.

2 files changed, 43 insertions, 22 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index b6f9c43..e6ab5ee 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -6018,8 +6018,7 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	/* Evaluate the cleanups.  */
 	FOR_EACH_VEC_ELT_REVERSE (cleanups, i, cleanup)
 	  cxx_eval_constant_expression (ctx, cleanup, false,
-					non_constant_p, overflow_p,
-					jump_target);
+					non_constant_p, overflow_p);
       }
       break;
 
@@ -6030,29 +6029,20 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
       if (!*non_constant_p)
 	/* Also evaluate the cleanup.  */
 	cxx_eval_constant_expression (ctx, TREE_OPERAND (t, 1), true,
-				      non_constant_p, overflow_p,
-				      jump_target);
+				      non_constant_p, overflow_p);
       break;
 
     case CLEANUP_STMT:
-      {
-	tree initial_jump_target = jump_target ? *jump_target : NULL_TREE;
-	r = cxx_eval_constant_expression (ctx, CLEANUP_BODY (t), lval,
-					  non_constant_p, overflow_p,
-					  jump_target);
-	if (!CLEANUP_EH_ONLY (t) && !*non_constant_p)
-	  {
-	    iloc_sentinel ils (loc);
-	    /* Also evaluate the cleanup.  If we weren't skipping at the
-	       start of the CLEANUP_BODY, change jump_target temporarily
-	       to &initial_jump_target, so that even a return or break or
-	       continue in the body doesn't skip the cleanup.  */
-	    cxx_eval_constant_expression (ctx, CLEANUP_EXPR (t), true,
-					  non_constant_p, overflow_p,
-					  jump_target ? &initial_jump_target
-					  : NULL);
-	  }
-      }
+      r = cxx_eval_constant_expression (ctx, CLEANUP_BODY (t), lval,
+					non_constant_p, overflow_p,
+					jump_target);
+      if (!CLEANUP_EH_ONLY (t) && !*non_constant_p)
+	{
+	  iloc_sentinel ils (loc);
+	  /* Also evaluate the cleanup.  */
+	  cxx_eval_constant_expression (ctx, CLEANUP_EXPR (t), true,
+					non_constant_p, overflow_p);
+	}
       break;
 
       /* These differ from cxx_eval_unary_expression in that this doesn't
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor9.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor9.C
new file mode 100644
index 0000000..975e5fc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-dtor9.C
@@ -0,0 +1,31 @@
+// PR c++/97790
+// { dg-do compile { target c++20 } }
+
+struct S
+{
+  int *d;
+  int n;
+  constexpr S () : d(new int[1]{}), n(1) {}
+  constexpr ~S () { delete [] d; }
+};
+
+constexpr S
+foo ()
+{
+  return S ();
+}
+
+constexpr int
+bar ()
+{
+  return foo ().n;
+}
+
+constexpr int
+baz ()
+{
+  return S ().n;
+}
+
+constexpr int a = baz ();
+constexpr int b = bar ();