2021-01-15  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98672
	* constexpr.c (potential_constant_expression_1) <case FOR_STMT>,
	<case WHILE_STMT>: If the condition isn't constant true, check if
	the loop body can contain a return stmt.

	* g++.dg/cpp1y/constexpr-98672.C: New test.

--- gcc/cp/constexpr.c.jj	2021-01-13 19:19:44.368469462 +0100
+++ gcc/cp/constexpr.c	2021-01-14 12:02:27.347042704 +0100
@@ -8190,7 +8190,17 @@ potential_constant_expression_1 (tree t,
 	  /* If we couldn't evaluate the condition, it might not ever be
 	     true.  */
 	  if (!integer_onep (tmp))
-	    return true;
+	    {
+	      /* Before returning true, check if the for body can contain
+		 a return.  */
+	      hash_set<tree> pset;
+	      check_for_return_continue_data data = { &pset, NULL_TREE };
+	      if (tree ret_expr
+		  = cp_walk_tree (&FOR_BODY (t), check_for_return_continue,
+				  &data, &pset))
+		*jump_target = ret_expr;
+	      return true;
+	    }
 	}
       if (!RECUR (FOR_EXPR (t), any))
 	return false;
@@ -8219,7 +8229,17 @@ potential_constant_expression_1 (tree t,
 	tmp = cxx_eval_outermost_constant_expr (tmp, true);
       /* If we couldn't evaluate the condition, it might not ever be true.  */
       if (!integer_onep (tmp))
-	return true;
+	{
+	  /* Before returning true, check if the while body can contain
+	     a return.  */
+	  hash_set<tree> pset;
+	  check_for_return_continue_data data = { &pset, NULL_TREE };
+	  if (tree ret_expr
+	      = cp_walk_tree (&WHILE_BODY (t), check_for_return_continue,
+			      &data, &pset))
+	    *jump_target = ret_expr;
+	  return true;
+	}
       if (!RECUR (WHILE_BODY (t), any))
 	return false;
       if (breaks (jump_target) || continues (jump_target))
--- gcc/testsuite/g++.dg/cpp1y/constexpr-98672.C.jj	2021-01-14 12:19:24.842438847 +0100
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-98672.C	2021-01-14 12:07:33.935551155 +0100
@@ -0,0 +1,35 @@
+// PR c++/98672
+// { dg-do compile { target c++14 } }
+
+void
+foo ()
+{
+}
+
+constexpr int
+bar ()
+{
+  for (int i = 0; i < 5; ++i)
+    return i;
+  foo ();
+  return 0;
+}
+
+constexpr int
+baz ()
+{
+  int i = 0;
+  while (i < 5)
+    {
+      if (i == 3)
+	return i;
+      else
+	++i;
+    }
+  foo ();
+  return 0;
+}
+
+constexpr int i = bar ();
+constexpr int j = baz ();
+static_assert (i == 0 && j == 3, "");