[algs] Use universal references for hb_pair()

Such that it carries lvalues inside.

src/Makefile.am

@@ -384,12 +384,16 @@ dump_use_data_SOURCES = dump-use-data.cc hb-ot-shape-complex-use-table.cc
 dump_use_data_CPPFLAGS = $(HBCFLAGS)
 dump_use_data_LDADD = libharfbuzz.la $(HBLIBS)
 
-COMPILED_TESTS = test-iter test-ot-tag test-unicode-ranges
+COMPILED_TESTS = test-algs test-iter test-ot-tag test-unicode-ranges
 COMPILED_TESTS_CPPFLAGS = $(HBCFLAGS) -DMAIN -UNDEBUG
 COMPILED_TESTS_LDADD = libharfbuzz.la $(HBLIBS)
 check_PROGRAMS += $(COMPILED_TESTS)
 TESTS += $(COMPILED_TESTS)
 
+test_algs_SOURCES = test-algs.cc hb-static.cc
+test_algs_CPPFLAGS = $(COMPILED_TESTS_CPPFLAGS)
+test_algs_LDADD = $(COMPILED_TESTS_LDADD)
+
 test_iter_SOURCES = test-iter.cc hb-static.cc
 test_iter_CPPFLAGS = $(COMPILED_TESTS_CPPFLAGS)
 test_iter_LDADD = $(COMPILED_TESTS_LDADD)

src/hb-algs.hh

@@ -59,7 +59,7 @@ struct hb_pair_t
   T2 second;
 };
 template <typename T1, typename T2> static inline hb_pair_t<T1, T2>
-hb_pair (T1 a, T2 b) { return hb_pair_t<T1, T2> (a, b); }
+hb_pair (T1&& a, T2&& b) { return hb_pair_t<T1, T2> (a, b); }
 
 static const struct
 {

src/test-algs.cc

@@ -0,0 +1,45 @@
+/*
+ * Copyright © 2019  Facebook, Inc.
+ *
+ *  This is part of HarfBuzz, a text shaping library.
+ *
+ * Permission is hereby granted, without written agreement and without
+ * license or royalty fees, to use, copy, modify, and distribute this
+ * software and its documentation for any purpose, provided that the
+ * above copyright notice and the following two paragraphs appear in
+ * all copies of this software.
+ *
+ * IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE TO ANY PARTY FOR
+ * DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
+ * ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN
+ * IF THE COPYRIGHT HOLDER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
+ * DAMAGE.
+ *
+ * THE COPYRIGHT HOLDER SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,
+ * BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
+ * ON AN "AS IS" BASIS, AND THE COPYRIGHT HOLDER HAS NO OBLIGATION TO
+ * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
+ *
+ * Facebook Author(s): Behdad Esfahbod
+ */
+
+#include "hb.hh"
+#include "hb-algs.hh"
+
+
+int
+main (int argc, char **argv)
+{
+  int i = 1;
+  auto p = hb_pair (1, i);
+
+  p.second = 2;
+  assert (i == 2);
+
+  const int c = 3;
+  auto pc = hb_pair (1, c);
+  assert (pc.second == 3);
+
+  return 0;
+}