Make OUStringLiteral ctor actually constexpr

...which had accidentally been broken with
87707670 "Make OUStringLiteral more useful".
(std::strlen unfortunately isn't constexpr, so need to use an explicit loop
instead.)

Change-Id: I9a820e2940b99a0c37124477810ef879d82c8325
Reviewed-on: https://gerrit.libreoffice.org/59344
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

include/rtl/stringutils.hxx

@@ -13,10 +13,13 @@
 #include "sal/config.h"
 
 #include <cstddef>
-#include <cstring>
 
 #include "sal/types.h"
 
+#if defined LIBO_INTERNAL_ONLY
+#include "config_global.h"
+#endif
+
 // The unittest uses slightly different code to help check that the proper
 // calls are made. The class is put into a different namespace to make
 // sure the compiler generates a different (if generating also non-inline)
@@ -165,8 +168,20 @@ struct ConstCharArrayDetector< const char[ N ], T >
     typedef T Type;
     static const std::size_t length = N - 1;
     static const bool ok = true;
-    static bool isValid(char const (& literal)[N])
-    { return std::strlen(literal) == length; }
+#if defined LIBO_INTERNAL_ONLY && HAVE_CXX14_CONSTEXPR
+    constexpr
+#endif
+    static bool isValid(char const (& literal)[N]) {
+        for (std::size_t i = 0; i != N - 1; ++i) {
+            if (literal[i] == '\0') {
+                return false;
+            }
+        }
+        return literal[N - 1] == '\0';
+    }
+#if defined LIBO_INTERNAL_ONLY
+    constexpr
+#endif
     static char const * toPointer(char const (& literal)[N]) { return literal; }
 };
 #if defined LIBO_INTERNAL_ONLY

sal/osl/unx/nlsupport.cxx

@@ -20,6 +20,7 @@
 #include <sal/config.h>
 
 #include <algorithm>
+#include <cstring>
 
 #include <osl/nlsupport.h>
 #include <osl/diagnose.h>

sal/qa/rtl/strings/test_oustring_stringliterals.cxx

@@ -41,6 +41,9 @@ private:
 
     void testcall( const char str[] );
 
+    // Check that OUStringLiteral ctor is actually constexpr:
+    static constexpr rtlunittest::OUStringLiteral dummy{"dummy"};
+
 CPPUNIT_TEST_SUITE(StringLiterals);
 CPPUNIT_TEST(checkCtors);
 CPPUNIT_TEST(checkUsage);