Bug 1278314 - avoid invalid copying of objects in PrefsHelper.h; r=darchons

In an expression such as:

  const auto& x = cond() ? AClass(...) : AClass();

the C++ standard specifies that the copy constructor of AClass is
invoked on the result of the conditional expression ([expr.cond]p6).
GCC does not honor this part of the specification, whereas clang does;
clang therefore complains about instances of code such as:

   const auto& jstrVal = type == widget::PrefsHelper::PREF_STRING ?
       jni::StringParam(strVal, aPrefName.Env()) :
       jni::StringParam(nullptr);

as jni::StringParam is not copy-constructable.

The simplest solution that does not introduce unnecessary allocation
uses mozilla::Maybe to hold the temporary objects and to hide some of
the details of constructing objects in-place.  The compiler may even be
able to optimize away some of the unnnecessary checks that Maybe
introduces (e.g. checking for whether the Maybe is a Some or None at
certain points).

widget/android/PrefsHelper.h

@@ -12,6 +12,7 @@
 #include "nsCOMPtr.h"
 #include "nsVariant.h"
 
+#include "mozilla/Maybe.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/Services.h"
 #include "mozilla/UniquePtr.h"
@@ -67,16 +68,19 @@ class PrefsHelper
                 return false;
         }
 
-        const auto& jstrVal = type == widget::PrefsHelper::PREF_STRING ?
-                jni::StringParam(strVal, aPrefName.Env()) :
-                jni::StringParam(nullptr);
+        Maybe<jni::StringParam> jstrVal;
+        jstrVal.emplace(nullptr);
+        if (type == widget::PrefsHelper::PREF_STRING) {
+            jstrVal.reset();
+            jstrVal.emplace(strVal, aPrefName.Env());
+        }
 
         if (aPrefHandler) {
             widget::PrefsHelper::CallPrefHandler(
-                    aPrefHandler, type, aPrefName, boolVal, intVal, jstrVal);
+                    aPrefHandler, type, aPrefName, boolVal, intVal, jstrVal.ref());
         } else {
             widget::PrefsHelper::OnPrefChange(
-                    aPrefName, type, boolVal, intVal, jstrVal);
+                    aPrefName, type, boolVal, intVal, jstrVal.ref());
         }
         return true;
     }
@@ -189,12 +193,15 @@ public:
                     continue;
             }
 
-            const auto& jstrVal = type == widget::PrefsHelper::PREF_STRING ?
-                    jni::StringParam(strVal, aCls.Env()) :
-                    jni::StringParam(nullptr);
+            Maybe<jni::StringParam> jstrVal;
+            jstrVal.emplace(nullptr);
+            if (type == widget::PrefsHelper::PREF_STRING) {
+                jstrVal.reset();
+                jstrVal.emplace(strVal, aCls.Env());
+            }
 
             widget::PrefsHelper::CallPrefHandler(
-                    aPrefHandler, type, nameStr, boolVal, intVal, jstrVal);
+                    aPrefHandler, type, nameStr, boolVal, intVal, jstrVal.ref());
         }
 
         widget::PrefsHelper::CallPrefHandler(
@@ -314,11 +321,14 @@ public:
                 return;
         }
 
-        const auto& jstrVal = type == widget::PrefsHelper::PREF_STRING ?
-                jni::StringParam(strVal) :
-                jni::StringParam(nullptr);
+        Maybe<jni::StringParam> jstrVal;
+        jstrVal.emplace(nullptr);
+        if (type == widget::PrefsHelper::PREF_STRING) {
+            jstrVal.reset();
+            jstrVal.emplace(strVal);
+        }
 
-        widget::PrefsHelper::OnPrefChange(name, type, boolVal, intVal, jstrVal);
+        widget::PrefsHelper::OnPrefChange(name, type, boolVal, intVal, jstrVal.ref());
     }
 };