Bug 1291397. Work around clang bug that they didn't actually manage to ship a fix for which causes it to give spurious warnings it shouldn't be giving, which are then fatal due to -Werror. r=dholbert

MozReview-Commit-ID: GbMEvoxGpH1

dom/bindings/Codegen.py

@@ -4690,14 +4690,24 @@ def getJSToNativeConversionInfo(type, descriptorProvider, failureCode=None,
         # For JS-implemented APIs, we refuse to allow passing objects that the
         # API consumer does not subsume. The extra parens around
         # ($${passedToJSImpl}) suppress unreachable code warnings when
-        # $${passedToJSImpl} is the literal `false`.
+        # $${passedToJSImpl} is the literal `false`.  But Apple is shipping a
+        # buggy clang (clang 3.9) in Xcode 8.3, so there even the parens are not
+        # enough.  So we manually disable some warnings in clang.
         if not isinstance(descriptorProvider, Descriptor) or descriptorProvider.interface.isJSImplemented():
             templateBody = fill(
                 """
+                #ifdef __clang__
+                #pragma clang diagnostic push
+                #pragma clang diagnostic ignored "-Wunreachable-code"
+                #pragma clang diagnostic ignored "-Wunreachable-code-return"
+                #endif // __clang__
                 if (($${passedToJSImpl}) && !CallerSubsumes($${val})) {
                   ThrowErrorMessage(cx, MSG_PERMISSION_DENIED_TO_PASS_ARG, "${sourceDescription}");
                   $*{exceptionCode}
                 }
+                #ifdef __clang__
+                #pragma clang diagnostic pop
+                #endif // __clang__
                 """,
                 sourceDescription=sourceDescription,
                 exceptionCode=exceptionCode) + templateBody
@@ -5896,14 +5906,24 @@ def getJSToNativeConversionInfo(type, descriptorProvider, failureCode=None,
         # For JS-implemented APIs, we refuse to allow passing objects that the
         # API consumer does not subsume. The extra parens around
         # ($${passedToJSImpl}) suppress unreachable code warnings when
-        # $${passedToJSImpl} is the literal `false`.
+        # $${passedToJSImpl} is the literal `false`.  But Apple is shipping a
+        # buggy clang (clang 3.9) in Xcode 8.3, so there even the parens are not
+        # enough.  So we manually disable some warnings in clang.
         if not isinstance(descriptorProvider, Descriptor) or descriptorProvider.interface.isJSImplemented():
             templateBody = fill(
                 """
+                #ifdef __clang__
+                #pragma clang diagnostic push
+                #pragma clang diagnostic ignored "-Wunreachable-code"
+                #pragma clang diagnostic ignored "-Wunreachable-code-return"
+                #endif // __clang__
                 if (($${passedToJSImpl}) && !CallerSubsumes($${val})) {
                   ThrowErrorMessage(cx, MSG_PERMISSION_DENIED_TO_PASS_ARG, "${sourceDescription}");
                   $*{exceptionCode}
                 }
+                #ifdef __clang__
+                #pragma clang diagnostic pop
+                #endif // __clang__
                 """,
                 sourceDescription=sourceDescription,
                 exceptionCode=exceptionCode) + templateBody