[LLVMbugs] [Bug 19061] New: "Protocol *" isn't properly loaded from ObjectiveC module

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=19061

            Bug ID: 19061
           Summary: "Protocol *" isn't properly loaded from ObjectiveC
                    module
           Product: clang
           Version: trunk
          Hardware: PC
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: Modules
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jordan_rose at apple.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Objective-C has a class named "Protocol" which represents the runtime metadata
about a Protocol. It is defined in <objc/Protocol.h>, which on OS X 10.9 is the
ObjectiveC.Protocol module.

The Protocol class is special because it can be generated by the @protocol
expression:

Protocol *codingProto = @protocol(NSCoding);
if ([obj conformsToProtocol:codingProto]) { /*...*/ }

The class is thus forward-declared in every translation unit as one of the
predefined types, roughly as "@class Protocol;".

With all of this set up, I can perform a lookup for "Protocol" and get back an
ObjCInterfaceDecl for this forward declaration. If I have imported the
ObjectiveC module and ask for the definition, I get it as expected. The trouble
comes when I import another module that #imports some (but not all) of the
headers from the ObjectiveC module. It is easiest to demonstrate with a fake
subclass of Protocol, something that requires a definition. (Please ignore the
fact that no one would actually do this; the real client is a tool built on
Clang.)

This program is correctly accepted by Clang:

@import ObjectiveC;
@interface Foo : Protocol
@end


This program is correctly rejected by Clang with a nice diagnostic:

@import Foundation;
@interface Foo : Protocol
@end

<stdin>:3:18: error: definition of 'Protocol' must be imported from module
      'ObjectiveC.Protocol' before it is required
@interface Foo : Protocol
                 ^

But this program gets the /wrong/ diagnostic:

@import AppKit;
@interface Foo : Protocol
@end

<stdin>:3:18: error: attempting to use the forward class 'Protocol' as
superclass
      of 'Foo'
@interface Foo : Protocol
~~~~~~~~~~~~~~   ^


After tracing through the code in ASTReader::updateOutOfDateIdentifier, it
looks like we search the AppKit module for the name "Protocol", find a match,
and stop the search there. ModuleManager::visit then notes that we found a
match, and therefore /shouldn't search anything visible via AppKit/.

I haven't gotten any further than that, but why does Foundation work, and what
can we do to fix this?

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