Improve diagnostic for hidden decls coming from non-modular headers

[bcardosolopes]

Bugzilla Link	32670
Version	trunk
OS	All
Attachments	testcase
CC	@bcardosolopes,@DougGregor,@zygoloid,@vgvassilev

Extended Description

The following code snippet leads to a very confusing error (also attached for easy reproduction):

MessageTypes.h:

#ifndef ROOT_MessageTypes
#define ROOT_MessageTypes

enum EMessageTypes {
kROOTD_OPEN = 2004, //filename follows + mode
};

#endif

module.modulemap:

module "libNet.so" {
requires cplusplus
module "TMessage.h" { header "TMessage.h" export * }
module "TNetFile.h" { header "TNetFile.h" export * }
export *
}

Output:

TNetFile.cxx:1:10: remark: building module 'libNet.so' as '/home/teemperor/Downloads/merge_enum/pcms/3MNDY7KURXTXS/libNet.so-141H35WZHCMDE.pcm' [-Rmodule-build]
#include "TNetFile.h"
^
TNetFile.cxx:1:10: remark: finished building module 'libNet.so' [-Rmodule-build]
TNetFile.cxx:2:10: error: declaration of 'kROOTD_OPEN' must be imported from module 'libNet.so.TMessage.h' before it is required
auto v = kROOTD_OPEN;
^
In module 'libNet.so' imported from TNetFile.cxx:1:
./MessageTypes.h:5:4: note: previous declaration is here
kROOTD_OPEN = 2004, //filename follows + mode
^
1 error generated.

test.sh:

#!/bin/bash

CLANG="/opt/clang/inst/bin/clang"

rm -rf ./pcms/

Normal clang invocation

#"$CLANG" -cc1 -triple x86_64-apple-macosx10.11.0 -fsyntax-only -I . -std=c++11 -x c++ TNetFile.cxx -nostdsysteminc

Modules clang invocation

"$CLANG" -cc1 -triple x86_64-apple-macosx10.11.0 -fsyntax-only -I . -std=c++11 -fmodules -fimplicit-module-maps -fmodules-cache-path=./pcms/ -fcolor-diagnostics -fdiagnostics-show-option -fdiagnostics-show-note-include-stack -x c++ TNetFile.cxx -nostdsysteminc -Rmodule-build

TMessage.h:

#ifndef ROOT_TMessage
#define ROOT_TMessage
#include "MessageTypes.h"
#endif

TNetFile.cxx:

#include "TNetFile.h"
auto v = kROOTD_OPEN;

TNetFile.h:

#ifndef ROOT_TNetFile
#define ROOT_TNetFile
#include "MessageTypes.h"
#endif

[bcardosolopes]

The issue here is that kROOTD_OPEN is hidden because it comes from a non-modular pulled into a module. Instead of the current error:

TNetFile.cpp:2:10: error: declaration of 'kROOTD_OPEN' must be imported from module 'libNet.so.TMessage.h' before it is required
auto v = kROOTD_OPEN;
^
In module 'libNet.so' imported from TNetFile.cpp:1:
Inputs/MessageTypes.h:5:3: note: previous declaration is here
kROOTD_OPEN = 2004, // filename follows + mode
^
1 error generated.

I think we could keep track of non-modular headers used in each module, so we can diagnose something along the lines of:

TNetFile.cpp:2:10: error: declaration of 'kROOTD_OPEN' must be imported from module 'libNet.so.TMessage.h' before it is required
auto v = kROOTD_OPEN;
^
In module 'libNet.so' imported from TNetFile.cpp:1:
TNetFile.cpp:2:10: note: 'TMessage.h' is a non-modular header, which hides 'kROOTD_OPEN'; provide a modular version of this header or directly include from it.
auto v = kROOTD_OPEN;
^
1 error generated.

What do you think? Suggestions?

[vgvassilev]

IIUC, libNet.so exports the include guards of MessageTypes.h and the subsequent request for 'textual' inclusion is a nop. As you said, it looks like the error comes from the fact the contents of the MessageTypes.h are unavailable because we included only TNetFile.h (and not TMessage.h) which contains TMessageTypes.h.

I think diagnosing this is definitely worthwhile if we cannot fix the leaking include guards of the 'non-activated' content from the module.