"""
[ObjC] Error out when using forward-declared protocol in a @protocol
expression

Clang emits invalid protocol metadata when a @protocol expression is used with a
forward-declared protocol. The protocol metadata is missing protocol conformance
list of the protocol since we don't have access to the definition of it in the
compiled translation unit. The linker then might end up picking the invalid
metadata when linking which will lead to incorrect runtime protocol conformance
checks.

This commit makes sure that Clang fails to compile code that uses a @protocol
expression with a forward-declared protocol. This ensures that Clang does not
emit invalid protocol metadata. I added an extra assert in CodeGen to ensure
that this kind of issue won't happen in other places.
"""


Fires on Chromium at least here:

https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.clang%2FToTiOS%2F2025%2F%2B%2Frecipes%2Fsteps%2Fcompile%2F0%2Fstdout


FAILED: obj/ios/chrome/browser/ssl/unit_tests/ios_ssl_error_handler_unittest.o 
../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF obj/ios/chrome/browser/ssl/unit_tests/ios_ssl_error_handler_unittest.o.d -DV8_DEPRECATION_WARNINGS -DNO_TCMALLOC -DCHROMIUM_BUILD -DFIELDTRIAL_TESTING_ENABLED -D_LIBCPP_HIDE_FROM_ABI=_LIBCPP_HIDDEN\ _LIBCPP_ALWAYS_INLINE -DCR_XCODE_VERSION=1000 -DCR_CLANG_REVISION=\"340123\" -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D_FORTIFY_SOURCE=2 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DNS_BLOCK_ASSERTIONS=1 -DWEBP_EXTERN=extern -DEXPAT_RELATIVE_PATH -DGTEST_API_= -DGTEST_HAS_POSIX_RE=0 -DGTEST_LANG_CXX11=1 -DGTEST_HAS_TR1_TUPLE=0 -DU_USING_ICU_NAMESPACE=0 -DU_ENABLE_DYLOAD=0 -DU_STATIC_IMPLEMENTATION -DICU_UTIL_DATA_IMPL=ICU_UTIL_DATA_FILE -DUCHAR_TYPE=uint16_t -DGOOGLE_PROTOBUF_NO_RTTI -DGOOGLE_PROTOBUF_NO_STATIC_INITIALIZER -DHAVE_PTHREAD -DWEBRTC_NON_STATIC_TRACE_EVENT_HANDLERS=0 -DWEBRTC_CHROMIUM_BUILD -DWEBRTC_POSIX -DWEBRTC_MAC -DWEBRTC_IOS -DABSL_ALLOCATOR_NOTHROW=1 -DNO_MAIN_THREAD_WRAPPING -DUSE_REMOTE_TEST_SERVER -DUNIT_TEST -I../.. -Igen -I../../third_party/libyuv/include -I../../third_party/libwebp/src -I../../third_party/webrtc_overrides -I../../testing/gtest/include -I../../third_party/libyuv/include -I../../third_party/usrsctp/usrsctplib -I../../third_party/webrtc -I../../third_party/googletest/custom -I../../third_party/googletest/src/googletest/include -I../../third_party/ced/src -I../../third_party/icu/source/common -I../../third_party/icu/source/i18n -I../../third_party/protobuf/src -Igen/protoc_out -I../../third_party/protobuf/src -I../../third_party/boringssl/src/include -I../../third_party/webrtc_overrides -I../../third_party/webrtc -I../../third_party/abseil-cpp -I../../third_party/googletest/custom -I../../third_party/googletest/src/googlemock/include -I../../third_party/ocmock -fno-strict-aliasing --param=ssp-buffer-size=4 -fstack-protector -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -fcolor-diagnostics -fmerge-all-constants -no-canonical-prefixes -arch armv7 -Wall -Werror -Wextra -Wimplicit-fallthrough -Wthread-safety -Wunguarded-availability -Wundeclared-selector -Wno-missing-field-initializers -Wno-unused-parameter -Wno-c++11-narrowing -Wno-covered-switch-default -Wno-unneeded-internal-declaration -Wno-undefined-var-template -Wno-nonportable-include-path -Wno-user-defined-warnings -Wno-unused-lambda-capture -Wno-null-pointer-arithmetic -Wno-enum-compare-switch -Wno-ignored-pragma-optimize -Oz -fno-omit-frame-pointer -gdwarf-2 -isysroot /b/c/xcode_ios_10l221o.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS12.0.sdk -stdlib=libc++ -miphoneos-version-min=10.0 -fvisibility=hidden -Xclang -load -Xclang ../../third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.dylib -Xclang -add-plugin -Xclang find-bad-constructs -Xclang -plugin-arg-find-bad-constructs -Xclang enforce-in-thirdparty-webkit -Xclang -plugin-arg-find-bad-constructs -Xclang check-enum-max-value -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -Wno-inconsistent-missing-override -std=c++14 -fobjc-call-cxx-cdtors -Wobjc-missing-property-synthesis -fno-exceptions -fno-rtti -fvisibility-inlines-hidden -fobjc-arc -c ../../ios/chrome/browser/ssl/ios_ssl_error_handler_unittest.mm -o obj/ios/chrome/browser/ssl/unit_tests/ios_ssl_error_handler_unittest.o
../../ios/chrome/browser/ssl/ios_ssl_error_handler_unittest.mm:50:26: error: @protocol is using a forward protocol declaration of 'CaptivePortalDetectorTabHelperDelegate'
        mockForProtocol:@protocol(CaptivePortalDetectorTabHelperDelegate)];
                         ^
../../ios/chrome/browser/ssl/captive_portal_detector_tab_helper.h:10:11: note: 'CaptivePortalDetectorTabHelperDelegate' declared here
@protocol CaptivePortalDetectorTabHelperDelegate;
          ^