Before this patch adding a new /D flag when compiling a source file that consumed a PCH with clang-cl would issue a diagnostic and then fail. With the patch, the diagnostic is still issued but the definition is accepted. This matches the msvc behavior. The fuzzy-pch-msvc.c is a clone of the existing fuzzy-pch.c tests with some msvc specific rework.
warning C4605: '/DBAR=int' specified on current command line, but was not specified when precompiled header was built
Output of the CHECK-BAR test prior to the code change:
<built-in>(1,9): warning: definition of macro 'BAR' does not match definition in precompiled header [-Wclang-cl-pch] #define BAR int ^ D:\repos\llvm\llvm-project\clang\test\PCH\fuzzy-pch-msvc.c(12,1): error: unknown type name 'BAR' BAR bar = 17; ^ D:\repos\llvm\llvm-project\clang\test\PCH\fuzzy-pch-msvc.c(23,4): error: BAR was not defined # error BAR was not defined ^ 1 warning and 2 errors generated.