Allow /D flags absent during PCH creation under msvc-compat

Summary:
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.

msvc diagnostic:
  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.

Reviewers: rnk, thakis, hans, zturner

Subscribers: mikerice, aganea, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72405

clang/lib/Lex/PPDirectives.cpp

@@ -2727,7 +2727,9 @@ void Preprocessor::HandleDefineDirective(
                              /*Syntactic=*/LangOpts.MicrosoftExt))
       Diag(MI->getDefinitionLoc(), diag::warn_pp_macro_def_mismatch_with_pch)
           << MacroNameTok.getIdentifierInfo();
-    return;
+    // Issue the diagnostic but allow the change if msvc extensions are enabled
+    if (!LangOpts.MicrosoftExt)
+      return;
   }
 
   // Finally, if this identifier already had a macro defined for it, verify that

clang/test/PCH/ms-pch-macro.c

@@ -0,0 +1,39 @@
+// Test -D and -U interaction with a PCH when -fms-extensions is enabled.
+
+// RUN: %clang_cc1 -DFOO %S/variables.h -emit-pch -o %t1.pch
+
+// RUN: not %clang_cc1 -DFOO=blah -DBAR=int -include-pch %t1.pch -pch-through-header=%S/variables.h %s 2> %t.err
+// RUN: FileCheck -check-prefix=CHECK-FOO %s < %t.err
+
+// RUN: not %clang_cc1 -UFOO -DBAR=int -include-pch %t1.pch %s -pch-through-header=%S/variables.h 2> %t.err
+// RUN: FileCheck -check-prefix=CHECK-NOFOO %s < %t.err
+
+// RUN: %clang_cc1 -include-pch %t1.pch -DBAR=int -pch-through-header=%S/variables.h -verify %s 2> %t.err
+
+// Enabling MS extensions should allow us to add BAR definitions.
+// RUN: %clang_cc1 -fms-extensions -DFOO %S/variables.h -emit-pch -o %t1.pch
+// RUN: %clang_cc1 -fms-extensions -include-pch %t1.pch -DBAR=int -pch-through-header=%S/variables.h -verify %s 2> %t.err
+
+#include "variables.h"
+
+BAR bar = 17;
+#ifndef _MSC_EXTENSIONS
+// expected-error@-2 {{unknown type name 'BAR'}}
+#endif
+
+#ifndef FOO
+#  error FOO was not defined
+#endif
+
+#if FOO != 1
+#  error FOO has the wrong definition
+#endif
+
+#if defined(_MSC_EXTENSIONS) && !defined(BAR)
+#  error BAR was not defined
+#endif
+
+// CHECK-FOO: definition of macro 'FOO' differs between the precompiled header ('1') and the command line ('blah')
+// CHECK-NOFOO: macro 'FOO' was defined in the precompiled header but undef'd on the command line
+
+// expected-warning@1 {{definition of macro 'BAR' does not match definition in precompiled header}}