Teach AttachPreviousImpl to inherit MSInheritanceAttr attribute

This commit teaches ASTDeclReader::attachPreviousDecl to successfully merge
two Decl's when one contains an inheritable attribute like the
MSInheritanceAttr. Usually, attributes that are needed to be present along the
redeclaration chain are attached during ASTReading from
ASTDeclReader::attachPreviousDecl, but no such thing is done for inheritable
attributes. Currently, only the logic for merging MSInheritanceAttr is
provided.

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -281,6 +281,9 @@ namespace clang {
     static Decl *getMostRecentDeclImpl(...);
     static Decl *getMostRecentDecl(Decl *D);
 
+    static void mergeInheritableAttributes(ASTReader &Reader, Decl *D,
+                                           Decl *Previous);
+
     template <typename DeclT>
     static void attachPreviousDeclImpl(ASTReader &Reader,
                                        Redeclarable<DeclT> *D, Decl *Previous,
@@ -3511,6 +3514,19 @@ Decl *ASTReader::getMostRecentExistingDecl(Decl *D) {
   return ASTDeclReader::getMostRecentDecl(D->getCanonicalDecl());
 }
 
+void ASTDeclReader::mergeInheritableAttributes(ASTReader &Reader, Decl *D,
+                                               Decl *Previous) {
+  InheritableAttr *NewAttr = nullptr;
+  ASTContext &Context = Reader.getContext();
+  const auto *IA = Previous->getAttr<MSInheritanceAttr>();
+
+  if (IA && !D->hasAttr<MSInheritanceAttr>()) {
+    NewAttr = cast<InheritableAttr>(IA->clone(Context));
+    NewAttr->setInherited(true);
+    D->addAttr(NewAttr);
+  }
+}
+
 template<typename DeclT>
 void ASTDeclReader::attachPreviousDeclImpl(ASTReader &Reader,
                                            Redeclarable<DeclT> *D,
@@ -3669,6 +3685,12 @@ void ASTDeclReader::attachPreviousDecl(ASTReader &Reader, Decl *D,
   if (auto *TD = dyn_cast<TemplateDecl>(D))
     inheritDefaultTemplateArguments(Reader.getContext(),
                                     cast<TemplateDecl>(Previous), TD);
+
+  // If any of the declaration in the chain contains an Inheritable attribute,
+  // it needs to be added to all the declarations in the redeclarable chain.
+  // FIXME: Only the logic of merging MSInheritableAttr is present, it should
+  // be extended for all inheritable attributes.
+  mergeInheritableAttributes(Reader, D, Previous);
 }
 
 template<typename DeclT>

clang/test/Modules/Inputs/inherit-attribute/a.h

@@ -0,0 +1,10 @@
+#ifndef FOO
+#define FOO
+
+class Foo {
+public:
+  void step(int v);
+  Foo();
+};
+
+#endif

clang/test/Modules/Inputs/inherit-attribute/b.h

@@ -0,0 +1,5 @@
+#include "a.h"
+
+void bar() {
+  &Foo::step;
+}

clang/test/Modules/Inputs/inherit-attribute/c.h

@@ -0,0 +1 @@
+#include "a.h"

clang/test/Modules/Inputs/inherit-attribute/module.modulemap

@@ -0,0 +1,3 @@
+module "b" { header "b.h" }
+
+module "c" { header "c.h" }

clang/test/Modules/inherit-attribute.cpp

@@ -0,0 +1,20 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -triple x86_64-pc-windows-msvc-unknown -I%S\Inputs\inherit-attribute -fmodules-cache-path=%t \
+// RUN: -fimplicit-module-maps -fmodules-local-submodule-visibility %s -ast-dump-all \
+// RUN: | FileCheck %s
+
+#include "b.h"
+#include "c.h"
+
+class Foo;
+
+Foo f;
+
+// CHECK:   CXXRecordDecl {{.*}} imported in b {{.*}} Foo
+// CHECK:   MSInheritanceAttr {{[^()]*$}}
+
+// CHECK:   CXXRecordDecl {{.*}} prev {{.*}} imported in c {{.*}} Foo
+// CHECK:   MSInheritanceAttr {{.*}} Inherited {{[^()]*$}}
+
+// CHECK:   CXXRecordDecl {{.*}} <line:9:1, col:7> col:7 referenced class Foo
+// CHECK:   MSInheritanceAttr {{.*}} Inherited {{[^()]*$}}