[modules] Fix Decl's Used invariant.

The Decl::isUsed has a value for every decl. In non-module builds it is very
difficult (but possible) to break this invariant but when we walk up the redecl
chain we find the neccessary information.

When deserializing the decls from a module it is much more difficult to update
correctly this invariant. The patch centralizes the information whether a decl
is used in the canonical decl marking the entire entity as being used.

Fixes https://llvm.org/bugs/show_bug.cgi?id=27401

Patch by Cristina Cristescu and me.

Thanks to Richard Smith who helped to debug and understand the issue!

Reviewed by Richard Smith.

llvm-svn: 267691

1 files changed, 1 insertions, 11 deletions

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 0f47e9e0994..065c236d6cc 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -13011,17 +13011,7 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
       UndefinedButUsed.insert(std::make_pair(Func->getCanonicalDecl(), Loc));
   }
 
-  // Normally the most current decl is marked used while processing the use and
-  // any subsequent decls are marked used by decl merging. This fails with
-  // template instantiation since marking can happen at the end of the file
-  // and, because of the two phase lookup, this function is called with at
-  // decl in the middle of a decl chain. We loop to maintain the invariant
-  // that once a decl is used, all decls after it are also used.
-  for (FunctionDecl *F = Func->getMostRecentDecl();; F = F->getPreviousDecl()) {
-    F->markUsed(Context);
-    if (F == Func)
-      break;
-  }
+  Func->markUsed(Context);
 }
 
 static void