Index: COFF/InputFiles.h
===================================================================
--- COFF/InputFiles.h
+++ COFF/InputFiles.h
@@ -207,8 +207,8 @@
 
   static std::vector<ImportFile *> Instances;
 
-  DefinedImportData *ImpSym = nullptr;
-  DefinedImportThunk *ThunkSym = nullptr;
+  Symbol *ImpSym = nullptr;
+  Symbol *ThunkSym = nullptr;
   std::string DLLName;
 
 private:
Index: COFF/InputFiles.cpp
===================================================================
--- COFF/InputFiles.cpp
+++ COFF/InputFiles.cpp
@@ -431,7 +431,8 @@
   // address pointed by the __imp_ symbol. (This allows you to call
   // DLL functions just like regular non-DLL functions.)
   if (Hdr->getType() == llvm::COFF::IMPORT_CODE)
-    ThunkSym = Symtab->addImportThunk(Name, ImpSym, Hdr->Machine);
+    ThunkSym = Symtab->addImportThunk(
+        Name, cast_or_null<DefinedImportData>(ImpSym), Hdr->Machine);
 }
 
 void BitcodeFile::parse() {
Index: COFF/SymbolTable.h
===================================================================
--- COFF/SymbolTable.h
+++ COFF/SymbolTable.h
@@ -92,9 +92,9 @@
   Symbol *addCommon(InputFile *F, StringRef N, uint64_t Size,
                     const llvm::object::coff_symbol_generic *S = nullptr,
                     CommonChunk *C = nullptr);
-  DefinedImportData *addImportData(StringRef N, ImportFile *F);
-  DefinedImportThunk *addImportThunk(StringRef Name, DefinedImportData *S,
-                                     uint16_t Machine);
+  Symbol *addImportData(StringRef N, ImportFile *F);
+  Symbol *addImportThunk(StringRef Name, DefinedImportData *S,
+                         uint16_t Machine);
 
   void reportDuplicate(Symbol *Existing, InputFile *NewFile);
 
Index: COFF/SymbolTable.cpp
===================================================================
--- COFF/SymbolTable.cpp
+++ COFF/SymbolTable.cpp
@@ -342,30 +342,29 @@
   return S;
 }
 
-DefinedImportData *SymbolTable::addImportData(StringRef N, ImportFile *F) {
+Symbol *SymbolTable::addImportData(StringRef N, ImportFile *F) {
   Symbol *S;
   bool WasInserted;
   std::tie(S, WasInserted) = insert(N);
   S->IsUsedInRegularObj = true;
   if (WasInserted || isa<Undefined>(S) || isa<Lazy>(S)) {
     replaceSymbol<DefinedImportData>(S, N, F);
-    return cast<DefinedImportData>(S);
+    return S;
   }
 
   reportDuplicate(S, F);
   return nullptr;
 }
 
-DefinedImportThunk *SymbolTable::addImportThunk(StringRef Name,
-                                               DefinedImportData *ID,
-                                               uint16_t Machine) {
+Symbol *SymbolTable::addImportThunk(StringRef Name, DefinedImportData *ID,
+                                    uint16_t Machine) {
   Symbol *S;
   bool WasInserted;
   std::tie(S, WasInserted) = insert(Name);
   S->IsUsedInRegularObj = true;
   if (WasInserted || isa<Undefined>(S) || isa<Lazy>(S)) {
     replaceSymbol<DefinedImportThunk>(S, Name, ID, Machine);
-    return cast<DefinedImportThunk>(S);
+    return S;
   }
 
   reportDuplicate(S, ID->File);
Index: COFF/Writer.cpp
===================================================================
--- COFF/Writer.cpp
+++ COFF/Writer.cpp
@@ -544,17 +544,24 @@
     if (Config->DLLOrder.count(DLL) == 0)
       Config->DLLOrder[DLL] = Config->DLLOrder.size();
 
-    if (DefinedImportThunk *Thunk = File->ThunkSym)
+    if (File->ThunkSym) {
+      if (!isa<DefinedImportThunk>(File->ThunkSym))
+        fatal(toString(*File->ThunkSym) + " was replaced");
+      DefinedImportThunk *Thunk = cast<DefinedImportThunk>(File->ThunkSym);
       if (File->ThunkLive)
         TextSec->addChunk(Thunk->getChunk());
+    }
 
+    if (File->ImpSym && !isa<DefinedImportData>(File->ImpSym))
+      fatal(toString(*File->ImpSym) + " was replaced");
+    DefinedImportData *ImpSym = cast_or_null<DefinedImportData>(File->ImpSym);
     if (Config->DelayLoads.count(StringRef(File->DLLName).lower())) {
       if (!File->ThunkSym)
         fatal("cannot delay-load " + toString(File) +
-              " due to import of data: " + toString(*File->ImpSym));
-      DelayIdata.add(File->ImpSym);
+              " due to import of data: " + toString(*ImpSym));
+      DelayIdata.add(ImpSym);
     } else {
-      Idata.add(File->ImpSym);
+      Idata.add(ImpSym);
     }
   }
 
Index: test/COFF/Inputs/otherFunc.s
===================================================================
--- test/COFF/Inputs/otherFunc.s
+++ test/COFF/Inputs/otherFunc.s
@@ -0,0 +1,7 @@
+.global otherFunc
+.global MessageBoxA
+.text
+otherFunc:
+  ret
+MessageBoxA:
+  ret
Index: test/COFF/thunk-replace.s
===================================================================
--- test/COFF/thunk-replace.s
+++ test/COFF/thunk-replace.s
@@ -0,0 +1,15 @@
+# REQUIRES: x86
+
+# RUN: llvm-mc -triple=x86_64-win32 %s -filetype=obj -o %t.main.obj
+# RUN: llvm-mc -triple=x86_64-win32 %p/Inputs/otherFunc.s -filetype=obj -o %t.other.obj
+# RUN: llvm-ar rcs %t.other.lib %t.other.obj
+# RUN: not lld-link -out:%t.exe -entry:main %t.main.obj %p/Inputs/std64.lib %t.other.lib -opt:noref 2>&1 | FileCheck %s
+# CHECK: MessageBoxA was replaced
+
+.global main
+.text
+main:
+  callq MessageBoxA
+  callq ExitProcess
+  callq otherFunc
+  ret