Index: COFF/Driver.cpp
===================================================================
--- COFF/Driver.cpp
+++ COFF/Driver.cpp
@@ -594,10 +594,6 @@
     }
   }
 
-  // Make sure we have resolved all symbols.
-  if (Symtab.reportRemainingUndefines())
-    return false;
-
   // Do LTO by compiling bitcode input files to a native COFF file
   // then link that file.
   if (auto EC = Symtab.addCombinedLTOObject()) {
@@ -605,6 +601,10 @@
     return false;
   }
 
+  // Make sure we have resolved all symbols.
+  if (Symtab.reportRemainingUndefines(/*Resolve=*/true))
+    return false;
+
   // Windows specific -- if no /subsystem is given, we need to infer
   // that from entry point name.
   if (Config->Subsystem == IMAGE_SUBSYSTEM_UNKNOWN) {
Index: COFF/InputFiles.cpp
===================================================================
--- COFF/InputFiles.cpp
+++ COFF/InputFiles.cpp
@@ -293,8 +293,11 @@
     if (SymbolDef == LTO_SYMBOL_DEFINITION_UNDEFINED) {
       SymbolBodies.push_back(new (Alloc) Undefined(SymName));
     } else {
-      bool Replaceable = (SymbolDef == LTO_SYMBOL_DEFINITION_TENTATIVE ||
-                          (Attrs & LTO_SYMBOL_COMDAT));
+      bool Replaceable =
+          (SymbolDef == LTO_SYMBOL_DEFINITION_TENTATIVE || // common
+           (Attrs & LTO_SYMBOL_COMDAT) ||                  // comdat
+           (SymbolDef == LTO_SYMBOL_DEFINITION_WEAK &&     // weak external
+            (Attrs & LTO_SYMBOL_ALIAS)));
       SymbolBodies.push_back(new (Alloc) DefinedBitcode(this, SymName,
                                                         Replaceable));
     }
Index: COFF/SymbolTable.h
===================================================================
--- COFF/SymbolTable.h
+++ COFF/SymbolTable.h
@@ -46,8 +46,9 @@
   std::error_code run();
   bool queueEmpty();
 
-  // Print an error message on undefined symbols.
-  bool reportRemainingUndefines();
+  // Print an error message on undefined symbols. If Resolve is true, try to
+  // resolve any undefined symbols and update the symbol table accordingly.
+  bool reportRemainingUndefines(bool Resolve);
 
   // Returns a list of chunks of selected symbols.
   std::vector<Chunk *> getChunks();
Index: COFF/SymbolTable.cpp
===================================================================
--- COFF/SymbolTable.cpp
+++ COFF/SymbolTable.cpp
@@ -118,7 +118,7 @@
   return ArchiveQueue.empty() && ObjectQueue.empty();
 }
 
-bool SymbolTable::reportRemainingUndefines() {
+bool SymbolTable::reportRemainingUndefines(bool Resolve) {
   bool Ret = false;
   for (auto &I : Symtab) {
     Symbol *Sym = I.second;
@@ -126,20 +126,19 @@
     if (!Undef)
       continue;
     StringRef Name = Undef->getName();
-    // A weak alias may have been resolved, so check for that. A weak alias
-    // may be a weak alias to another symbol, so check recursively.
-    for (SymbolBody *A = Undef->WeakAlias; A;
-         A = cast<Undefined>(A)->WeakAlias) {
-      if (auto *D = dyn_cast<Defined>(A->repl())) {
+    // A weak alias may have been resolved, so check for that.
+    if (Defined *D = Undef->resolveWeakAlias()) {
+      if (Resolve)
         Sym->Body = D;
-        goto next;
-      }
+      continue;
     }
     // If we can resolve a symbol by removing __imp_ prefix, do that.
     // This odd rule is for compatibility with MSVC linker.
     if (Name.startswith("__imp_")) {
       Symbol *Imp = find(Name.substr(strlen("__imp_")));
       if (Imp && isa<Defined>(Imp->Body)) {
+        if (!Resolve)
+          continue;
         auto *D = cast<Defined>(Imp->Body);
         auto *S = new (Alloc) DefinedLocalImport(Name, D);
         LocalImportChunks.push_back(S->getChunk());
@@ -151,11 +150,11 @@
     // Remaining undefined symbols are not fatal if /force is specified.
     // They are replaced with dummy defined symbols.
     if (Config->Force) {
-      Sym->Body = new (Alloc) DefinedAbsolute(Name, 0);
+      if (Resolve)
+        Sym->Body = new (Alloc) DefinedAbsolute(Name, 0);
       continue;
     }
     Ret = true;
-    next:;
   }
   return Ret;
 }
@@ -295,6 +294,12 @@
   if (BitcodeFiles.empty())
     return std::error_code();
 
+  // Diagnose any undefined symbols early, but do not resolve weak externals,
+  // as resolution breaks the invariant that each Symbol points to a unique
+  // SymbolBody, which we rely on to replace DefinedBitcode symbols correctly.
+  if (reportRemainingUndefines(/*Resolve=*/false))
+    return make_error_code(LLDError::BrokenFile);
+
   // Create an object file and add it to the symbol table by replacing any
   // DefinedBitcode symbols with the definitions in the object file.
   LTOCodeGenerator CG;
@@ -306,21 +311,12 @@
   for (SymbolBody *Body : Obj->getSymbols()) {
     if (!Body->isExternal())
       continue;
-    // Find an existing Symbol. We should not see any new undefined symbols at
-    // this point.
+    // We should not see any new undefined symbols at this point, but we'll
+    // diagnose them later in reportRemainingUndefines().
     StringRef Name = Body->getName();
     Symbol *Sym = insert(Body);
-    if (Sym->Body == Body && !isa<Defined>(Body)) {
-      llvm::errs() << "LTO: undefined symbol: " << Name << '\n';
-      return make_error_code(LLDError::BrokenFile);
-    }
 
     if (isa<DefinedBitcode>(Sym->Body)) {
-      // The symbol should now be defined.
-      if (!isa<Defined>(Body)) {
-        llvm::errs() << "LTO: undefined symbol: " << Name << '\n';
-        return make_error_code(LLDError::BrokenFile);
-      }
       Sym->Body = Body;
       continue;
     }
@@ -349,9 +345,6 @@
     return make_error_code(LLDError::BrokenFile);
   }
 
-  // New runtime library symbol references may have created undefined references.
-  if (reportRemainingUndefines())
-    return make_error_code(LLDError::BrokenFile);
   return std::error_code();
 }
 
@@ -371,9 +364,12 @@
         CG->addMustPreserveSymbol(Body->getName());
 
   // Likewise for other symbols that must be preserved.
-  for (Undefined *U : Config->GCRoot)
-    if (isa<DefinedBitcode>(U->repl()))
-      CG->addMustPreserveSymbol(U->getName());
+  for (Undefined *U : Config->GCRoot) {
+    if (auto *S = dyn_cast<DefinedBitcode>(U->repl()))
+      CG->addMustPreserveSymbol(S->getName());
+    else if (auto *S = dyn_cast_or_null<DefinedBitcode>(U->resolveWeakAlias()))
+      CG->addMustPreserveSymbol(S->getName());
+  }
 
   CG->setModule(BitcodeFiles[0]->releaseModule());
   for (unsigned I = 1, E = BitcodeFiles.size(); I != E; ++I)
Index: COFF/Symbols.h
===================================================================
--- COFF/Symbols.h
+++ COFF/Symbols.h
@@ -254,6 +254,11 @@
   // If it remains undefined, it'll be replaced with whatever the
   // Alias pointer points to.
   SymbolBody *WeakAlias = nullptr;
+
+  // If this symbol is external weak, try to resolve it to a defined
+  // symbol by searching the chain of fallback symbols. Returns the symbol if
+  // successful, otherwise returns null.
+  Defined *resolveWeakAlias();
 };
 
 // Windows-specific classes.
Index: COFF/Symbols.cpp
===================================================================
--- COFF/Symbols.cpp
+++ COFF/Symbols.cpp
@@ -236,5 +236,14 @@
   return std::move(Obj);
 }
 
+Defined *Undefined::resolveWeakAlias() {
+  // A weak alias may be a weak alias to another symbol, so check recursively.
+  for (SymbolBody *A = WeakAlias; A; A = cast<Undefined>(A)->WeakAlias) {
+    if (auto *D = dyn_cast<Defined>(A->repl()))
+      return D;
+  }
+  return nullptr;
+}
+
 } // namespace coff
 } // namespace lld
Index: test/COFF/Inputs/entry-mangled.ll
===================================================================
--- /dev/null
+++ test/COFF/Inputs/entry-mangled.ll
@@ -0,0 +1,6 @@
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc18.0.0"
+
+define void @"\01?main@@YAHXZ"() {
+  ret void
+}
Index: test/COFF/Inputs/weak-external.ll
===================================================================
--- /dev/null
+++ test/COFF/Inputs/weak-external.ll
@@ -0,0 +1,6 @@
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+define void @g() {
+  ret void
+}
Index: test/COFF/Inputs/weak-external2.ll
===================================================================
--- /dev/null
+++ test/COFF/Inputs/weak-external2.ll
@@ -0,0 +1,6 @@
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+define void @f() {
+  ret void
+}
Index: test/COFF/Inputs/weak-external3.ll
===================================================================
--- /dev/null
+++ test/COFF/Inputs/weak-external3.ll
@@ -0,0 +1,8 @@
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+@f = weak alias void()* @g
+
+define void @g() {
+  ret void
+}
Index: test/COFF/entry-mangled.test
===================================================================
--- test/COFF/entry-mangled.test
+++ test/COFF/entry-mangled.test
@@ -1,5 +1,7 @@
 # RUN: yaml2obj < %s > %t.obj
 # RUN: lld -flavor link2 /out:%t.exe /entry:main %t.obj
+# RUN: llvm-as -o %t.lto.obj %S/Inputs/entry-mangled.ll
+# RUN: lld -flavor link2 /out:%t.exe /entry:main %t.lto.obj
 
 ---
 header:
Index: test/COFF/weak-external.test
===================================================================
--- test/COFF/weak-external.test
+++ test/COFF/weak-external.test
@@ -1,5 +1,12 @@
 # RUN: yaml2obj %s > %t.obj
-# RUN: lld -flavor link2 /out:%t.exe /entry:g /subsystem:console %t.obj
+# RUN: llvm-as -o %t.lto.obj %S/Inputs/weak-external.ll
+# RUN: lld -flavor link2 /out:%t1.exe /entry:g /subsystem:console %t.obj
+# RUN: lld -flavor link2 /out:%t2.exe /entry:g /subsystem:console /lldmap:%t2.map %t.obj %t.lto.obj
+# RUN: FileCheck %s < %t2.map
+
+# CHECK: lto-llvm{{.*}}:
+# CHECK-NOT: :
+# CHECK: {{ g$}}
 
 ---
 header:          
Index: test/COFF/weak-external2.test
===================================================================
--- test/COFF/weak-external2.test
+++ test/COFF/weak-external2.test
@@ -1,5 +1,6 @@
 # RUN: yaml2obj %s > %t.obj
-# RUN: lld -flavor link2 /out:%t.exe /entry:g /subsystem:console %t.obj
+# RUN: llvm-as -o %t.lto.obj %S/Inputs/weak-external2.ll
+# RUN: lld -flavor link2 /out:%t.exe /entry:g /subsystem:console %t.obj %t.lto.obj
 
 ---
 header:          
@@ -13,7 +14,7 @@
 symbols:         
   - Name:            'f'
     Value:           0
-    SectionNumber:   1
+    SectionNumber:   0
     SimpleType:      IMAGE_SYM_TYPE_NULL
     ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
     StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
Index: test/COFF/weak-external3.test
===================================================================
--- test/COFF/weak-external3.test
+++ test/COFF/weak-external3.test
@@ -1,5 +1,17 @@
 # RUN: yaml2obj %s > %t.obj
-# RUN: lld -flavor link2 /out:%t.exe /entry:g /subsystem:console %t.obj
+# RUN: llvm-as -o %t.lto.obj %S/Inputs/weak-external3.ll
+# RUN: lld -flavor link2 /out:%t1.exe /entry:f /subsystem:console /lldmap:%t1.map %t.lto.obj
+# RUN: FileCheck --check-prefix=CHECK1 %s < %t1.map
+# RUN: lld -flavor link2 /out:%t2.exe /entry:f /subsystem:console /lldmap:%t2.map %t.obj %t.lto.obj
+# RUN: FileCheck --check-prefix=CHECK2 %s < %t2.map
+
+# CHECK1: lto-llvm{{.*}}:
+# CHECK1-NOT: :
+# CHECK1: {{ g$}}
+
+# CHECK2: weak-external3{{.*}}:
+# CHECK2-NOT: :
+# CHECK2: {{ f$}}
 
 ---
 header:          
@@ -17,13 +29,4 @@
     SimpleType:      IMAGE_SYM_TYPE_NULL
     ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
     StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
-  - Name:            'g'
-    Value:           0
-    SectionNumber:   0
-    SimpleType:      IMAGE_SYM_TYPE_NULL
-    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
-    StorageClass:    IMAGE_SYM_CLASS_WEAK_EXTERNAL
-    WeakExternal:    
-      TagIndex:        0
-      Characteristics: IMAGE_WEAK_EXTERN_SEARCH_LIBRARY
 ...