Index: ELF/Symbols.h
===================================================================
--- ELF/Symbols.h
+++ ELF/Symbols.h
@@ -253,18 +253,22 @@
 // Undefined symbol.
 class Undefined : public SymbolBody {
   typedef SymbolBody::Kind Kind;
-  bool CanKeepUndefined;
+  bool Optional;
+  // If it was ever seen in live sections.
+  bool Live = false;
 
 protected:
   Undefined(Kind K, StringRef N, bool IsWeak, uint8_t Visibility, bool IsTls);
 
 public:
-  Undefined(StringRef N, bool IsWeak, uint8_t Visibility,
-            bool CanKeepUndefined);
+  Undefined(StringRef N, bool IsWeak, uint8_t Visibility, bool Opt);
 
   static bool classof(const SymbolBody *S) { return S->isUndefined(); }
 
-  bool canKeepUndefined() const { return CanKeepUndefined; }
+  bool canKeepUndefined() const {
+    return Optional || (Config->GcSections && !Live);
+  }
+  void setLive() { Live = true; }
 };
 
 template <class ELFT> class UndefinedElf : public Undefined {
Index: ELF/Symbols.cpp
===================================================================
--- ELF/Symbols.cpp
+++ ELF/Symbols.cpp
@@ -171,13 +171,12 @@
 Undefined::Undefined(SymbolBody::Kind K, StringRef N, bool IsWeak,
                      uint8_t Visibility, bool IsTls)
     : SymbolBody(K, N, IsWeak, Visibility, IsTls, /*IsFunction*/ false),
-      CanKeepUndefined(false) {}
+      Optional(false) {}
 
-Undefined::Undefined(StringRef N, bool IsWeak, uint8_t Visibility,
-                     bool CanKeepUndefined)
+Undefined::Undefined(StringRef N, bool IsWeak, uint8_t Visibility, bool Opt)
     : Undefined(SymbolBody::UndefinedKind, N, IsWeak, Visibility,
                 /*IsTls*/ false) {
-  this->CanKeepUndefined = CanKeepUndefined;
+  this->Optional = Opt;
 }
 
 template <typename ELFT>
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -292,6 +292,17 @@
     SymbolBody *Body = File.getSymbolBody(SymIndex);
     uint32_t Type = RI.getType(Config->Mips64EL);
 
+    auto Name = C.getSectionName();
+    if (Body && Body->isUndefined()) {
+      if (auto *U = dyn_cast<Undefined>(Body->repl()))
+        U->setLive();
+
+      // Set "used" bit for --as-needed.
+      if (!Body->isWeak())
+        if (auto *S = dyn_cast<SharedSymbol<ELFT>>(Body->repl()))
+          S->File->IsUsed = true;
+    }
+
     // Ignore "hint" relocation because it is for optional code optimization.
     if (Target->isHintRel(Type))
       continue;
@@ -299,11 +310,6 @@
     if (Target->isGotRelative(Type))
       HasGotOffRel = true;
 
-    // Set "used" bit for --as-needed.
-    if (Body && Body->isUndefined() && !Body->isWeak())
-      if (auto *S = dyn_cast<SharedSymbol<ELFT>>(Body->repl()))
-        S->File->IsUsed = true;
-
     if (Body)
       Body = Body->repl();
 
Index: test/ELF/gc-sections-unref-undef.s
===================================================================
--- test/ELF/gc-sections-unref-undef.s
+++ test/ELF/gc-sections-unref-undef.s
@@ -0,0 +1,26 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld %t -o %t.out -e main --gc-sections
+# RUN: llvm-readobj %t.out > /dev/null
+
+## Don't allow undefines if GC is not enabled
+# RUN: not ld.lld %t -o %t.out -e main 2>&1 | FileCheck %s
+# CHECK: undefined symbol: _UNDEFINED
+
+.text
+.section .text._unreferenced_function,"ax",@progbits
+.globl _unreferenced_function
+.type  _unreferenced_function,@function
+_unreferenced_function:
+ callq _UNDEFINED
+
+.section .text._function,"ax",@progbits
+.globl _function
+.type  _function,@function
+_function:
+
+.section .text.main,"ax",@progbits
+.globl main
+.type  main,@function
+main:  
+ callq _function
Index: test/ELF/gc-sections-unref-undef2.s
===================================================================
--- test/ELF/gc-sections-unref-undef2.s
+++ test/ELF/gc-sections-unref-undef2.s
@@ -0,0 +1,26 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: not ld.lld %t -o %t.out -e main --gc-sections 2>&1 | FileCheck %s
+# CHECK: undefined symbol: _UNDEFINED
+
+## This checks that if there are section with unreferenced
+## undefined and the section where it is used, then
+## we still can see the error.
+.text
+.section .text._unreferenced_function,"ax",@progbits
+.globl _unreferenced_function
+.type  _unreferenced_function,@function
+_unreferenced_function:
+ callq _UNDEFINED
+
+.section .text._function,"ax",@progbits
+.globl _function
+.type  _function,@function
+_function:
+ callq _UNDEFINED
+
+.section .text.main,"ax",@progbits
+.globl main
+.type  main,@function
+main:  
+ callq _function