Index: ELF/GdbIndex.cpp
===================================================================
--- ELF/GdbIndex.cpp
+++ ELF/GdbIndex.cpp
@@ -17,6 +17,7 @@
 #include "GdbIndex.h"
 #include "Memory.h"
 #include "llvm/DebugInfo/DWARF/DWARFDebugPubTable.h"
+#include "llvm/Object/Decompressor.h"
 #include "llvm/Object/ELFObjectFile.h"
 
 using namespace llvm;
@@ -33,6 +34,7 @@
                                  .Case(".debug_ranges", &RangeSection)
                                  .Case(".debug_line", &LineSection)
                                  .Default(nullptr)) {
+      Sec->maybeUncompress();
       M->Data = toStringRef(Sec->Data);
       M->Sec = Sec;
       continue;
Index: ELF/InputSection.h
===================================================================
--- ELF/InputSection.h
+++ ELF/InputSection.h
@@ -165,7 +165,7 @@
 
   InputSection *getLinkOrderDep() const;
 
-  void uncompress();
+  void maybeUncompress();
 
   // Returns a source location string. Used to construct an error message.
   template <class ELFT> std::string getLocation(uint64_t Offset);
Index: ELF/InputSection.cpp
===================================================================
--- ELF/InputSection.cpp
+++ ELF/InputSection.cpp
@@ -210,9 +210,15 @@
   return Sec ? Sec->getParent() : nullptr;
 }
 
-// Uncompress section contents. Note that this function is called
-// from parallelForEach, so it must be thread-safe.
-void InputSectionBase::uncompress() {
+// Uncompress section contents if required. Note that this function
+// is called from parallelForEach, so it must be thread-safe.
+void InputSectionBase::maybeUncompress() {
+  if (UncompressBuf)
+    return;
+
+  if (!Decompressor::isCompressedELFSection(Flags, Name))
+    return;
+
   Decompressor Dec = check(Decompressor::create(Name, toStringRef(Data),
                                                 Config->IsLE, Config->Is64));
 
Index: ELF/SyntheticSections.cpp
===================================================================
--- ELF/SyntheticSections.cpp
+++ ELF/SyntheticSections.cpp
@@ -2300,8 +2300,7 @@
   parallelForEach(InputSections, [](InputSectionBase *S) {
     if (!S->Live)
       return;
-    if (Decompressor::isCompressedELFSection(S->Flags, S->Name))
-      S->uncompress();
+    S->maybeUncompress();
     if (auto *MS = dyn_cast<MergeInputSection>(S))
       MS->splitIntoPieces();
   });
Index: test/ELF/compressed-debug-conflict.s
===================================================================
--- /dev/null
+++ test/ELF/compressed-debug-conflict.s
@@ -0,0 +1,29 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple i686-linux-gnu -compress-debug-sections=zlib %s -o %t.o
+# RUN: llvm-readobj -sections %t.o | FileCheck -check-prefix=OBJ %s
+# RUN: not ld.lld %t.o %t.o -o %tout 2>&1 | FileCheck -check-prefix=ERROR %s
+
+# OBJ:      Sections [
+# OBJ:        Section {
+# OBJ:          Index: 3
+# OBJ-NEXT:     Name: .debug_line (16)
+# OBJ-NEXT:     Type: SHT_PROGBITS (0x1)
+# OBJ-NEXT:     Flags [ (0x800)
+# OBJ-NEXT:       SHF_COMPRESSED (0x800)
+# OBJ-NEXT:     ]
+
+# ERROR:      error: duplicate symbol: main
+# ERROR-NEXT: >>> defined at reduced.c:2 (/tmp/reduced.c:2)
+# ERROR-NEXT: >>>
+# ERROR-NEXT: >>> defined at reduced.c:2 (/tmp/reduced.c:2)
+# ERROR-NEXT: >>>
+
+	.text
+	.file	"reduced.c"
+	.globl	main
+main:
+	.file	1 "/tmp" "reduced.c"
+	.loc	1 2 0
+	xorl	%eax, %eax
+	retl
+	.file	2 "/tmp/repeat/repeat/repeat/repeat" "repeat.h"