diff --git a/lld/COFF/InputFiles.cpp b/lld/COFF/InputFiles.cpp
--- a/lld/COFF/InputFiles.cpp
+++ b/lld/COFF/InputFiles.cpp
@@ -505,7 +505,13 @@
 
   assert(leader->data && "Comdat leader without SectionChunk?");
   leaderChunk = leader->getChunk();
-  leaderSelection = leaderChunk->selection;
+  if (leaderChunk->selection == IMAGE_COMDAT_SELECT_LTO) {
+    // If the leader is only a LTO symbol, we don't know e.g. its final size
+    // yet, so we can't do the full strict comdat selection checking yet.
+    selection = leaderSelection;
+  } else {
+    leaderSelection = leaderChunk->selection;
+  }
 
   if ((selection == IMAGE_COMDAT_SELECT_ANY &&
        leaderSelection == IMAGE_COMDAT_SELECT_LARGEST) ||
@@ -550,6 +556,7 @@
     break;
 
   case IMAGE_COMDAT_SELECT_ANY:
+  case IMAGE_COMDAT_SELECT_LTO:
     // Nothing to do.
     break;
 
@@ -558,8 +565,10 @@
       if (!config->mingw) {
         symtab->reportDuplicate(leader, this);
       } else {
-        const coff_aux_section_definition *leaderDef = findSectionDef(
-            leaderChunk->file->getCOFFObj(), leaderChunk->getSectionNumber());
+        const coff_aux_section_definition *leaderDef = nullptr;
+        if (leaderChunk->file)
+          leaderDef = findSectionDef(leaderChunk->file->getCOFFObj(),
+                                     leaderChunk->getSectionNumber());
         if (!leaderDef || leaderDef->Length != def->Length)
           symtab->reportDuplicate(leader, this);
       }
@@ -1050,9 +1059,12 @@
 class FakeSectionChunk {
 public:
   FakeSectionChunk(const coff_section *section) : chunk(nullptr, section) {
-    // FIXME: comdats from LTO files don't know their selection; treat them
-    // as "any".
-    chunk.selection = IMAGE_COMDAT_SELECT_ANY;
+    // Comdats from LTO files can't be fully treated as regular comdats
+    // yet; we don't e.g. know what size they are going to have, so we can't
+    // do proper checking against a regular comdat of type
+    // IMAGE_COMDAT_SELECT_SAME_SIZE. Therefore, while the symbol isn't yet
+    // LTO compiled, bail out from the nitpicky part of comdat resolution.
+    chunk.selection = IMAGE_COMDAT_SELECT_LTO;
   }
 
   SectionChunk chunk;
diff --git a/lld/test/COFF/lto-comdat-samesize.ll b/lld/test/COFF/lto-comdat-samesize.ll
new file mode 100644
--- /dev/null
+++ b/lld/test/COFF/lto-comdat-samesize.ll
@@ -0,0 +1,36 @@
+; REQUIRES: x86
+
+; RUN: split-file %s %t.dir
+; RUN: llvm-as %t.dir/other.ll -o %t.other.bc
+; RUN: llc -filetype=obj -o %t.main.obj %t.dir/main.ll
+
+; RUN: lld-link -out:%t.exe -subsystem:console %t.other.bc %t.main.obj
+
+#--- main.ll
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-windows-msvc19.14.0"
+
+$comdatData = comdat samesize
+
+@comdatData = weak_odr dso_local global i32 42, comdat
+
+define dso_local void @mainCRTStartup() {
+entry:
+  tail call void @other()
+  ret void
+}
+
+declare dso_local void @other()
+
+#--- other.ll
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-windows-msvc19.14.0"
+
+$comdatData = comdat samesize
+
+@comdatData = weak_odr dso_local global i32 42, comdat
+
+define dso_local void @other() {
+entry:
+  ret void
+}
diff --git a/llvm/include/llvm/BinaryFormat/COFF.h b/llvm/include/llvm/BinaryFormat/COFF.h
--- a/llvm/include/llvm/BinaryFormat/COFF.h
+++ b/llvm/include/llvm/BinaryFormat/COFF.h
@@ -410,7 +410,9 @@
   IMAGE_COMDAT_SELECT_EXACT_MATCH,
   IMAGE_COMDAT_SELECT_ASSOCIATIVE,
   IMAGE_COMDAT_SELECT_LARGEST,
-  IMAGE_COMDAT_SELECT_NEWEST
+  IMAGE_COMDAT_SELECT_NEWEST,
+  // Not a real value in files, but usable as dummy value within LLD.
+  IMAGE_COMDAT_SELECT_LTO = 0xff
 };
 
 // Auxiliary Symbol Formats