Index: lib/Transforms/IPO/FunctionImport.cpp
===================================================================
--- lib/Transforms/IPO/FunctionImport.cpp
+++ lib/Transforms/IPO/FunctionImport.cpp
@@ -543,7 +543,14 @@
         // recorded in the index using the original name.
         // FIXME: This may not be needed once PR27866 is fixed.
         const auto &GS = DefinedGlobals.find(GlobalValue::getGUID(OrigName));
-        assert(GS != DefinedGlobals.end());
+        // If we don't find the original name, then conservatively return
+        // true, indicating that it must be preserved. One example where
+        // we might not find it is when the linked in local copy for an alias
+        // conflicts with the declaration created for the aliased preempted
+        // weak value, and the ValueSymbolTable appends a unique identifier
+        // to distinguish them.
+        if (GS == DefinedGlobals.end())
+          return true;
         Linkage = GS->second->linkage();
       } else {
         Linkage = GS->second->linkage();
Index: test/tools/gold/X86/Inputs/thinlto_alias2.ll
===================================================================
--- /dev/null
+++ test/tools/gold/X86/Inputs/thinlto_alias2.ll
@@ -0,0 +1,8 @@
+$c2 = comdat any
+
+define weak_odr i32 @f1(i8*) unnamed_addr comdat($c2) {
+  ret i32 41
+}
+
+@r2 = global i32(i8*)* @f1
+@a2 = alias i32(i8*), i32(i8*)* @f1
Index: test/tools/gold/X86/thinlto_alias2.ll
===================================================================
--- /dev/null
+++ test/tools/gold/X86/thinlto_alias2.ll
@@ -0,0 +1,22 @@
+; RUN: opt -module-summary %s -o %t.o
+; RUN: opt -module-summary %p/Inputs/thinlto_alias2.ll -o %t2.o
+
+; RUN: %gold -m elf_x86_64 -plugin %llvmshlibdir/LLVMgold.so \
+; RUN:     -shared \
+; RUN:     --plugin-opt=thinlto \
+; RUN:     --plugin-opt=-import-instr-limit=0 \
+; RUN:     --plugin-opt=save-temps \
+; RUN:     -o %t3.o %t.o %t2.o
+; RUN: llvm-dis %t2.o.opt.bc -o - | FileCheck %s
+
+$c1 = comdat any
+
+define weak_odr i32 @f1(i8*) unnamed_addr comdat($c1) {
+  ret i32 42
+}
+
+@a1 = alias i32 (i8*), i32 (i8*)* @f1
+
+; CHECK: @a2 = alias i32 (i8*), i32 (i8*)* @f1.1.llvm.0
+; CHECK: declare i32 @f1(i8*) unnamed_addr
+; CHECK: define hidden i32 @f1.1.llvm.0(i8* nocapture readnone) unnamed_addr #0 comdat($c2) {