Index: llvm/trunk/lib/Linker/IRMover.cpp
===================================================================
--- llvm/trunk/lib/Linker/IRMover.cpp
+++ llvm/trunk/lib/Linker/IRMover.cpp
@@ -241,18 +241,27 @@
   // These are types that LLVM itself will unique.
   bool IsUniqued = !isa<StructType>(Ty) || cast<StructType>(Ty)->isLiteral();
 
-#ifndef NDEBUG
   if (!IsUniqued) {
+    StructType *STy = cast<StructType>(Ty);
+    // This is actually a type from the destination module, this can be reached
+    // when this type is loaded in another module, added to DstStructTypesSet,
+    // and then we reach the same type in another module where it has not been
+    // added to MappedTypes. (PR37684)
+    if (STy->getContext().isODRUniquingDebugTypes() && !STy->isOpaque() &&
+        DstStructTypesSet.hasType(STy))
+      return *Entry = STy;
+
+#ifndef NDEBUG
     for (auto &Pair : MappedTypes) {
       assert(!(Pair.first != Ty && Pair.second == Ty) &&
              "mapping to a source type");
     }
-  }
 #endif
 
-  if (!IsUniqued && !Visited.insert(cast<StructType>(Ty)).second) {
-    StructType *DTy = StructType::create(Ty->getContext());
-    return *Entry = DTy;
+    if (!Visited.insert(STy).second) {
+      StructType *DTy = StructType::create(Ty->getContext());
+      return *Entry = DTy;
+    }
   }
 
   // If this is not a recursive type, then just map all of the elements and
Index: llvm/trunk/test/LTO/X86/Inputs/type-mapping-bug2.ll
===================================================================
--- llvm/trunk/test/LTO/X86/Inputs/type-mapping-bug2.ll
+++ llvm/trunk/test/LTO/X86/Inputs/type-mapping-bug2.ll
@@ -0,0 +1,16 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @c() !dbg !6 {
+  unreachable
+}
+
+!llvm.module.flags = !{!0, !1}
+!llvm.dbg.cu = !{!2}
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = !{i32 1, !"ThinLTO", i32 0}
+!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !3, retainedTypes: !4)
+!3 = !DIFile(filename: "f2", directory: "")
+!4 = !{!5}
+!5 = !DICompositeType(tag: DW_TAG_class_type, file: !3, flags: DIFlagFwdDecl, identifier: "SHARED")
+!6 = distinct !DISubprogram(unit: !2)
Index: llvm/trunk/test/LTO/X86/type-mapping-bug2.ll
===================================================================
--- llvm/trunk/test/LTO/X86/type-mapping-bug2.ll
+++ llvm/trunk/test/LTO/X86/type-mapping-bug2.ll
@@ -0,0 +1,42 @@
+; RUN: opt -module-summary -o %t0.o %S/Inputs/type-mapping-bug2.ll
+; RUN: opt -module-summary -o %t1.o %s
+; RUN: llvm-lto2 run -o %t2 %t0.o %t1.o -r %t0.o,c,px -r %t1.o,a,px -r %t1.o,b,px
+;
+; Test for the issue described in https://bugs.llvm.org/show_bug.cgi?id=37684
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; T1 will be linked against T2 because T2 was already loaded in %t0.o due to
+; the declaration for @b being imported due to !13
+%"T1" = type {}
+%"T2" = type {}
+
+define %"T1" @a() {
+  unreachable
+}
+
+define i1 @b(%"T2"*) {
+  unreachable
+}
+
+!llvm.module.flags = !{!0, !1}
+!llvm.dbg.cu = !{!2}
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = !{i32 1, !"ThinLTO", i32 0}
+!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !3, retainedTypes: !4)
+!3 = !DIFile(filename: "f1", directory: "")
+!4 = !{!5, !9}
+!5 = !DICompositeType(tag: DW_TAG_class_type, file: !3, templateParams: !6, scope: !8)
+!6 = !{!7}
+
+; The reference to @b and T2 that will be loaded in %t0.o
+
+!7 = !DITemplateValueParameter(value: i1 (%"T2"*)* @b)
+!8 = distinct !DISubprogram(unit: !2)
+
+; This DICompositeType is uniqued against !5 in Inputs/type-mapping-bug2.ll,
+; causing !7 and hence %T2 to be loaded into it's module
+
+!9 = !DICompositeType(tag: DW_TAG_array_type, identifier: "SHARED", scope: !8)
+