Index: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -144,6 +144,17 @@
   New->takeName(&AI);
   New->setUsedWithInAlloca(AI.isUsedWithInAlloca());
 
+  // Replace all metadata uses of AI with New. Most metadata uses don't care
+  // about the allocated type, and tmpcast created below is likely to dissappear
+  // soon.
+  if (ValueAsMetadata *MDUse = ValueAsMetadata::getIfExists(&AI)) {
+    LLVMContext &Ctx = New->getContext();
+    if (MetadataAsValue *MDVal = MetadataAsValue::getIfExists(Ctx, MDUse)) {
+      MDVal->replaceAllUsesWith(
+          MetadataAsValue::get(Ctx, ValueAsMetadata::get(New)));
+    }
+  }
+
   // If the allocation has multiple real uses, insert a cast and change all
   // things that used it to use the new cast.  This will also hack on CI, but it
   // will die soon.
Index: llvm/test/Transforms/InstCombine/alloca-cast-debuginfo.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/alloca-cast-debuginfo.ll
@@ -0,0 +1,87 @@
+; RUN: opt < %s -S -instcombine -instcombine-lower-dbg-declare=0 | FileCheck %s
+
+; In this example, instcombine wants to turn "local" into an i64, since that's
+; how it is stored. It should keep the debug info referring to the alloca when
+; it does the replacement.
+
+; C source:
+; struct Foo {
+;   int x, y;
+; };
+; void escape(const void*);
+; void f(struct Foo *p) {
+;   struct Foo local;
+;   *(__int64 *)&local = *(__int64 *)p;
+;   escape(&local);
+; }
+
+; ModuleID = '<stdin>'
+source_filename = "t.c"
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc19.11.25508"
+
+%struct.Foo = type { i32, i32 }
+
+define void @f(%struct.Foo* %p) !dbg !11 {
+entry:
+  %local = alloca %struct.Foo, align 4
+  %0 = bitcast %struct.Foo* %local to i8*, !dbg !24
+  call void @llvm.dbg.declare(metadata %struct.Foo* %local, metadata !22, metadata !DIExpression()), !dbg !25
+  %1 = bitcast %struct.Foo* %p to i64*, !dbg !26
+  %2 = load i64, i64* %1, align 8, !dbg !26, !tbaa !27
+  %3 = bitcast %struct.Foo* %local to i64*, !dbg !31
+  store i64 %2, i64* %3, align 4, !dbg !32, !tbaa !27
+  %4 = bitcast %struct.Foo* %local to i8*, !dbg !33
+  call void @escape(i8* %4), !dbg !34
+  %5 = bitcast %struct.Foo* %local to i8*, !dbg !35
+  ret void, !dbg !35
+}
+
+; CHECK-LABEL: define void @f(%struct.Foo* %p)
+; CHECK: %local = alloca i64, align 8
+; CHECK: call void @llvm.dbg.declare(metadata i64* %local, metadata !22, metadata !DIExpression())
+
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+declare void @escape(i8*)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!6, !7, !8, !9}
+!llvm.ident = !{!10}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 6.0.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3)
+!1 = !DIFile(filename: "t.c", directory: "C:\5Csrc\5Cllvm-project\5Cbuild", checksumkind: CSK_MD5, checksum: "d7473625866433067a75fd7d03d2abf7")
+!2 = !{}
+!3 = !{!4}
+!4 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !5, size: 64)
+!5 = !DIBasicType(name: "long long int", size: 64, encoding: DW_ATE_signed)
+!6 = !{i32 2, !"CodeView", i32 1}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{i32 1, !"wchar_size", i32 2}
+!9 = !{i32 7, !"PIC Level", i32 2}
+!10 = !{!"clang version 6.0.0 "}
+!11 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 5, type: !12, isLocal: false, isDefinition: true, scopeLine: 5, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !20)
+!12 = !DISubroutineType(types: !13)
+!13 = !{null, !14}
+!14 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !15, size: 64)
+!15 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "Foo", file: !1, line: 1, size: 64, elements: !16)
+!16 = !{!17, !19}
+!17 = !DIDerivedType(tag: DW_TAG_member, name: "x", scope: !15, file: !1, line: 2, baseType: !18, size: 32)
+!18 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!19 = !DIDerivedType(tag: DW_TAG_member, name: "y", scope: !15, file: !1, line: 2, baseType: !18, size: 32, offset: 32)
+!20 = !{!21, !22}
+!21 = !DILocalVariable(name: "p", arg: 1, scope: !11, file: !1, line: 5, type: !14)
+!22 = !DILocalVariable(name: "local", scope: !11, file: !1, line: 6, type: !15)
+!23 = !DILocation(line: 5, column: 20, scope: !11)
+!24 = !DILocation(line: 6, column: 3, scope: !11)
+!25 = !DILocation(line: 6, column: 14, scope: !11)
+!26 = !DILocation(line: 7, column: 24, scope: !11)
+!27 = !{!28, !28, i64 0}
+!28 = !{!"long long", !29, i64 0}
+!29 = !{!"omnipotent char", !30, i64 0}
+!30 = !{!"Simple C/C++ TBAA"}
+!31 = !DILocation(line: 7, column: 3, scope: !11)
+!32 = !DILocation(line: 7, column: 22, scope: !11)
+!33 = !DILocation(line: 8, column: 10, scope: !11)
+!34 = !DILocation(line: 8, column: 3, scope: !11)
+!35 = !DILocation(line: 9, column: 1, scope: !11)