diff --git a/llvm/include/llvm/AsmParser/LLParser.h b/llvm/include/llvm/AsmParser/LLParser.h
--- a/llvm/include/llvm/AsmParser/LLParser.h
+++ b/llvm/include/llvm/AsmParser/LLParser.h
@@ -179,8 +179,10 @@
           Lex(F, SM, Err, Context), M(M), Index(Index), Slots(Slots),
           BlockAddressPFS(nullptr) {}
     bool Run(
-        bool UpgradeDebugInfo, DataLayoutCallbackTy DataLayoutCallback =
-                                   [](StringRef) { return std::nullopt; });
+        bool UpgradeDebugInfo,
+        DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
+          return std::nullopt;
+        });
 
     bool parseStandaloneConstantValue(Constant *&C, const SlotMapping *Slots);
 
@@ -318,8 +320,8 @@
     bool parseTopLevelEntities();
     bool validateEndOfModule(bool UpgradeDebugInfo);
     bool validateEndOfIndex();
-    bool parseTargetDefinitions();
-    bool parseTargetDefinition();
+    bool parseTargetDefinitions(DataLayoutCallbackTy DataLayoutCallback);
+    bool parseTargetDefinition(std::string &TentativeDLStr, LocTy &DLStrLoc);
     bool parseModuleAsm();
     bool parseSourceFileName();
     bool parseUnnamedType();
diff --git a/llvm/include/llvm/AsmParser/Parser.h b/llvm/include/llvm/AsmParser/Parser.h
--- a/llvm/include/llvm/AsmParser/Parser.h
+++ b/llvm/include/llvm/AsmParser/Parser.h
@@ -29,7 +29,7 @@
 class SMDiagnostic;
 class Type;
 
-typedef llvm::function_ref<std::optional<std::string>(StringRef)>
+typedef llvm::function_ref<std::optional<std::string>(StringRef, StringRef)>
     DataLayoutCallbackTy;
 
 /// This function is a main interface to the LLVM Assembly Parser. It parses
@@ -86,7 +86,7 @@
 ParsedModuleAndIndex parseAssemblyFileWithIndex(
     StringRef Filename, SMDiagnostic &Err, LLVMContext &Context,
     SlotMapping *Slots = nullptr,
-    DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+    DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
       return std::nullopt;
     });
 
@@ -127,7 +127,7 @@
 std::unique_ptr<Module> parseAssembly(
     MemoryBufferRef F, SMDiagnostic &Err, LLVMContext &Context,
     SlotMapping *Slots = nullptr,
-    DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+    DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
       return std::nullopt;
     });
 
@@ -169,7 +169,7 @@
 bool parseAssemblyInto(
     MemoryBufferRef F, Module *M, ModuleSummaryIndex *Index, SMDiagnostic &Err,
     SlotMapping *Slots = nullptr,
-    DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+    DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
       return std::nullopt;
     });
 
diff --git a/llvm/include/llvm/Bitcode/BitcodeReader.h b/llvm/include/llvm/Bitcode/BitcodeReader.h
--- a/llvm/include/llvm/Bitcode/BitcodeReader.h
+++ b/llvm/include/llvm/Bitcode/BitcodeReader.h
@@ -34,7 +34,11 @@
 class MemoryBuffer;
 class ModuleSummaryIndex;
 
-typedef llvm::function_ref<std::optional<std::string>(StringRef)>
+// Callback to override the data layout string of an imported bitcode module.
+// The first argument is the target triple, the second argument the data layout
+// string from the input, or a default string. It will be used if the callback
+// returns std::nullopt.
+typedef llvm::function_ref<std::optional<std::string>(StringRef, StringRef)>
     DataLayoutCallbackTy;
 
   // These functions are for converting Expected/Error values to
@@ -101,14 +105,18 @@
     /// bodies. If ShouldLazyLoadMetadata is true, lazily load metadata as well.
     /// If IsImporting is true, this module is being parsed for ThinLTO
     /// importing into another module.
-    Expected<std::unique_ptr<Module>> getLazyModule(LLVMContext &Context,
-                                                    bool ShouldLazyLoadMetadata,
-                                                    bool IsImporting);
+    Expected<std::unique_ptr<Module>> getLazyModule(
+        LLVMContext &Context, bool ShouldLazyLoadMetadata, bool IsImporting,
+        DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
+          return std::nullopt;
+        });
 
     /// Read the entire bitcode module and return it.
     Expected<std::unique_ptr<Module>> parseModule(
-        LLVMContext &Context, DataLayoutCallbackTy DataLayoutCallback =
-                                  [](StringRef) { return std::nullopt; });
+        LLVMContext &Context,
+        DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
+          return std::nullopt;
+        });
 
     /// Returns information about the module to be used for LTO: whether to
     /// compile with ThinLTO, and whether it has a summary.
@@ -145,10 +153,12 @@
   /// deserialization of function bodies. If ShouldLazyLoadMetadata is true,
   /// lazily load metadata as well. If IsImporting is true, this module is
   /// being parsed for ThinLTO importing into another module.
-  Expected<std::unique_ptr<Module>>
-  getLazyBitcodeModule(MemoryBufferRef Buffer, LLVMContext &Context,
-                       bool ShouldLazyLoadMetadata = false,
-                       bool IsImporting = false);
+  Expected<std::unique_ptr<Module>> getLazyBitcodeModule(
+      MemoryBufferRef Buffer, LLVMContext &Context,
+      bool ShouldLazyLoadMetadata = false, bool IsImporting = false,
+      DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
+        return std::nullopt;
+      });
 
   /// Like getLazyBitcodeModule, except that the module takes ownership of
   /// the memory buffer if successful. If successful, this moves Buffer. On
@@ -175,7 +185,7 @@
   /// Read the specified bitcode file, returning the module.
   Expected<std::unique_ptr<Module>> parseBitcodeFile(
       MemoryBufferRef Buffer, LLVMContext &Context,
-      DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+      DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
         return std::nullopt;
       });
 
diff --git a/llvm/include/llvm/CodeGen/MIRParser/MIRParser.h b/llvm/include/llvm/CodeGen/MIRParser/MIRParser.h
--- a/llvm/include/llvm/CodeGen/MIRParser/MIRParser.h
+++ b/llvm/include/llvm/CodeGen/MIRParser/MIRParser.h
@@ -34,7 +34,7 @@
 class SMDiagnostic;
 class StringRef;
 
-typedef llvm::function_ref<std::optional<std::string>(StringRef)>
+typedef llvm::function_ref<std::optional<std::string>(StringRef, StringRef)>
     DataLayoutCallbackTy;
 
 /// This class initializes machine functions by applying the state loaded from
@@ -52,9 +52,8 @@
   /// A new, empty module is created if the LLVM IR isn't present.
   /// \returns nullptr if a parsing error occurred.
   std::unique_ptr<Module>
-  parseIRModule(DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
-    return std::nullopt;
-  });
+  parseIRModule(DataLayoutCallbackTy DataLayoutCallback =
+                    [](StringRef, StringRef) { return std::nullopt; });
 
   /// Parses MachineFunctions in the MIR file and add them to the given
   /// MachineModuleInfo \p MMI.
diff --git a/llvm/include/llvm/IRReader/IRReader.h b/llvm/include/llvm/IRReader/IRReader.h
--- a/llvm/include/llvm/IRReader/IRReader.h
+++ b/llvm/include/llvm/IRReader/IRReader.h
@@ -27,7 +27,7 @@
 class SMDiagnostic;
 class LLVMContext;
 
-typedef llvm::function_ref<std::optional<std::string>(StringRef)>
+typedef llvm::function_ref<std::optional<std::string>(StringRef, StringRef)>
     DataLayoutCallbackTy;
 
 /// If the given MemoryBuffer holds a bitcode image, return a Module
@@ -55,7 +55,7 @@
 /// \param DataLayoutCallback Override datalayout in the llvm assembly.
 std::unique_ptr<Module> parseIR(
     MemoryBufferRef Buffer, SMDiagnostic &Err, LLVMContext &Context,
-    DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+    DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
       return std::nullopt;
     });
 
@@ -65,7 +65,7 @@
 /// \param DataLayoutCallback Override datalayout in the llvm assembly.
 std::unique_ptr<Module> parseIRFile(
     StringRef Filename, SMDiagnostic &Err, LLVMContext &Context,
-    DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+    DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
       return std::nullopt;
     });
 }
diff --git a/llvm/lib/AsmParser/LLParser.cpp b/llvm/lib/AsmParser/LLParser.cpp
--- a/llvm/lib/AsmParser/LLParser.cpp
+++ b/llvm/lib/AsmParser/LLParser.cpp
@@ -95,11 +95,8 @@
         "Can't read textual IR with a Context that discards named Values");
 
   if (M) {
-    if (parseTargetDefinitions())
+    if (parseTargetDefinitions(DataLayoutCallback))
       return true;
-
-    if (auto LayoutOverride = DataLayoutCallback(M->getTargetTriple()))
-      M->setDataLayout(*LayoutOverride);
   }
 
   return parseTopLevelEntities() || validateEndOfModule(UpgradeDebugInfo) ||
@@ -353,11 +350,19 @@
 // Top-Level Entities
 //===----------------------------------------------------------------------===//
 
-bool LLParser::parseTargetDefinitions() {
-  while (true) {
+bool LLParser::parseTargetDefinitions(DataLayoutCallbackTy DataLayoutCallback) {
+  // Delay parsing of the data layout string until the target triple is known.
+  // Then, pass both the the target triple and the tentative data layout string
+  // to DataLayoutCallback, allowing to override the DL string.
+  // This enables importing modules with invalid DL strings.
+  std::string TentativeDLStr = M->getDataLayoutStr();
+  LocTy DLStrLoc;
+
+  bool Done = false;
+  while (!Done) {
     switch (Lex.getKind()) {
     case lltok::kw_target:
-      if (parseTargetDefinition())
+      if (parseTargetDefinition(TentativeDLStr, DLStrLoc))
         return true;
       break;
     case lltok::kw_source_filename:
@@ -365,9 +370,25 @@
         return true;
       break;
     default:
-      return false;
+      Done = true;
     }
   }
+  // Run the override callback to potentially change the data layout string, and
+  // parse the data layout string. Parsing of the data layout is done separately
+  // in the two cases below because the error handling differs: For invalid
+  // data layout strings parsed from the module file, we want to report an error
+  // referencing the relevant source location. For layout strings created by
+  // callbacks, we let Module::setDataLayout handle potential errors.
+  if (auto LayoutOverride =
+          DataLayoutCallback(M->getTargetTriple(), TentativeDLStr)) {
+    M->setDataLayout(*LayoutOverride);
+  } else {
+    Expected<DataLayout> MaybeDL = DataLayout::parse(TentativeDLStr);
+    if (!MaybeDL)
+      return error(DLStrLoc, toString(MaybeDL.takeError()));
+    M->setDataLayout(MaybeDL.get());
+  }
+  return false;
 }
 
 bool LLParser::parseTopLevelEntities() {
@@ -471,7 +492,8 @@
 /// toplevelentity
 ///   ::= 'target' 'triple' '=' STRINGCONSTANT
 ///   ::= 'target' 'datalayout' '=' STRINGCONSTANT
-bool LLParser::parseTargetDefinition() {
+bool LLParser::parseTargetDefinition(std::string &TentativeDLStr,
+                                     LocTy &DLStrLoc) {
   assert(Lex.getKind() == lltok::kw_target);
   std::string Str;
   switch (Lex.Lex()) {
@@ -488,13 +510,9 @@
     Lex.Lex();
     if (parseToken(lltok::equal, "expected '=' after target datalayout"))
       return true;
-    LocTy Loc = Lex.getLoc();
-    if (parseStringConstant(Str))
+    DLStrLoc = Lex.getLoc();
+    if (parseStringConstant(TentativeDLStr))
       return true;
-    Expected<DataLayout> MaybeDL = DataLayout::parse(Str);
-    if (!MaybeDL)
-      return error(Loc, toString(MaybeDL.takeError()));
-    M->setDataLayout(MaybeDL.get());
     return false;
   }
 }
diff --git a/llvm/lib/AsmParser/Parser.cpp b/llvm/lib/AsmParser/Parser.cpp
--- a/llvm/lib/AsmParser/Parser.cpp
+++ b/llvm/lib/AsmParser/Parser.cpp
@@ -91,9 +91,10 @@
                                                   SMDiagnostic &Err,
                                                   LLVMContext &Context,
                                                   SlotMapping *Slots) {
-  return ::parseAssemblyWithIndex(F, Err, Context, Slots,
-                                  /*UpgradeDebugInfo*/ true,
-                                  [](StringRef) { return std::nullopt; });
+  return ::parseAssemblyWithIndex(
+      F, Err, Context, Slots,
+      /*UpgradeDebugInfo*/ true,
+      [](StringRef, StringRef) { return std::nullopt; });
 }
 
 static ParsedModuleAndIndex
@@ -150,7 +151,7 @@
   // index, but we need to initialize it.
   LLVMContext unusedContext;
   return LLParser(F.getBuffer(), SM, Err, nullptr, &Index, unusedContext)
-      .Run(true, [](StringRef) { return std::nullopt; });
+      .Run(true, [](StringRef, StringRef) { return std::nullopt; });
 }
 
 std::unique_ptr<ModuleSummaryIndex>
diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -821,7 +821,7 @@
   Error parseAttrKind(uint64_t Code, Attribute::AttrKind *Kind);
   Error parseModule(
       uint64_t ResumeBit, bool ShouldLazyLoadMetadata = false,
-      DataLayoutCallbackTy DataLayoutCallback = [](StringRef) {
+      DataLayoutCallbackTy DataLayoutCallback = [](StringRef, StringRef) {
         return std::nullopt;
       });
 
@@ -4192,21 +4192,35 @@
   // Parts of bitcode parsing depend on the datalayout.  Make sure we
   // finalize the datalayout before we run any of that code.
   bool ResolvedDataLayout = false;
-  auto ResolveDataLayout = [&] {
+  // In order to support importing modules with illegal data layout strings,
+  // delay parsing the data layout string until after upgrades and overrides
+  // have been applied, allowing to fix illegal data layout strings.
+  // Initialize to the current module's layout string in case none is specified.
+  std::string TentativeDataLayoutStr = TheModule->getDataLayoutStr();
+
+  auto ResolveDataLayout = [&]() -> Error {
     if (ResolvedDataLayout)
-      return;
+      return Error::success();
 
-    // datalayout and triple can't be parsed after this point.
+    // Datalayout and triple can't be parsed after this point.
     ResolvedDataLayout = true;
 
-    // Upgrade data layout string.
-    std::string DL = llvm::UpgradeDataLayoutString(
-        TheModule->getDataLayoutStr(), TheModule->getTargetTriple());
-    TheModule->setDataLayout(DL);
+    // Auto-upgrade the layout string
+    TentativeDataLayoutStr = llvm::UpgradeDataLayoutString(
+        TentativeDataLayoutStr, TheModule->getTargetTriple());
+
+    // Apply override
+    if (auto LayoutOverride = DataLayoutCallback(TheModule->getTargetTriple(),
+                                                 TentativeDataLayoutStr))
+      TentativeDataLayoutStr = *LayoutOverride;
+
+    // Now the layout string is finalized in TentativeDataLayoutStr. Parse it.
+    Expected<DataLayout> MaybeDL = DataLayout::parse(TentativeDataLayoutStr);
+    if (!MaybeDL)
+      return MaybeDL.takeError();
 
-    if (auto LayoutOverride =
-            DataLayoutCallback(TheModule->getTargetTriple()))
-      TheModule->setDataLayout(*LayoutOverride);
+    TheModule->setDataLayout(MaybeDL.get());
+    return Error::success();
   };
 
   // Read all the records for this module.
@@ -4220,7 +4234,8 @@
     case BitstreamEntry::Error:
       return error("Malformed block");
     case BitstreamEntry::EndBlock:
-      ResolveDataLayout();
+      if (Error Err = ResolveDataLayout())
+        return Err;
       return globalCleanup();
 
     case BitstreamEntry::SubBlock:
@@ -4285,7 +4300,8 @@
           return Err;
         break;
       case bitc::FUNCTION_BLOCK_ID:
-        ResolveDataLayout();
+        if (Error Err = ResolveDataLayout())
+          return Err;
 
         // If this is the first function body we've seen, reverse the
         // FunctionsWithBodies list.
@@ -4384,13 +4400,8 @@
     case bitc::MODULE_CODE_DATALAYOUT: {  // DATALAYOUT: [strchr x N]
       if (ResolvedDataLayout)
         return error("datalayout too late in module");
-      std::string S;
-      if (convertToString(Record, 0, S))
+      if (convertToString(Record, 0, TentativeDataLayoutStr))
         return error("Invalid record");
-      Expected<DataLayout> MaybeDL = DataLayout::parse(S);
-      if (!MaybeDL)
-        return MaybeDL.takeError();
-      TheModule->setDataLayout(MaybeDL.get());
       break;
     }
     case bitc::MODULE_CODE_ASM: {  // ASM: [strchr x N]
@@ -4436,7 +4447,8 @@
         return Err;
       break;
     case bitc::MODULE_CODE_FUNCTION:
-      ResolveDataLayout();
+      if (Error Err = ResolveDataLayout())
+        return Err;
       if (Error Err = parseFunctionRecord(Record))
         return Err;
       break;
@@ -4465,6 +4477,7 @@
     }
     Record.clear();
   }
+  return Error::success();
 }
 
 Error BitcodeReader::parseBitcodeInto(Module *M, bool ShouldLazyLoadMetadata,
@@ -7942,9 +7955,10 @@
 
 Expected<std::unique_ptr<Module>>
 BitcodeModule::getLazyModule(LLVMContext &Context, bool ShouldLazyLoadMetadata,
-                             bool IsImporting) {
+                             bool IsImporting,
+                             DataLayoutCallbackTy DataLayoutCallback) {
   return getModuleImpl(Context, false, ShouldLazyLoadMetadata, IsImporting,
-                       [](StringRef) { return std::nullopt; });
+                       DataLayoutCallback);
 }
 
 // Parse the specified bitcode buffer and merge the index into CombinedIndex.
@@ -8090,12 +8104,14 @@
 
 Expected<std::unique_ptr<Module>>
 llvm::getLazyBitcodeModule(MemoryBufferRef Buffer, LLVMContext &Context,
-                           bool ShouldLazyLoadMetadata, bool IsImporting) {
+                           bool ShouldLazyLoadMetadata, bool IsImporting,
+                           DataLayoutCallbackTy DataLayoutCallback) {
   Expected<BitcodeModule> BM = getSingleModule(Buffer);
   if (!BM)
     return BM.takeError();
 
-  return BM->getLazyModule(Context, ShouldLazyLoadMetadata, IsImporting);
+  return BM->getLazyModule(Context, ShouldLazyLoadMetadata, IsImporting,
+                           DataLayoutCallback);
 }
 
 Expected<std::unique_ptr<Module>> llvm::getOwningLazyBitcodeModule(
diff --git a/llvm/lib/CodeGen/MIRParser/MIRParser.cpp b/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
--- a/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
+++ b/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
@@ -234,7 +234,8 @@
     // Create an empty module when the MIR file is empty.
     NoMIRDocuments = true;
     auto M = std::make_unique<Module>(Filename, Context);
-    if (auto LayoutOverride = DataLayoutCallback(M->getTargetTriple()))
+    if (auto LayoutOverride =
+            DataLayoutCallback(M->getTargetTriple(), M->getDataLayoutStr()))
       M->setDataLayout(*LayoutOverride);
     return M;
   }
@@ -257,7 +258,8 @@
   } else {
     // Create an new, empty module.
     M = std::make_unique<Module>(Filename, Context);
-    if (auto LayoutOverride = DataLayoutCallback(M->getTargetTriple()))
+    if (auto LayoutOverride =
+            DataLayoutCallback(M->getTargetTriple(), M->getDataLayoutStr()))
       M->setDataLayout(*LayoutOverride);
     NoLLVMIR = true;
   }
diff --git a/llvm/test/Assembler/invalid-datalayout-override.ll b/llvm/test/Assembler/invalid-datalayout-override.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Assembler/invalid-datalayout-override.ll
@@ -0,0 +1,7 @@
+; Check that importing this file gives an error due to the broken data layout:
+; RUN: not llvm-as < %s 2>&1 | FileCheck %s
+; Check that specifying a valid data layout allows to import this file:
+; RUN: llvm-as -data-layout "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128" < %s
+
+target datalayout = "A16777216"
+; CHECK: Invalid address space, must be a 24-bit integer
diff --git a/llvm/tools/llc/llc.cpp b/llvm/tools/llc/llc.cpp
--- a/llvm/tools/llc/llc.cpp
+++ b/llvm/tools/llc/llc.cpp
@@ -530,8 +530,8 @@
 
   // If user just wants to list available options, skip module loading
   if (!SkipModule) {
-    auto SetDataLayout =
-        [&](StringRef DataLayoutTargetTriple) -> std::optional<std::string> {
+    auto SetDataLayout = [&](StringRef DataLayoutTargetTriple,
+                             StringRef OldDLStr) -> std::optional<std::string> {
       // If we are supposed to override the target triple, do so now.
       std::string IRTargetTriple = DataLayoutTargetTriple.str();
       if (!TargetTriple.empty())
diff --git a/llvm/tools/llvm-as/llvm-as.cpp b/llvm/tools/llvm-as/llvm-as.cpp
--- a/llvm/tools/llvm-as/llvm-as.cpp
+++ b/llvm/tools/llvm-as/llvm-as.cpp
@@ -121,7 +121,7 @@
 
   // Parse the file now...
   SMDiagnostic Err;
-  auto SetDataLayout = [](StringRef) -> std::optional<std::string> {
+  auto SetDataLayout = [](StringRef, StringRef) -> std::optional<std::string> {
     if (ClDataLayout.empty())
       return std::nullopt;
     return ClDataLayout;
diff --git a/llvm/tools/llvm-reduce/ReducerWorkItem.cpp b/llvm/tools/llvm-reduce/ReducerWorkItem.cpp
--- a/llvm/tools/llvm-reduce/ReducerWorkItem.cpp
+++ b/llvm/tools/llvm-reduce/ReducerWorkItem.cpp
@@ -407,8 +407,8 @@
     std::unique_ptr<MIRParser> MParser =
         createMIRParser(std::move(FileOrErr.get()), Ctxt);
 
-    auto SetDataLayout =
-        [&](StringRef DataLayoutTargetTriple) -> std::optional<std::string> {
+    auto SetDataLayout = [&](StringRef DataLayoutTargetTriple,
+                             StringRef OldDLStr) -> std::optional<std::string> {
       // If we are supposed to override the target triple, do so now.
       std::string IRTargetTriple = DataLayoutTargetTriple.str();
       if (!TargetTriple.empty())
diff --git a/llvm/tools/opt/opt.cpp b/llvm/tools/opt/opt.cpp
--- a/llvm/tools/opt/opt.cpp
+++ b/llvm/tools/opt/opt.cpp
@@ -542,7 +542,7 @@
   std::unique_ptr<ToolOutputFile> RemarksFile = std::move(*RemarksFileOrErr);
 
   // Load the input module...
-  auto SetDataLayout = [](StringRef) -> std::optional<std::string> {
+  auto SetDataLayout = [](StringRef, StringRef) -> std::optional<std::string> {
     if (ClDataLayout.empty())
       return std::nullopt;
     return ClDataLayout;
diff --git a/llvm/unittests/AsmParser/AsmParserTest.cpp b/llvm/unittests/AsmParser/AsmParserTest.cpp
--- a/llvm/unittests/AsmParser/AsmParserTest.cpp
+++ b/llvm/unittests/AsmParser/AsmParserTest.cpp
@@ -10,8 +10,10 @@
 #include "llvm/AsmParser/Parser.h"
 #include "llvm/AsmParser/SlotMapping.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/IR/DataLayout.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
+#include "llvm/Support/Error.h"
 #include "llvm/Support/SourceMgr.h"
 #include "gtest/gtest.h"
 
@@ -400,4 +402,39 @@
   ASSERT_TRUE(Read == 4);
 }
 
+TEST(AsmParserTest, InvalidDataLayoutStringCallback) {
+  LLVMContext Ctx;
+  SMDiagnostic Error;
+  // Note the invalid i8:7 part
+  // Overalign i32 as marker so we can check that indeed this DL was used,
+  // and not some default.
+  StringRef InvalidDLStr =
+      "e-m:e-p:64:64-i8:7-i16:16-i32:64-i64:64-f80:128-n8:16:32:64";
+  StringRef FixedDLStr =
+      "e-m:e-p:64:64-i8:8-i16:16-i32:64-i64:64-f80:128-n8:16:32:64";
+  Expected<DataLayout> ExpectedFixedDL = DataLayout::parse(FixedDLStr);
+  ASSERT_TRUE(!ExpectedFixedDL.takeError());
+  DataLayout FixedDL = ExpectedFixedDL.get();
+  std::string Source = ("target datalayout = \"" + InvalidDLStr + "\"\n").str();
+  MemoryBufferRef SourceBuffer(Source, "<string>");
+
+  // Check that we reject the source without a DL override.
+  SlotMapping Mapping1;
+  auto Mod1 = parseAssembly(SourceBuffer, Error, Ctx, &Mapping1);
+  EXPECT_TRUE(Mod1 == nullptr);
+
+  // Check that we pass the correct DL str to the callback,
+  // that fixing the DL str from the callback works,
+  // and that the resulting module has the correct DL.
+  SlotMapping Mapping2;
+  auto Mod2 = parseAssembly(
+      SourceBuffer, Error, Ctx, &Mapping2,
+      [&](StringRef Triple, StringRef DLStr) -> std::optional<std::string> {
+        EXPECT_EQ(DLStr, InvalidDLStr);
+        return std::string{FixedDLStr};
+      });
+  ASSERT_TRUE(Mod2 != nullptr);
+  EXPECT_EQ(Mod2->getDataLayout(), FixedDL);
+}
+
 } // end anonymous namespace