diff --git a/llvm/include/llvm/MC/MCAsmLayout.h b/llvm/include/llvm/MC/MCAsmLayout.h
--- a/llvm/include/llvm/MC/MCAsmLayout.h
+++ b/llvm/include/llvm/MC/MCAsmLayout.h
@@ -10,6 +10,7 @@
 #define LLVM_MC_MCASMLAYOUT_H
 
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
 
 namespace llvm {
@@ -31,6 +32,9 @@
   /// List of sections in layout order.
   llvm::SmallVector<MCSection *, 16> SectionOrder;
 
+  /// Set of fragments currently being laid out.
+  mutable SmallPtrSet<MCFragment *, 4> LayingOutFragmentsSet;
+
   /// The last fragment which was laid out, or 0 if nothing has been laid
   /// out. Fragments are always laid out in order, so all fragments with a
   /// lower ordinal will be valid.
@@ -49,6 +53,10 @@
   /// Get the assembler object this is a layout for.
   MCAssembler &getAssembler() const { return Assembler; }
 
+  bool isFragmentValidating(const MCFragment *F) const {
+    return LayingOutFragmentsSet.find(F) != LayingOutFragmentsSet.end();
+  }
+
   /// Invalidate the fragments starting with F because it has been
   /// resized. The fragment's size should have already been updated, but
   /// its bundle padding will be recomputed.
diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -397,6 +397,10 @@
   assert((!Prev || isFragmentValid(Prev)) &&
          "Attempt to compute fragment before its predecessor!");
 
+  bool InsertionSuccess{};
+  std::tie(std::ignore, InsertionSuccess) = LayingOutFragmentsSet.insert(F);
+  assert(InsertionSuccess && "Already being laid out!");
+
   ++stats::FragmentLayouts;
 
   // Compute fragment offset and size.
@@ -404,6 +408,7 @@
     F->Offset = Prev->Offset + getAssembler().computeFragmentSize(*this, *Prev);
   else
     F->Offset = 0;
+  LayingOutFragmentsSet.erase(F);
   LastValidFragment[F->getParent()] = F;
 
   // If bundling is enabled and this fragment has instructions in it, it has to
diff --git a/llvm/lib/MC/MCExpr.cpp b/llvm/lib/MC/MCExpr.cpp
--- a/llvm/lib/MC/MCExpr.cpp
+++ b/llvm/lib/MC/MCExpr.cpp
@@ -547,8 +547,10 @@
   if (!Asm->getWriter().isSymbolRefDifferenceFullyResolved(*Asm, A, B, InSet))
     return;
 
-  if (SA.getFragment() == SB.getFragment() && !SA.isVariable() &&
-      !SA.isUnset() && !SB.isVariable() && !SB.isUnset()) {
+  MCFragment *FA = SA.getFragment();
+  MCFragment *FB = SB.getFragment();
+  if (FA == FB && !SA.isVariable() && !SA.isUnset() && !SB.isVariable() &&
+      !SB.isUnset()) {
     Addend += (SA.getOffset() - SB.getOffset());
 
     // Pointers to Thumb symbols need to have their low-bit set to allow
@@ -570,12 +572,16 @@
   if (!Layout)
     return;
 
-  const MCSection &SecA = *SA.getFragment()->getParent();
-  const MCSection &SecB = *SB.getFragment()->getParent();
+  const MCSection &SecA = *FA->getParent();
+  const MCSection &SecB = *FB->getParent();
 
   if ((&SecA != &SecB) && !Addrs)
     return;
 
+  if (Layout &&
+      (Layout->isFragmentValidating(FA) || Layout->isFragmentValidating(FB)))
+    return;
+
   // Eagerly evaluate.
   Addend += Layout->getSymbolOffset(A->getSymbol()) -
             Layout->getSymbolOffset(B->getSymbol());
diff --git a/llvm/test/MC/AsmParser/pr45805.s b/llvm/test/MC/AsmParser/pr45805.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/AsmParser/pr45805.s
@@ -0,0 +1,10 @@
+// RUN: not llvm-mc --filetype=obj %s -o /dev/null 2>&1 | FileCheck %s
+
+
+fct_end:
+
+// CHECK: pr45805.s:7:7: error: expected assembly-time absolute expression
+.fill (data_start - fct_end), 1, 42
+
+data_start:
+.long 0x42424242