diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -6210,9 +6210,11 @@
   }
   case Intrinsic::dbg_assign: {
     // Debug intrinsics are handled seperately in assignment tracking mode.
-    assert(AssignmentTrackingEnabled &&
-           "expected assignment tracking to be enabled");
-    return;
+    if (AssignmentTrackingEnabled)
+      return;
+    // If assignment tracking hasn't been enabled then fall through and treat
+    // the dbg.assign as a dbg.value.
+    [[fallthrough]];
   }
   case Intrinsic::dbg_value: {
     // Debug intrinsics are handled seperately in assignment tracking mode.
diff --git a/llvm/test/DebugInfo/assignment-tracking/X86/assignment-tracking-not-enabled.ll b/llvm/test/DebugInfo/assignment-tracking/X86/assignment-tracking-not-enabled.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/DebugInfo/assignment-tracking/X86/assignment-tracking-not-enabled.ll
@@ -0,0 +1,81 @@
+; RUN: llc %s -stop-after=finalize-isel -o - \
+; RUN: | FileCheck %s --implicit-check-not=DBG_
+
+;; Check that SelectionDAG downgrades dbg.assigns to dbg.values if assignment
+;; tracking isn't enabled (e.g. if the module flag
+;; "debug-info-assignment-tracking" is missing / false).
+
+;; With assignment tracking enabled we'd see the variable put into the stack
+;; slot side table because the variable is always located in its stack
+;; slot. Check there's no debug-info saved there:
+;; CHECK: stack:
+;; CHECK-NEXT: - { id: 0, name: x, type: default, offset: 0, size: 4, alignment: 4,
+;; CHECK-NEXT:     stack-id: default, callee-saved-register: '', callee-saved-restored: true,
+;; CHECK-NEXT:     debug-info-variable: '', debug-info-expression: '', debug-info-location: '' }
+
+;; With assignment tracking disabled we should see the dbg.assigns downgraded
+;; to dbg.values, which become DBG _VALUEs/_INSTR_REFs.
+; CHECK: bb.0.entry:
+; CHECK: DBG_VALUE $noreg
+
+; CHECK: bb.1.if.then:
+; CHECK: DBG_INSTR_REF
+
+; CHECK: bb.2.if.else:
+; CHECK: DBG_VALUE 2
+
+target triple = "x86_64-unknown-unknown"
+
+@g = dso_local global i32 0, align 4, !dbg !0
+
+define dso_local noundef i32 @_Z3funv() #0 !dbg !15 {
+entry:
+  %x = alloca i32, align 4, !DIAssignID !19
+  call void @llvm.dbg.assign(metadata i1 undef, metadata !20, metadata !DIExpression(), metadata !19, metadata ptr %x, metadata !DIExpression()), !dbg !21
+  %0 = load i32, ptr @g, align 4
+  %tobool = icmp ne i32 %0, 0
+  br i1 %tobool, label %if.then, label %if.else
+
+if.then:                                          ; preds = %entry
+  %call = call noundef i32 @_Z3getv()
+  store i32 %call, ptr %x, align 4, !DIAssignID !27
+  call void @llvm.dbg.assign(metadata i32 %call, metadata !20, metadata !DIExpression(), metadata !27, metadata ptr %x, metadata !DIExpression()), !dbg !21
+  br label %if.end
+
+if.else:                                          ; preds = %entry
+  store i32 2, ptr %x, align 4, !DIAssignID !30
+  call void @llvm.dbg.assign(metadata i32 2, metadata !20, metadata !DIExpression(), metadata !30, metadata ptr %x, metadata !DIExpression()), !dbg !21
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  %1 = load i32, ptr %x, align 4
+  ret i32 %1
+}
+
+declare void @llvm.dbg.assign(metadata, metadata, metadata, metadata, metadata, metadata) #1
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+declare noundef i32 @_Z3getv() #2
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!6, !7}
+!llvm.ident = !{!14}
+
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "g", scope: !2, file: !3, line: 1, type: !5, isLocal: false, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !3, producer: "clang version 17.0.0)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, globals: !4, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "test.cpp", directory: "/")
+!4 = !{!0}
+!5 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!6 = !{i32 7, !"Dwarf Version", i32 5}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!14 = !{!"clang version 17.0.0"}
+!15 = distinct !DISubprogram(name: "fun", linkageName: "_Z3funv", scope: !3, file: !3, line: 3, type: !16, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !18)
+!16 = !DISubroutineType(types: !17)
+!17 = !{!5}
+!18 = !{}
+!19 = distinct !DIAssignID()
+!20 = !DILocalVariable(name: "x", scope: !15, file: !3, line: 4, type: !5)
+!21 = !DILocation(line: 0, scope: !15)
+!23 = distinct !DILexicalBlock(scope: !15, file: !3, line: 5, column: 7)
+!27 = distinct !DIAssignID()
+!30 = distinct !DIAssignID()