Index: llvm/lib/Target/AArch64/MCTargetDesc/AArch64MCTargetDesc.cpp
===================================================================
--- llvm/lib/Target/AArch64/MCTargetDesc/AArch64MCTargetDesc.cpp
+++ llvm/lib/Target/AArch64/MCTargetDesc/AArch64MCTargetDesc.cpp
@@ -57,7 +57,18 @@
       CPU = "apple-a12";
   }
 
-  return createAArch64MCSubtargetInfoImpl(TT, CPU, /*TuneCPU*/ CPU, FS);
+  // Most of the NEON instruction set isn't supported in streaming mode on SME
+  // targets, but if no CPU is specified the default is "generic", which
+  // implies NEON. If streaming mode is enabled and the CPU is generic, disable
+  // NEON, only if it isn't explicitly specified in the feature flags.
+  bool HasNEON = FS.contains("neon");
+  MCSubtargetInfo *STI =
+      createAArch64MCSubtargetInfoImpl(TT, CPU, /*TuneCPU*/ CPU, FS);
+  if (CPU.equals("generic") && !HasNEON &&
+      STI->hasFeature(AArch64::FeatureNEON) &&
+      STI->hasFeature(AArch64::FeatureStreamingSVE))
+    STI->ToggleFeature(AArch64::FeatureNEON);
+  return STI;
 }
 
 void AArch64_MC::initLLVMToCVRegMapping(MCRegisterInfo *MRI) {
Index: llvm/test/MC/AArch64/SME/feature-negative.s
===================================================================
--- /dev/null
+++ llvm/test/MC/AArch64/SME/feature-negative.s
@@ -0,0 +1,5 @@
+// RUN: not llvm-mc -triple=aarch64 -mattr=+streaming-sve < %s 2>&1 | FileCheck %s
+
+// Verify NEON is disabled when targeting streaming mode.
+dup b0, v0.b[0]
+// CHECK: error: instruction requires: neon
Index: llvm/test/MC/AArch64/SME/feature.s
===================================================================
--- llvm/test/MC/AArch64/SME/feature.s
+++ llvm/test/MC/AArch64/SME/feature.s
@@ -8,4 +8,12 @@
 
 // Verify +sme flags imply +bf16
 bfdot z0.s, z1.h, z2.h
-// CHECK-INST: bfdot z0.s, z1.h, z2.h
+// CHECK: bfdot z0.s, z1.h, z2.h
+
+// Most of the NEON instruction set is illegal in streaming mode. If no CPU is
+// specified the default is generic which implies NEON, but if streaming-sve
+// (implied by +sme) is enabled it's disabled, only if it's not explicitly
+// enabled, verify this.
+.arch_extension simd
+add v0.8b, v1.8b, v2.8b
+// CHECK: add v0.8b, v1.8b, v2.8b