Index: llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
===================================================================
--- llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
+++ llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
@@ -2815,6 +2815,8 @@
   const char *Name;
   const FeatureBitset Features;
 } ExtensionMap[] = {
+    {"dummy_extension",
+     {}}, // This is a dummy extension used for testing error outputs.
     {"crc", {AArch64::FeatureCRC}},
     {"sm4", {AArch64::FeatureSM4}},
     {"sha3", {AArch64::FeatureSHA3}},
@@ -5142,7 +5144,7 @@
         continue;
 
       if (Extension.Features.none())
-        report_fatal_error("unsupported architectural extension: " + Name);
+        return Error(ArchLoc, "unsupported architectural extension: " + Name);
 
       FeatureBitset ToggleFeatures = EnableFeature
                                          ? (~Features & Extension.Features)
Index: llvm/test/MC/AsmParser/AArch64/directive-feature.s
===================================================================
--- /dev/null
+++ llvm/test/MC/AsmParser/AArch64/directive-feature.s
@@ -0,0 +1,5 @@
+# RUN: not llvm-mc -triple aarch64 %s 2>&1 > /dev/null| FileCheck %s --check-prefix=CHECK-ERROR
+
+# CHECK-ERROR:  error: unsupported architectural extension: dummy_extension
+TEST1:
+        .arch v8+dummy_extension