[AsmParser] Return an error in the case of empty symbol ref in an expression

[thegameg]

Summary

The following instruction:

str q28, [x0, #1*6*4*@]

contains a @ which is parsed as an empty symbol. The parser returns true but has no error, so the assembler continues by ignoring the instruction.

This check was introduced by D26728 to avoid asserting.

[t.p.northover]

Looks fine to me.

lib/MC/MCParser/AsmParser.cpp

if (!MAI.useParensForSymbolVariant()) {
       Split = std::make_pair(Identifier, VName);
     }
 
     EndLoc = SMLoc::getFromPointer(Identifier.end());
 
     // This is a symbol reference.
     StringRef SymbolName = Identifier;
     if (SymbolName.empty())
-      return true;
+      return Error(getLexer().getLoc(), "expected a symbol reference");
 
     MCSymbolRefExpr::VariantKind Variant = MCSymbolRefExpr::VK_None;
 
     // Lookup the symbol variant if used.
     if (!Split.second.empty()) {
       Variant = MCSymbolRefExpr::getVariantKindForName(Split.second);
       if (Variant != MCSymbolRefExpr::VK_Invalid) {
         SymbolName = Split.first;

test/MC/AArch64/expr-bad-symbol.s

+// RUN: not llvm-mc -triple aarch64-- %s 2>&1 | FileCheck %s
+
+  .text
+_foo:
+  str q28, [x0, #1*6*4*@]
+// CHECK: error: expected a symbol reference