[clang][Interp] Emit negated integers directly as constants

[aaron.ballman]

This is missing test coverage (possibly around integer promotion behavior as well).

clang/lib/AST/Interp/ByteCodeExprGen.cpp

1525

Are you sure about IsUnsigned? http://eel.is/c++draft/expr.unary.op#8 reads to me like -0ULL will remain unsigned as a result.

[tbaeder]

This was just to make the byte code a little more readable, but given the amount of pending patches I already have, I'll abandon this and maybe revisit at a later point, thanks!

clang/lib/AST/Interp/ByteCodeExprGen.cpp

case UO_PreDec: { // --x
     return this->emitStore(*T, E);
   }
   case UO_LNot: // !x
     if (!this->visit(SubExpr))
       return false;
     // The Inv doesn't change anything, so skip it if we don't need the result.
     return DiscardResult ? this->emitPop(*T, E) : this->emitInvBool(E);
   case UO_Minus: // -x
+    // Emit negated integers directly.
+    if (const auto *IL = dyn_cast<IntegerLiteral>(SubExpr)) {
+      if (DiscardResult)
+        return true;
+      APSInt V = APSInt(IL->getValue(), /*IsUnsigned=*/false);
+      return this->emitConst(-V, E);
+    }
     if (!this->visit(SubExpr))
       return false;
     return DiscardResult ? this->emitPop(*T, E) : this->emitNeg(*T, E);
   case UO_Plus:  // +x
     if (!this->visit(SubExpr)) // noop
       return false;
     return DiscardResult ? this->emitPop(*T, E) : true;
   case UO_AddrOf: // &x