[Sema] Treat literal minus as a unary operator for apex precedence group purposes

Author: xwu

include/swift/AST/DiagnosticsSema.def

@@ -3386,7 +3386,7 @@ ERROR(construct_protocol_by_name,none,
 ERROR(unknown_binop,none,
       "operator is not a known binary operator", ())
 ERROR(unordered_adjacent_unary_operator,none,
-      "operator in precedence group %0 and adjacent unary operator have"
+      "operator in precedence group %0 and adjacent unary operator have "
       "undefined relative precedence; use grouping parentheses",
       (Identifier))
 ERROR(non_associative_adjacent_operators,none,

lib/Sema/TypeCheckExpr.cpp

@@ -291,6 +291,16 @@ static Expr *makeBinOp(ASTContext &Ctx, Expr *Op, Expr *LHS, Expr *RHS,
                          opPrecedence->getName())
       .highlight(prefixUnary->getFn()->getSourceRange());
     }
+    // Prefix '-' (but not '+') is lexed as part of a number literal rather than
+    // as a standalone operator.
+    else if (auto *numberLiteral = dyn_cast<NumberLiteralExpr>(LHS)) {
+      if (numberLiteral->isNegative()) {
+        Ctx.Diags.diagnose(Op->getLoc(),
+                           diag::unordered_adjacent_unary_operator,
+                           opPrecedence->getName())
+        .highlight(numberLiteral->getMinusLoc());
+      }
+    }
     if (auto *postfixUnary = dyn_cast<PostfixUnaryExpr>(RHS)) {
       Ctx.Diags.diagnose(Op->getLoc(),
                          diag::unordered_adjacent_unary_operator,