expressions: for "if" expressions, make both branches optional

... using, also for the "then" branch, the empty list as default.
In this way, this statement not only more symmetric, but also
allows shorter representations of some typical expressions.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/buildtool/build_engine/expression/evaluator.cpp b/src/buildtool/build_engine/expression/evaluator.cpp
index 11299bb5..d53259ba 100644
--- a/src/buildtool/build_engine/expression/evaluator.cpp
+++ b/src/buildtool/build_engine/expression/evaluator.cpp
@@ -495,7 +495,7 @@ auto IfExpr(SubExprEvaluator&& eval,
             ExpressionPtr const& expr,
             Configuration const& env) -> ExpressionPtr {
     if (ValueIsTrue(EvalArgument(expr, "cond", eval, env))) {
-        return EvalArgument(expr, "then", eval, env);
+        return eval(expr->Get("then", list_t{}), env);
     }
     return eval(expr->Get("else", list_t{}), env);
 }