value expression as variability condition (#456)

src/resolver/solver.ts

@@ -770,11 +770,21 @@ export default class Solver {
          * Assume that expression is a value expression that returns a boolean
          * Thus, {@param expression} can be in reality also of type {@link ValueExpression}
          */
-        const result = this.evaluateValueExpression(expression, context)
+        const result = this.evaluateValueExpressionAsLogic(expression, context)
         assert.isBoolean(result)
         return this.transformLogicExpression(result, context)
     }
 
+    evaluateValueExpressionAsLogic(expression: ValueExpression, context: ExpressionContext): boolean {
+        const result = this.evaluateValueExpression(expression, context)
+
+        if (check.isBoolean(result)) return result
+        if (check.isString(result)) return true
+        if (check.isNumber(result)) return result > 0
+
+        throw new Error(`Unexpected value expression treated as logic "${utils.pretty(expression)}"`)
+    }
+
     evaluateValueExpression(expression: ValueExpression, context: ExpressionContext): InputAssignmentValue {
         if (check.isObject(expression) && !check.isArray(expression)) {
             if (check.isDefined(expression._cached_result)) return expression._cached_result