Fix compile error with single explicit assert in switch expression branch (#1845)

docs/release_notes.adoc

@@ -28,8 +28,9 @@ include::include.adoc[]
 * Fix mocking issue with the ByteBuddy MockMaker when using multiple classloaders in Java 21 spockIssue:2017[]
 * Fix mocking of final classes via `@SpringBean` and `@SpringSpy` spockIssue:1960[]
 * Size of data providers is no longer calculated multiple times but only once
-* Fix exception when using `@RepeatUntilFailure` with a data provider with unknown iteration amount. spockPull:2031[]
+* Fix exception when using `@RepeatUntilFailure` with a data provider with unknown iteration amount spockPull:2031[]
 * Clarified documentation about data providers and `size()` calls spockIssue:2022[]
+* Fix compile error with single explicit assert in switch expression branch spockIssue:1845[]
 
 == 2.4-M4 (2024-03-21)
 

spock-core/src/main/java/org/spockframework/compiler/AbstractDeepBlockRewriter.java

@@ -149,6 +149,16 @@
       currSpecialMethodCall = NoSpecialMethodCall.INSTANCE; // unrelated closure terminates currSpecialMethodCall scope
     }
     try {
+      Statement code = expr.getCode();
+      // the code of a closure is not necessarily a block statement but could for example
+      // also be a single assert statement in cases like `case 3 -> assert 1 == 1`
+      // to uniformly treat the closure code as block statement and later be able to
+      // add statements to it, wrap non-block statements in a block statement here
+      if (!(code instanceof BlockStatement)) {
+        BlockStatement block = new BlockStatement();
+        block.addStatement(code);
+        expr.setCode(block);
+      }
       doVisitClosureExpression(expr);
     } finally {
       currClosure = oldClosure;

spock-specs/src/test-groovy-ge-4.0/groovy/org/spockframework/smoke/condition/ConditionG4Spec.groovy

@@ -13,4 +13,14 @@ class ConditionG4Spec extends Specification {
     (0..<5) == [0, 1, 2, 3, 4]
     (0<..<5) == [1, 2, 3, 4]
   }
+
+  @Issue("https://github.com/spockframework/spock/issues/1845")
+  def "explicit assert in switch expression"() {
+    expect:
+    def b = 3
+    !!switch (b) {
+      case 3 -> assert 1 == 1
+      default -> assert 1 == 1
+    }
+  }
 }