Fix regression in blocks returning multiple meshes

Author: nicklockwood

ShapeScript/Interpreter.swift

@@ -857,7 +857,12 @@ extension Definition {
                                     "value cannot be assigned a name"
                             )
                         }
-                    } else if context.name.isEmpty {
+                    } else if context.name.isEmpty,
+                              // Manage backwards compatibility for blocks that return
+                              // multiple meshes to be used inside difference block
+                              !children.contains(where: { $0.type == .mesh }) ||
+                              children.contains(where: { ![.mesh, .path].contains($0.type) })
+                    {
                         return .tuple(children.map {
                             switch $0 {
                             case let .path(path):

ShapeScriptTests/InterpreterTests.swift

@@ -1388,6 +1388,19 @@ class InterpreterTests: XCTestCase {
         }
     }
 
+    func testBlockReturnsGroupedMeshes() throws {
+        let program = try parse("""
+        define foo {
+            cube { size 0.5 }
+            cube { size 0.8 }
+        }
+        foo
+        """)
+        let context = EvaluationContext(source: program.source, delegate: nil)
+        XCTAssertNoThrow(try program.evaluate(in: context))
+        XCTAssertEqual(context.children.count, 1)
+    }
+
     // MARK: Ranges
 
     func testRange() {