#6189: Add first unit test checking the root cause

test/DeclManager.cpp

@@ -1724,4 +1724,30 @@ decl/temporaryGame/1
     fs::remove_all(temporaryFmPath);
 }
 
+// #6189: Assigning a new syntax block on a previously missing declaration didn't clear
+// the internal _syntaxBlockInvalidated flag of EditableDeclarations, which caused
+// the new syntax block contents to be immediately overwritten by generateSyntax()
+TEST_F(DeclManagerTest, AssigningSyntaxBlockOverridesModification)
+{
+    GlobalDeclarationManager().registerDeclType("testdecl", std::make_shared<TestDeclarationCreator>());
+    GlobalDeclarationManager().registerDeclFolder(decl::Type::TestDecl, TEST_DECL_FOLDER, ".decl");
+
+    // Find an existing decl
+    auto decl = std::static_pointer_cast<TestDeclaration>(
+        GlobalDeclarationManager().findDeclaration(decl::Type::TestDecl, "decl/numbers/3"));
+
+    decl->setKeyValue("description", "This decl is missing and has been modified");
+    decl->setFileInfo(vfs::FileInfo(TEST_DECL_FOLDER, "_autogenerated.decl", vfs::Visibility::NORMAL));
+
+    auto newSyntax = decl->getBlockSyntax();
+    newSyntax.contents = R"(
+    "description" "assigned"
+)";
+
+    // Assigning a new syntax block after parsing should work even if the decl has been modified
+    decl->setBlockSyntax(newSyntax);
+
+    EXPECT_EQ(decl->getKeyValue("description"), "assigned") << "Assigned syntax block didn't take effect";
+}
+
 }