runtime error (panic) when flattening an included Taskfile with a default task defined

[mgbowman]

I'm excited for the new flatten feature that was recently merged to main and wanted to give it a try and came across this SIGSEGV panic...

# Taskfile.yml
version: '3'
includes:
  base: 
    taskfile: Taskfile.base.yml
    flatten: true

tasks:
  foo:
    - echo "foo"
---
# Taskfile.base.yml
version: '3'
tasks:
  default: # <-- this appears to be the culprit
    - echo "bar"

This panics ...

$ task -a
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x98 pc=0x865a21]

goroutine 6 [running]:
github.com/go-task/task/v3/taskfile/ast.(*Tasks).Merge(0xc00018c5a8, {{{0xc00011c740, 0x1, 0x1}, 0xc0001fa780}}, 0xc0001b25b0, 0xc00005e700?)
        /home/vscode/go/pkg/mod/github.com/go-task/task/v3@v3.38.1-0.20240826211905-a72b65b3b23c/taskfile/ast/tasks.go:122 +0x221
github.com/go-task/task/v3/taskfile/ast.(*Taskfile).Merge(0xc00018c500, 0xc00018cf00, 0xc0001b25b0)
        /home/vscode/go/pkg/mod/github.com/go-task/task/v3@v3.38.1-0.20240826211905-a72b65b3b23c/taskfile/ast/taskfile.go:58 +0x1eb
github.com/go-task/task/v3/taskfile/ast.(*TaskfileGraph).Merge.func1()
        /home/vscode/go/pkg/mod/github.com/go-task/task/v3@v3.38.1-0.20240826211905-a72b65b3b23c/taskfile/ast/graph.go:92 +0xdc
golang.org/x/sync/errgroup.(*Group).Go.func1()
        /home/vscode/go/pkg/mod/golang.org/x/sync@v0.8.0/errgroup/errgroup.go:78 +0x56
created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 1
        /home/vscode/go/pkg/mod/golang.org/x/sync@v0.8.0/errgroup/errgroup.go:75 +0x96

Looks like a nil pointer at taskfiles/ast/tasks.go:122

---

If I change the included task to bar ...

# Taskfile.base.yml
version: '3'
tasks:
  bar:
    - echo "bar"

...everything works as expected...

$ task -a
task: Available tasks for this project:
* bar:       
* foo: 

---

• Task version: v3.38.1-0.20240826211905-a72b65b3b23c (h1:01ZCV3qmpxFkx17tDEDO4qQ/2uzXorCUl3lDtl56nVk=)
• Operating system: Linux colima 6.5.0-15-generic #15-Ubuntu SMP PREEMPT_DYNAMIC Tue Jan 9 17:03:36 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux
• Experiments enabled: none

[vmaerten]

Hi!
Thanks for testing new features 🚀 and reporting this
I'll be fixed soon!