Why does Composer say "Conclusion: don't install" when (seemingly) no obstacles are present?

Question

The following composer.json:

{
    "type": "project",
    "minimum-stability": "dev",
    "require": {
        "jasny/bootstrap": ">=3.1.3",
        "2amigos/yii2-file-input-widget": "*"
    }
}

leads to the following output of composer update:

Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.                               

  Problem 1
    - 2amigos/yii2-file-input-widget 0.1.2 requires jasny/bootstrap v3.1.0 -> satisfiable by jasny/bootstrap[v3.1.0] but these conflict with your requirements or minimum-stability.
    - 2amigos/yii2-file-input-widget 0.1.1 requires jasny/bootstrap v3.1.0 -> satisfiable by jasny/bootstrap[v3.1.0] but these conflict with your requirements or minimum-stability.
    - 2amigos/yii2-file-input-widget 0.1.0 requires jasny/bootstrap v3.1.0 -> satisfiable by jasny/bootstrap[v3.1.0] but these conflict with your requirements or minimum-stability.
    - 2amigos/yii2-file-input-widget 1.0.0 requires jasny/bootstrap ~3.1.0 -> satisfiable by jasny/bootstrap[v3.1.3].
    - 2amigos/yii2-file-input-widget 1.0.1 requires jasny/bootstrap ~3.1.0 -> satisfiable by jasny/bootstrap[v3.1.3].
    - 2amigos/yii2-file-input-widget 1.0.2 requires jasny/bootstrap ~3.1.0 -> satisfiable by jasny/bootstrap[v3.1.3].
    - 2amigos/yii2-file-input-widget dev-master requires jasny/bootstrap ~3.1.0 -> satisfiable by jasny/bootstrap[v3.1.3].
    - 2amigos/yii2-file-input-widget 1.0.x-dev requires jasny/bootstrap ~3.1.0 -> satisfiable by jasny/bootstrap[v3.1.3].
    - Conclusion: don't install jasny/bootstrap v3.1.3
    - Installation request for 2amigos/yii2-file-input-widget * -> satisfiable by 2amigos/yii2-file-input-widget[0.1.0, 0.1.1, 0.1.2, 1.0.0, 1.0.1, 1.0.2, dev-master, 1.0.x-dev].

Why?

---

Ok, I understand, why versions 0.1.0–0.1.2 of 2amigos/yii2-file-input-widget can't be installed — because they require jasny/bootstrap of exact version 3.1.0, which conflicts with >=3.1.3 requirement in composer.json (and therefore composer clarifies: but these conflict with your requirements or minimum-stability).

But, from common-sense view, composer can install version 1.0.0 or later of 2amigos/yii2-file-input-widget (which requires jasny/bootstrap of version ~3.1.0, which is satisfiable by 3.1.3). Neither I see any obstacles for installing 2amigos/yii2-file-input-widget 1.0.0+ together with jasny/bootstrap 3.1.3, nor composer writes any explicit clarification about it. Still it says: Conclusion: don't install jasny/bootstrap v3.1.3 — why?

Workaround 1

It seems it works with "prefer-stable": true.

Workaround 2

I actually found the other workaround some time ago: I replace "2amigos/yii2-file-input-widget": "*" with "2amigos/yii2-file-input-widget": "1.0.2", do composer update and then replace it back and do composer update again — and it works.

---

The question is why it works in so strange way: why it works with more restrictions, but fails with less restrictions (with no clarification). E.g. if it works with 1.0.2 (or with prefer-stable) works — then why it doesn't with * (or without prefer-stable)?

Answer 1

Change the order and it should work, e.g.

{
    "minimum-stability": "dev",
    "require": {
        "2amigos/yii2-file-input-widget": "*",
        "jasny/bootstrap": ">=3.1.3"
    }
}

Why? I don't know. Most likely a Composer's bug.

I've tested two configurations with different order on the empty folder using composer install command (Composer v1.6.3), the original order fails, however the order above works. I've reported the issue at GH-7215.