We currently have three different implementations of the
`resolveThemeValue()` Theme method:
1. The _core_ version which only resolves based on the exact CSS
variable name. This is the v4-only version.
2. A _compat light_ version which attempts to translate a dot-notation
keypath to a CSS variable name.
3. A _full compat_ version which resolves the legacy theme object which
is used whenever a `@plugin` or `@config` is added.
Unfortunately, there are some issues with this approach that we've
encountered when testing the CSS `theme()` function while upgrading
Tailwind Templates to v4 using our upgrading toolkit.
1. The _compat light_ resolver was trying to convert `theme(spacing.1)`
to tuple syntax. Tuples require a nested property access, though, and
instead this should be convert to `theme(--spacing-1)`.
2. We currently only load _full compat_ version if `@plugin` or
`@config` directives are used. It is possible, though, that we need the
full compat mapping in other cases as well. One example we encountered
is `theme(fontWeight.semibold)` which maps to a dictionary in the
default theme that that we do not have an equivalent for in v4
variables.
To fix both issues, we decided to remove the _compat light_ version of
the `theme()` function in favor of adding this behavior to an upgrade
codemod. Instead, the second layer now only decides wether it can use
the _core_ version or wether it needs to upgrade to the _full compat_
version. Since typos would trigger a _hard error_, we do not think this
has unintended performance consequences if someone wants to use the
_core_ version only (they would get an error regardless which they need
to fix in order to continue).
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