Fixes#1614.
## Context
A non-abstract `class` (or `module`) was allowed to declare `abstract`
properties and methods.
Because such an enclosing type is instantiable, an `abstract` member
there can never be guaranteed
an implementation — so the contradiction surfaced only as a runtime
error when the member was
accessed (`Cannot invoke abstract method`), or not at all.
This makes it a compile-time error to declare an `abstract` member
unless its enclosing class or
module is also `abstract`. This is consistent with how Pkl already
rejects instantiating an abstract
class, and mirrors how Java and Kotlin treat abstract members.
## Before
```pkl
class Foo {
abstract bar: Int
}
res = new Foo { bar = 5 } // evaluated successfully (should fail)
```
```pkl
class Foo {
abstract function bar(): Int
}
res = new Foo {} // evaluated successfully; res.bar() failed only at runtime
```
## After
```
–– Pkl Error ––
Cannot define an abstract member in a non-abstract class.
2 | abstract bar: Int
^^^^^^^^
at Foo
A member can only be `abstract` if its enclosing class is also `abstract`.
```
## Implementation
- `AstBuilder` now validates, while building the AST, that a
non-abstract class/module declares no
`abstract` members. The check runs in both `visitClass` and
`visitModule`, and the error points at
the `abstract` keyword.
- Adds the `abstractMemberInNonAbstractClass` error message.
## Scope: classes and modules
The issue describes classes; I applied the same rule to modules as well,
since a module is a class
in Pkl and a non-abstract module is likewise directly evaluatable. Happy
to narrow this to classes
only if you'd prefer — it's a one-line change either way.
The `moduleMethodModifiers` pkl-doc test fixture declared an abstract
method at non-abstract module
level (relying on the old behavior); it's updated to an `abstract
module`, and its expected
documentation output is regenerated.
## Tests
- New `LanguageSnippetTests` error cases: abstract property in a class,
abstract method in a class,
and abstract member in a module.
- `./gradlew build` passes (`pkl-core` and `pkl-doc` included).
---------
Co-authored-by: Vinayak <vinayak@vama.app>
Co-authored-by: Daniel Chao <daniel.h.chao@gmail.com>
