RFR: 8339296: Record deconstruction pattern in switch fails to compile [v2]

[Jan Lahoda]

> Consider code like this:
> 
>     int nestedSwitchesInArgumentPosition(Object o1) {
>         return id(switch (o1) {
>             case R(var o2) -> switch (o2) {
>                 case R(String s) -> s;
>                 default -> "n";
>             };
>             default -> "";
>         });
>     }
> 
>     int id(String s) {
>         return s.length();
>     }
> 
>     int id(int i) {
>         return i;
>     }
> 
> 
> Compiling this fails with a `StackOverflowError`, because:
>  - there are speculative attribution happening for the switch expressions,
>  - "completes normally" is computed during these speculative attribution, which have parts of the AST unfilled - specifically the nested `case R(String s)`
>  - so, `Attr.PostAttrAnalyzer` fills in the missing types. In particular, the `String s` binding will get the `Symtab.unknownType`.
>  - `Flow.makePatternDescription` will eventually ask `Types.isSubtype(..., unknownType)`. This is guaranteed to fail with the `StackOverflowError`, as:
>  - `unknownType.isPartial()` returns `true`, so `Types.isSubtype(t, s)` (`s` is the `unknownType`) calls `Types.isSuperType(s, t)`, and `Types.isSuperType(s, t)` does not contain any special handling for the `unknownType`, so it will delegate to `Types.isSubtype(t, s)`, leading to an infinite recursion.
> 
> It may be possible to side-step the issue by not computing the completes normally property during speculative attribution, or move that computation outside of `Attr`. It may be good to do, for performance reasons.
> 
> But, that does not seem to solve the underlying issue with `unknownType`. Many methods in `Types` misbehave in weird ways when the `unknownType` is passed to them.
> 
> The proposal herein is to attempt to resolve that. In particular, the `UnknownType` is proposed to extend the `ErrorType`, dropping the (internal) `UNKNOWN` type tag. The uses of the `UNKNOWN` type tag appear to be equivalent to handling of the `ERROR` type tag anyway. And the special handling of the `unknownType` appear to usually use ` == syms.unknownType`:
> https://github.com/openjdk/jdk/blob/0c36177fead8b64a4cee9da3c895e3799f8ba231/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Types.java#L904
> 
> After this change, the `unknownType` should behave as an erroneous type, unless specifically requested otherwise.
> 
> The intent is that the public API behavior for the `unknownType` should remain the same.

Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:

  Cleanup - removing the unknownType.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20990/files
  - new: https://git.openjdk.org/jdk/pull/20990/files/71942f9d..4ef410fe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20990&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20990&range=00-01

  Stats: 21 lines in 2 files changed: 0 ins; 20 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20990.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20990/head:pull/20990

PR: https://git.openjdk.org/jdk/pull/20990