Daniel Sun created GROOVY-12446:
-----------------------------------
Summary: STC: stop writing UnionTypeClassNode into inferred-return
signatures
Key: GROOVY-12446
URL: https://issues.apache.org/jira/browse/GROOVY-12446
Project: Groovy
Issue Type: Improvement
Reporter: Daniel Sun
h2. Context
{{UnionTypeClassNode}} is a checker-only {{ClassNode}}. It represents a flow
type such as {{x instanceof A}} or {{x instanceof B}}. Its Javadoc says it must
not leak outside the type checker.
{{SignatureCodecVersion1}} encodes an inferred return type as a Base64
{{DataOutputStream}} payload for {{@TypeChecked.TypeCheckingInfo}}. The first
UTF string in that payload is the simple name of the stored {{ClassNode}}
subclass.
GROOVY-12445 locked the unknown-version rule and left union payload layout as
separate work. The compiler has not written {{@TypeCheckingInfo}}
inferred-return annotations since GROOVY-6401. There is no production reader
today. The codec still has to define what that payload may contain.
h2. Problem
{{doEncode}} wrote the tag {{UnionTypeClassNode}}, then the arm count, then
each arm. {{doDecode}} rebuilt a {{UnionTypeClassNode}}. That round-trips a
checker-only node through a class-file annotation.
A union name is synthetic and is not a binary name. A later reader that treated
the decoded node as a loadable type would see a name {{Class.forName}} cannot
resolve.
h2. Contract
Version {{1}} stays version {{1}}. Newly encoded signatures never start with
the {{UnionTypeClassNode}} tag.
On encode, a union is replaced with a type that has a stable binary name, then
that type is encoded:
* the lowest upper bound of the arms, unwrapped until a real class or interface
remains
* {{Object}} if there is no such type: empty arms, a
{{LowestUpperBoundClassNode}} whose superclass is {{Object}}, or a name that is
empty or starts with '<'
A {{LowestUpperBoundClassNode}} supplied directly still round-trips, including
its interfaces. The primitive encoding tail (array flag, type descriptor,
generics) is unchanged.
On decode, a historical payload whose first tag is {{UnionTypeClassNode}} is
still consumed. The arms are decoded, then widened by the same rule. An arm
count of {{0}} or less decodes as {{Object}}.
||Input||Encode writes||Decode returns||
|disjoint types, e.g. {{String}} and {{List}}|{{Object}}; first tag is not
{{UnionTypeClassNode}}|{{Object}}|
|{{RuntimeException}} and {{Exception}}|{{Exception}}|{{Exception}}|
|two types that share one interface|that interface|that interface|
|historical {{UnionTypeClassNode}} tag|(read path only)|same widening|
|direct {{LowestUpperBoundClassNode}}|LUB tag, as today|same LUB node|
Callers of {{SignatureCodecVersion1.decode}} that expected a union node now
receive the shared supertype. Method descriptors are unchanged, so {{japicmp}}
does not report this.
h2. Touch points
* {{org.codehaus.groovy.transform.stc.SignatureCodecVersion1}} — encode and
historical decode
*
{{src/test/groovy/org/codehaus/groovy/transform/stc/SignatureCodecTest.groovy}}
— stable-bound and historical-payload tests
* {{COMPATIBILITY.md}} — behavioural-change note
h2. Scope
This issue covers union payload layout only. The unknown-version rule is
GROOVY-12445. Rewiring {{addTypeCheckingInfoAnnotation}} onto
{{startMethodInference}}, and a new compiler diagnostic, remain separate work.
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