Carry minted type identity through interface dispatch

baml_language/CHANGELOG.md

@@ -29,6 +29,8 @@ This changelog covers the independent `baml_language` release line. It does not
 
 ### Fixes
 
+- Carried a constructed type's identity through interface dispatch: `type.of<T>()` inside an implements-block or inherited default method is now the exact `type` value the caller minted for a `reflect.class.new` or `reflect.enum.new` declaration, instead of an `==`-distinct re-derivation of the same type. Identity-keyed patterns — a registry keyed by type, a comparison against a stored type — match inside an impl body instead of silently missing. Statically instantiated type parameters, derived types (`t.array()`, `type.meta(…)`), and declarations from a compiled `reflect.Package` keep re-deriving their value, so they are unaffected. ([#4516](https://github.com/BoundaryML/baml/pull/4516)) - Antonio Sarosi
+- Carried a runtime-minted type's identity through interface dispatch: `type.of<T>()` inside an implements-block or inherited default method is now the exact `type` value the caller minted, instead of an `==`-distinct re-derivation of the same type. Identity-keyed patterns — a registry keyed by type, a comparison against a stored type — match inside an impl body instead of silently missing. Statically instantiated type parameters and derived types (`t.array()`, `type.meta(…)`) are unaffected. ([#4516](https://github.com/BoundaryML/baml/pull/4516)) - Antonio Sarosi
 - Rejected an inline `unreflect(value)` type argument whose runtime type would escape into the expression's published type — `Agent<unreflect(t)>.new(...)`, `Holder<unreflect(t)> { ... }`, any callee returning `Wrapper<T>` or `T[]` — with E0168, which suggests naming it first (`type Out = unreflect(value)`). The class-literal spelling had reached MIR lowering with an error-recovery type argument and panicked. Results typed by the parameter itself (`sap.parse<unreflect(t)>`, `Fn$parse<unreflect(t)>`) and results that never mention it (`Fn$render_prompt<unreflect(t)>`) are unchanged. ([#4518](https://github.com/BoundaryML/baml/pull/4518)) - Antonio Sarosi
 - Fixed optional-chained method calls dropping their type arguments: `x?.m<T>()` — along with inferred, class-generic, and interface-dispatched forms — now seeds the callee's type-argument frame exactly like `x.m<T>()` instead of failing at runtime with an internal frame type-arg error. An optional-chained call to a builtin I/O method (`file?.text()`) also takes the same inline sys-op path as the plain spelling, so its omitted defaulted arguments are materialized rather than reaching the engine unset. ([#4495](https://github.com/BoundaryML/baml/pull/4495)) - Antonio Sarosi
 - Rejected class-literal construction of the builtin companion carriers (`baml.Int`, `baml.Map`, `baml.String`, and the other zero-field method carriers) with E0166 instead of building a meaningless empty instance; `baml.Map {}` and `baml.Array {}` had reached MIR lowering with unsolved phantom type arguments and panicked. ([#4493](https://github.com/BoundaryML/baml/pull/4493)) - Antonio Sarosi