feat: infer generic type arguments (make the turbofish optional)

[math3usmartins]

Makes the ::<> turbofish optional wherever a generic call's or new's type arguments are determined by the values passed.

[greptile-apps]

Greptile Summary

This PR makes the ::<> turbofish optional for generic calls (new, free-function, static-method, and instance-method) by inferring type arguments from the call-site argument types. Inference is conservative and sound: it unifies parameter declared types against concrete argument types (literals, new X(...), $this->prop, typed parameters, and — in the method compiler — tracked local/return types), and falls back to the existing explicit-turbofish requirement when it cannot produce a complete, unambiguous, concrete tuple.

• NewInferencePass (bottom-up leaveNode traversal) annotates bare new expressions before the instantiation collector sees them; GenericMethodCompiler gains a parallel inferCallTypeArgs/typeOf path for function, static, and instance method calls.
• TypeInference is a pure unifier extracted into its own class, covering conflict detection, subtype threading via TypeHierarchy, prefix/gap rules, and a pairArgsToParams helper that mirrors PHP's own named/positional/spread/variadic argument rules.
• Existing tests whose "missing type argument" cases relied on inferable calls were updated to use patterns where T appears only in the return type (not inferable from arguments), preserving the error-path coverage.

Confidence Score: 5/5

Safe to merge. The inference implementation is conservative and sound: it only annotates a bare call when it can fully prove a concrete type tuple, and falls back to the existing explicit-turbofish requirement otherwise, so no existing behavior changes for calls where inference cannot determine the type.

The previous review's concern about nested new being processed top-down is directly addressed by moving inference to leaveNode, with an explanatory comment. The pure TypeInference class is well unit-tested with every fallback case covered, and the end-to-end integration tests exercise compile, check, and runtime execution. Existing error-path tests were correctly updated to use non-inferable patterns (T only in the return type) rather than deleted. No data loss, soundness, or pipeline ordering issues were found.

Files Needing Attention: No files require special attention. The core TypeInference.php, NewInferencePass.php, and the anonymous visitor additions to GenericMethodCompiler.php are all covered by unit and integration tests.

Important Files Changed

Filename	Overview
src/Transpiler/Monomorphize/TypeInference.php	New pure unifier class. Algorithm is sound: unify walks parameter/argument trees in lock-step, assemblePrefix enforces the "concrete-prefix + defaultable-tail / no-hole" invariant, and pairArgsToParams correctly mirrors PHP's named/positional/spread/variadic argument matching.
src/Transpiler/Monomorphize/NewInferencePass.php	New bottom-up leaveNode visitor that infers bare new type arguments. The previous-review P1 (nested generics inferred in wrong order due to top-down traversal) is properly resolved: inner new nodes are annotated before the outer reads them via LiteralTyper::typeOfNew.
src/Transpiler/Monomorphize/GenericMethodCompiler.php	Adds inferCallTypeArgs/typeOf/tryInferFuncCall to the anonymous visitor, wiring inference into both the static/instance method and free-function call paths. typeParamNames is correctly accumulated through scope nesting and restored from snapshots.
src/Transpiler/Monomorphize/LiteralTyper.php	New context-free ExpressionTyper for scalar literals, array literals, and new X(...). typeOfNew correctly returns null for bare (un-turbofished) generics and non-concrete args, so it is safe to call during bottom-up inference.
src/Transpiler/Monomorphize/Compiler.php	Inserts NewInferencePass::run() at the same pipeline position in both compile() and check() — after collectDefinitions, before collectInstantiations — keeping the two modes in parity.
src/Transpiler/Monomorphize/ExpressionTyper.php	New interface providing the seam between the pure inference core and its two implementations (LiteralTyper and the anonymous visitor in GenericMethodCompiler). Clean design.
test/Transpiler/Monomorphize/TypeInferenceTest.php	New unit tests for the pure inference core with comprehensive coverage of all fallback cases (conflict, ambiguous supertype path, hole, unwitnessed required param, unknown arg type).
test/Transpiler/Monomorphize/GenericInferenceIntegrationTest.php	New end-to-end integration tests covering free-function, static-method, and instance-method inference, plus enclosing type-parameter guard cases and runtime execution fixtures.
test/Transpiler/Monomorphize/EnclosingParamBoundIntegrationTest.php	Existing tests updated to keep the "missing type argument" error path exercised: changed from pick<T>(T $x) (now successfully inferred) to pick<T>(int $n): T (T only in return type, cannot be inferred).
test/fixture/check/bare_new_missing_arg/source/Box.xphp	Updated to remove the constructor T parameter so new Box() with no args cannot be inferred, keeping the missing-arg error fixture relevant post-inference.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Source .xphp files] --> B[collectDefinitions\nbuilt template registry]
    B --> C[NewInferencePass.run\nbottom-up leaveNode]
    C --> D{bare new X found?}
    D -- yes --> E[TypeInference.infer\nunify constructor params\nvs argument types]
    E --> F{complete concrete\ntuple inferred?}
    F -- yes --> G[attach ATTR_GENERIC_ARGS\n+ ATTR_TEMPLATE_FQN to Name node]
    F -- no --> H[leave new bare\nfall back to synthesis/error]
    G --> I[collectInstantiations\nsees annotated new as explicit turbofish]
    H --> I
    I --> J[GenericMethodCompiler.process\nrewrite call sites]
    J --> K{explicit turbofish\nor first-class callable?}
    K -- explicit --> L[dispatch as normal]
    K -- bare call --> M[inferCallTypeArgs\nvia typeOf ExpressionTyper]
    M --> N{inferred?}
    N -- yes --> O[annotate node\nrecurse rewriteFuncCall]
    N -- no --> P[reportMissingTurbofishArguments\nor pad all-defaults]
    O --> L

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