Optimize distributive conditional type instantiation for trivial extends types

[DukeDeSouth]

Human View

Summary

Adds a fast path in getConditionalTypeInstantiation for distributive conditional types where the extends type resolves to never, any, or unknown. In these cases, the branch outcome is trivially determinable for all union members, so we skip the expensive per-member getConditionalType call (which involves type instantiation, permissive instantiation, and structural comparison) and directly apply the predetermined branch.

The Problem

When distributing a conditional type like Exclude<BigUnion, never> (= T extends never ? never : T) over a large union (e.g., React HTML props with 250+ members), the current code calls getConditionalType for each member of the union. Each call:

1. Instantiates the check type (+1 instantiateType call)
2. Instantiates the extends type (+1 instantiateType call)
3. Computes permissive instantiations
4. Runs isTypeAssignableTo structural comparison
5. Instantiates the result branch (+1 instantiateType call)

For extends never, every member goes to the false branch — all this work is redundant. In complex HOC patterns (withRouter + generic enhance + React props), multiple layers of Omit/Pick/Exclude compound this, potentially exhausting the 5M instantiationCount budget and producing a false positive TS2589 error.

The Fix

Before distributing over union members, resolve the extends type. If it's:

• never: All non-never members go to false branch → apply false branch directly
• any/unknown: All members go to true branch → apply true branch directly

Safety guards (to prevent false negatives on genuinely infinite types):

• Only applies when !forConstraint (constraint resolution has different semantics)
• Only applies when len(root.inferTypeParameters) == 0 (infer types need inference logic)
• Only applies when distribution type is a union
• Falls through to the original getConditionalType path for all other cases
• Does not change any instantiation limits

Testing

• All existing submodule tests pass (0 regressions)
• limitDeepInstantiations still correctly errors on genuinely infinite types
• All recursiveConditionalTypes / recursiveConditionalCrash tests pass
• All conditionalType* and mappedType* tests pass

Fixes microsoft/TypeScript#34933
Related: #988, #2609

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AI View (DCCE Protocol v1.0)

Metadata

• Generator: Claude (Anthropic) via Cursor IDE
• Methodology: AI-assisted development with human oversight and review

AI Contribution Summary

• Solution design and implementation

Verification Steps Performed

1. Reproduced the reported issue
2. Analyzed source code to identify root cause
3. Implemented and tested the fix

Human Review Guidance

• Review the implementation for correctness and completeness
• Verify test coverage matches the described scenarios

Made with M7 Cursor

[DukeDeSouth]

@microsoft-github-policy-service agree