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Advanced DType Semantics

Status: canonical PLIB-211 / #3726 reference record, updated 2026-03-16 after landing the first bounded advanced-dtype semantics report in crates/psionic-core/src/lib.rs.

This document records the current bounded advanced-dtype semantics surface for Psionic.

Canonical Runner

Run the advanced-dtype harness from the repo root:

scripts/release/check-psionic-advanced-dtype-semantics.sh

What Landed

psionic-core now exposes:

  • ExtendedDType
  • ExtendedDTypeClass
  • DTypePromotionCaseResult
  • DTypeCastCaseResult
  • DTypeBackendCapabilityCaseResult
  • AdvancedDTypeSemanticsReport
  • builtin_advanced_dtype_semantics_report()

Current Honest Posture

Today Psionic has a first-class advanced-dtype vocabulary, but it does not claim full runtime closure for that vocabulary.

The bounded seeded surface now makes three things explicit:

  • selected promotion rules across wider integers, float8, half/bfloat, and complex numbers
  • selected cast rules, including explicit support for widening and explicit refusal for complex-to-real imaginary-drop paths
  • backend-family capability truth showing that richer dtypes can exist in meta execution while current runtime backends remain bounded to the compact DType subset

Why This Matters

This report prevents two failure modes:

  • pretending "advanced dtype support" exists just because the type names exist
  • smuggling richer dtype assumptions into later autocast, quantization, or export work without one typed truth surface

The point of this issue is to make richer dtype behavior machine-legible while keeping the current runtime boundary honest.