whitepaper: 366/178 — wave5 defect tables + PDF rebuild

defects/swift/patch/swift-0002-requirement-machine-set.md

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+# swift-0001: isInMinimizationDomain — O(P) linear scan in O(R) rule loop
+
+## Severity: HIGH
+
+## Location
+- `lib/AST/RequirementMachine/RewriteSystem.cpp:484`
+- Called from loops in:
+  - `lib/AST/RequirementMachine/HomotopyReduction.cpp:611,659,753`
+  - `lib/AST/RequirementMachine/MinimalConformances.cpp:326`
+  - `lib/AST/RequirementMachine/Diagnostics.cpp:349`
+  - `lib/AST/RequirementMachine/PropertyUnification.cpp:133`
+
+## Description
+`RewriteSystem::isInMinimizationDomain()` performs a linear `std::find` scan over
+`Protos` (an `ArrayRef<const ProtocolDecl *>`) on every call. This function is invoked
+inside loops over all rewrite `Rules` (size scales with protocol graph complexity).
+
+**Complexity:** O(R × P) where R = rule count, P = protocol count in connected component.
+For a deeply hierarchical protocol graph with N protocols, both R and P are O(N),
+yielding O(N²) total cost during requirement signature minimization.
+
+## Root Cause
+```cpp
+// RewriteSystem.cpp:478
+bool RewriteSystem::isInMinimizationDomain(const ProtocolDecl *proto) const {
+  // ...
+  if (std::find(Protos.begin(), Protos.end(), proto) != Protos.end())
+    return true;
+  return false;
+}
+```
+
+`Protos` is `ArrayRef<const ProtocolDecl *>` — a plain pointer array. `std::find` is O(P).
+Called inside every rule-scanning loop; no caching or set-based lookup.
+
+## Fix
+Replace `ArrayRef<const ProtocolDecl *> Protos` with `llvm::DenseSet<const ProtocolDecl *> ProtoSet`
+populated once at construction, or cache a `llvm::SmallPtrSet` alongside the ArrayRef.
+Then `isInMinimizationDomain` becomes an O(1) hash set lookup.
+
+```cpp
+// In RewriteSystem.h, add:
+llvm::SmallPtrSet<const ProtocolDecl *, 8> ProtoSet;
+
+// In RewriteSystem::init or wherever Protos is set:
+ProtoSet.insert(Protos.begin(), Protos.end());
+
+// In isInMinimizationDomain:
+bool RewriteSystem::isInMinimizationDomain(const ProtocolDecl *proto) const {
+  if (proto == nullptr && Protos.empty())
+    return true;
+  return ProtoSet.count(proto) > 0;
+}
+```
+
+## Impact
+Protocol-heavy Swift code (many protocols with complex inheritance) hits O(N²) during
+generic signature computation. Manifest as slow compilation on codebases with many protocols
+or deep protocol hierarchies (e.g., Combine/SwiftUI framework chains).