UNDF: UNDF-2026-000000545

swift-0001: QualifiedLookupRequest — O(2^D) diamond re-traversal via unguarded protocol superclass push

CWE-407 — Algorithmic Complexity: O(2^D) recursive diamond re-traversal

Field	Value
ID	swift-0001
Severity	MEDIUM
Ecosystem	swift
Package	lib/AST
File	`lib/AST/NameLookup.cpp`
Lines	2858–2861
Complexity	O(2^D) on diamond protocol+class hierarchies
Hot path	qualified name lookup (every member access, every type check)

Defect

QualifiedLookupRequest::evaluate (NameLookup.cpp) walks a nominal type's inheritance graph via a BFS worklist. When the current node is a ProtocolDecl that has a class superclass, lines 2858–2861 push that superclass onto the stack without checking whether it was already visited:

// lib/AST/NameLookup.cpp lines 2858–2861 (DEFECT)
if (auto superclassDecl = protoDecl->getSuperclassDecl()) {
    visited.insert(superclassDecl);   // .second NOT checked
    stack.push_back(superclassDecl);  // unconditional push → O(2^D)
}

The identical operation 27 lines earlier (line 2833–2834, for the ClassDecl arm) is written correctly:

// line 2833–2834 (CORRECT — nearby, same function)
if (visited.insert(superclassDecl).second)
    stack.push_back(superclassDecl);

Diamond scenario

Consider a protocol diamond where two protocols both have the same class superclass Base:

protocol PA: Base {}   // PA.getSuperclassDecl() == Base
protocol PB: Base {}   // PB.getSuperclassDecl() == Base
protocol P: PA, PB {}  // inherits both

When looking up a member on P:

BFS pops P → enqueues PA, PB via addNominalType (correctly guarded).
BFS pops PA → protoDecl->getSuperclassDecl() returns Base. visited.insert(Base) succeeds, Base pushed. stack = [PB, Base]
BFS pops Base → processed normally.
BFS pops PB → protoDecl->getSuperclassDecl() returns Base again. visited.insert(Base) silently returns {iter, false} (already present) but the return value is ignored → Base pushed again. stack = [Base] — Base re-processed.

For a depth-D diamond this re-visits Base 2^D times.

Hot path: QualifiedLookupRequest is evaluated on every qualified member expression (.foo, .bar), every protocol conformance check, and every type-checker constraint solve. In large codebases with deep protocol hierarchies this fires frequently per compilation.

Fix

// lib/AST/NameLookup.cpp lines 2858–2861 (AFTER — O(D))
if (auto superclassDecl = protoDecl->getSuperclassDecl()) {
    if (visited.insert(superclassDecl).second)   // guard: only if newly inserted
        stack.push_back(superclassDecl);
}

One-line change matching the existing correct pattern on line 2833.

Speedup

Diamond depth (D)	Shared superclass visits (before)	After	Speedup
2	2	1	2×
5	16	1	16×
10	512	1	512×
15	16,384	1	16,384×

(Speedup is specifically for the superclass re-traversal component; total lookup speedup depends on hierarchy breadth.)

Context

// Full function context: lib/AST/NameLookup.cpp
QualifiedLookupResult
QualifiedLookupRequest::evaluate(Evaluator &eval, const DeclContext *DC,
                                 SmallVector<NominalTypeDecl *, 4> typeDecls,
                                 DeclNameRef member, NLOptions options) const {
  SmallVector<NominalTypeDecl *, 4> stack;
  llvm::SmallPtrSet<NominalTypeDecl *, 4> visited;    // line 2753

  auto addNominalType = [&](NominalTypeDecl *nominal) {
    if (!visited.insert(nominal).second)               // CORRECT guard
      return false;
    stack.push_back(nominal);
    return true;
  };
  ...
  while (!stack.empty()) {
    auto current = stack.back();
    stack.pop_back();
    ...
    if (auto classDecl = dyn_cast<ClassDecl>(current)) {
      if (visitSuperclass) {
        if (auto superclassDecl = classDecl->getSuperclassDecl())
          if (visited.insert(superclassDecl).second)   // line 2833: CORRECT
            stack.push_back(superclassDecl);
      }
    }
    ...
    if (auto *protoDecl = dyn_cast<ProtocolDecl>(current)) {
      if (!sawClassDecl) {
        if (auto superclassDecl = protoDecl->getSuperclassDecl()) {
          visited.insert(superclassDecl);              // line 2860: DEFECT — .second not checked
          stack.push_back(superclassDecl);             // line 2861: unconditional push
        }
      }
    }
  }
}

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