Looks like it's solving constraints in the typechecker:

  8140 swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*)  (in swift-frontend) + 125  [0x110560f9d]
    8140 (anonymous namespace)::StmtChecker::typeCheckASTNode(swift::ASTNode&)  (in swift-frontend) + 1043  [0x11055dcc3]
      8140 (anonymous namespace)::DeclChecker::visit(swift::Decl*)  (in swift-frontend) + 4497  [0x1104e0721]
        8140 swift::TypeChecker::typeCheckPatternBinding(swift::PatternBindingDecl*, unsigned int, swift::Type)  (in swift-frontend) + 250  [0x11049648a]
          8140 swift::TypeChecker::typeCheckBinding(swift::Pattern*&, swift::Expr*&, swift::DeclContext*, swift::Type, swift::PatternBindingDecl*, unsigned int)  (in swift-frontend) + 140  [0x1104962bc]
            8140 swift::TypeChecker::typeCheckExpression(swift::constraints::SolutionApplicationTarget&, swift::OptionSet<swift::TypeCheckExprFlags, unsigned int>)  (in swift-frontend) + 897  [0x110495e71]
              8140 swift::constraints::ConstraintSystem::solve(swift::constraints::SolutionApplicationTarget&, swift::FreeTypeVariableBinding)  (in swift-frontend) + 974  [0x11032cb1e]
                8140 swift::constraints::ConstraintSystem::solve(llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::FreeTypeVariableBinding)  (in swift-frontend) + 52  [0x11032d8b4]
                  8140 swift::constraints::ConstraintSystem::solveImpl(llvm::SmallVectorImpl<swift::constraints::Solution>&)  (in swift-frontend) + 372  [0x11032aa14]
                    8135 swift::constraints::ComponentStep::take(bool)  (in swift-frontend) + 2911  [0x1103393af]
                    + 4015 swift::constraints::ConstraintSystem::finalize()  (in swift-frontend) + 5258,5080,...  [0x110325a7a,0x1103259c8,...]
                    + 1819 swift::constraints::ConstraintSystem::finalize()  (in swift-frontend) + 5291  [0x110325a9b]