Immediately receiving "error: fatalError" with "The compiler is unable to type-check this expression in reasonable time"

[art-divin]

Description

While tinkering with swift-syntax I have changed the signature of the following method:

 private func visitImpl<NodeType: SyntaxProtocol>(
    _ node: Syntax,
    _ nodeType: NodeType.Type,
    _ visit: (NodeType) -> SyntaxVisitorContinueKind,
    _ visitPost: (NodeType) -> Void
  ) {
    let node = node.cast(NodeType.self)
    let needsChildren = (visit(node) == .visitChildren)
    // Avoid calling into visitChildren if possible.
    if needsChildren && !node.raw.layoutView!.children.isEmpty {
      visitChildren(node)
    }
    visitPost(node)
  }

to variant without generics:

private func visitImpl(
    _ node: Syntax,
    _ nodeType: SyntaxProtocol.Type,
    _ visit: (SyntaxProtocol) -> SyntaxVisitorContinueKind,
    _ visitPost: (SyntaxProtocol) -> Void
  ) {
    let node = node.cast(nodeType)
    let needsChildren = (visit(node) == .visitChildren)
    // Avoid calling into visitChildren if possible.
    if needsChildren && !node.raw.layoutView!.children.isEmpty {
      visitChildren(node)
    }
    visitPost(node)
  }

I then immediately receive "error: fatalError" with "The compiler is unable to type-check this expression in reasonable time"

Steps to reproduce

1. clone https://github.com/art-divin/swift-syntax/ and checkout branch "resolve-compilation-time-issues" with commit 16c35c5ffb529185adf1af8f0d3fe38c96b89ecb
2. cd into cloned directory
3. swift build to see error: fatalError or use Xcode to see The compiler is unable to type-check this expression in reasonable time

Expected behavior

Code should compile

Environment

• Swift compiler version info swift-driver version: 1.87.1 Apple Swift version 5.9 (swiftlang-5.9.0.128.108 clang-1500.0.40.1)
• Xcode version info Xcode 15.0.1 (Build version 15A507)
• Deployment target: Target: arm64-apple-macosx14.0

[art-divin]

👋🏻

Update

I have looked into it, and the error happens because there are too many overrides of the method, thus the condition in ConstraintSystem.cpp on line 4361:

if (isTooComplex(viable))
      return SolutionResult::forTooComplex(getTooComplexRange());

fails and returns that error.

In my opinion, the error is not the one developer can expect, i.e. the message should state that what TypeChecker says in the log:

...
Constraint restrictions:
      AccessorDeclListSyntax.Type to any SyntaxProtocol.Type is [metatype-to-existential-metatype]
    Trailing closure matching:
      locator@0x140eff900 [UnresolvedDot@/Users/art-divin/Documents/Projects/SwiftTypeChecker/swift-syntax/Sources/SwiftSyntax/generated/SyntaxVisitor.swift:7069:12 → apply argument]: forward
      locator@0x140effa90 [UnresolvedDot@/Users/art-divin/Documents/Projects/SwiftTypeChecker/swift-syntax/Sources/SwiftSyntax/generated/SyntaxVisitor.swift:7069:61 → apply argument]: forward
      locator@0x119199510 [UnresolvedDot@/Users/art-divin/Documents/Projects/SwiftTypeChecker/swift-syntax/Sources/SwiftSyntax/generated/SyntaxVisitor.swift:7069:73 → apply argument]: forward
      locator@0x1191ae608 [Call@/Users/art-divin/Documents/Projects/SwiftTypeChecker/swift-syntax/Sources/SwiftSyntax/generated/SyntaxVisitor.swift:7069:12 → apply argument]: forward
    Fixes:
      [fix: allow function type mismatch] @ locator@0x11a88f2c8 [Call@/Users/art-divin/Documents/Projects/SwiftTypeChecker/swift-syntax/Sources/SwiftSyntax/generated/SyntaxVisitor.swift:7069:12 → apply argument → comparing call argument #2 to parameter #2]
...

Meaning that there were no candidates found in overrides which satisfy such type constraint:
AccessorDeclListSyntax.Type to any SyntaxProtocol.Type

[art-divin]

👋🏻 Hi,

Update

After setting unsigned ExpressionTimeoutThreshold = 600 * 1024; unsigned SolverMemoryThreshold = std::numeric_limits<unsigned int>::max(); and unsigned SolverBindingThreshold = std::numeric_limits<unsigned int>::max();, I was able to let the compiler run as long as it needed to calm down and generate the expected error, i.e. the error I would expect to see in Xcode as a developer, rather than the "reasonable time" error (Kind::TooComplex).

SyntaxVisitor.swift:7069:12: error: no 'visit' candidates produce the expected type '(any SyntaxProtocol) -> SyntaxVisitorContinueKind' for parameter #2
      self.visitImpl2($0, AccessorDeclListSyntax.self, self.visit, self.visitPost)

I am unsure how to make it that with the existing constraints for SolverMemoryThreshold (512 * 1024 * 1024), SolverBindingThreshold (1024 * 1024), ExpressionTimeoutThreshold (600), the compiler would still generate the expected error rather that "timing out" due to the number of overloaded functions.

[art-divin]

👋🏻 Update

I have found that SolverMemoryThreshold is the limiting factor, and so, with the number of overrides - 558 - CSSolver.cpp cannot find neither the resolution, nor the correct error message - that no candidates with the types were found. If to set SolverMemoryThreshold to std::numeric_limits<unsigned int>::max();, then the process ends without the SolutionResult::TooComplex result.