Assertion failed: (hasInterfaceType() && "No interface type was set"), function getInterfaceType, file /Volumes/Data/swift-public/swift/lib/AST/Decl.cpp, line 1784.

2.  While verifying VarDecl 'c' at <stdin>:6:7

@xedin, any ideas?

@belkadan Thanks, I will take a look, I think it's related to how we set interface types for parameters in closures, which are allowed to have no types...

@swift-ci create

The problem is that we type check capture lists before constraint solving, which is usually OK because they can only reference bindings in other statements and outer scopes. It's a problem in single-expression closures because we might not have assigned a type yet.

See TypeCheckConstraints.cpp,

      // For capture lists, we typecheck the decls they contain.
      if (auto captureList = dyn_cast<CaptureListExpr>(expr)) {
        // Validate the capture list.
        for (auto capture : captureList->getCaptureList()) {
          TC.typeCheckDecl(capture.Init, true);
          TC.typeCheckDecl(capture.Init, false);
          TC.typeCheckDecl(capture.Var, true);
          TC.typeCheckDecl(capture.Var, false);
        }
        return finish(true, expr);
      }

For correct behavior, we would type check the Init expression as part of solving the outer constraint system.

I mentioned that in the radar instead of JIRA 🙁

I looked at this further and it's pretty involved.

• Pre-check needs to walk into capture list initializer expressions when walking the outer expression.

• Constraint generation similarly needs to walk into capture list initializers, and set the type of the pattern and capture VarDecl to the result of generating constraints for the initializer.

• CSApply similarly needs to walk initializers.

In principle this is no different than how single-expression closures work, it just needs to be plumbed through in a few places.