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$ swift --version
Apple Swift version 4.0.3 (swiftlang-900.0.74.1 clang-900.0.39.2)
Target: x86_64-apple-macosx10.9

Issue Description:

Description

When running the swift or swiftc command line tools on the following code, the compiler crashes while type-checking an init() declaration.

The following code declares a nested class inside a protocol, which is invalid.
But it also declares a subclass of the nested class, and the compiler exits with a Segmentation fault: 11.

protocol Proto {
  class Nested {
    // init() {}
  }
}

class Subclass: Proto.Nested {
  // override init() {}
}

The compiler type-checks init() even if not explicitly written in the code.

Steps to recreate the issue

Run one of the following commands :

swift file.swift

or

swiftc file.swift

Notes

I have not tested if the issue exists when running/compiling the code in an Xcode project or Playground.

I joined the stack trace produced (stderr.txt).