Open Radar

 
Summary:
Recursive type constraints seem to kill Swift in a variety of interesting ways.


Steps to Reproduce:
1. This:
protocol AlgebraicType {
	typealias Term
}

enum Expr<T>: AlgebraicType {
	typealias Term = T

	case Var(String)
	case Abstraction(String, T)
	case Application(T, T)
}

struct Term<E: AlgebraicType where Expr.Term == Term> {
	let expression: E
}


Expected Results:
should not crash the compiler


Actual Results:
but totally does:
term.swift:14:36: error: reference to generic type 'Expr' requires arguments in <...>
struct Term<E: AlgebraicType where Expr.Term == Term> {
                                   ^
term.swift:6:6: note: generic type 'Expr' declared here
enum Expr<T>: AlgebraicType {
     ^
0  swift                    0x000000010c4dde0b llvm::sys::PrintStackTrace(__sFILE*) + 43
1  swift                    0x000000010c4de54b SignalHandler(int) + 379
2  libsystem_platform.dylib 0x00007fff84de5f1a _sigtramp + 26
3  libc++.1.dylib           0x00007fff8e9dffff std::__1::basic_string<wchar_t, std::__1::char_traits<wchar_t>, std::__1::allocator<wchar_t> >::__get_pointer() + 19
4  swift                    0x000000010ac96487 swift::DeclContext::lookupQualified(swift::Type, swift::DeclName, unsigned int, swift::LazyResolver*, llvm::SmallVectorImpl<swift::ValueDecl*>&) const + 71
5  swift                    0x000000010ac95894 swift::UnqualifiedLookup::UnqualifiedLookup(swift::DeclName, swift::DeclContext*, swift::LazyResolver*, bool, swift::SourceLoc, bool, bool) + 2228
6  swift                    0x000000010ac98074 swift::UnqualifiedLookup::UnqualifiedLookup(swift::DeclName, swift::DeclContext*, swift::LazyResolver*, bool, swift::SourceLoc, bool, bool) + 36
7  swift                    0x000000010ab9109f swift::TypeChecker::lookupUnqualified(swift::DeclContext*, swift::DeclName, swift::SourceLoc, swift::OptionSet<swift::NameLookupFlags, unsigned int>) + 223
…
Stack dump:
0.	Program arguments: /Applications/Xcode-7-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift -frontend -c -primary-file term.swift -target x86_64-apple-darwin14.3.0 -enable-objc-interop -module-name term -o /var/folders/9y/f7n4rvx901nctm_5d16c1wm40000gn/T/term-a36ff2.o 
1.	While type-checking 'Term' at term.swift:14:1
2.	While resolving type E at [term.swift:15:18 - line:15:18] RangeText="E"
<unknown>:0: error: unable to execute command: Segmentation fault: 11
<unknown>:0: error: swift frontend command failed due to signal (use -v to see invocation)


Regression:
This also crashed Swift 1.2 but with a shorter backtrace.

Notes:
N/A