Substitution can fail in some invalid cases.
Substitution can fail in some invalid cases.

Arguably, this code should not be run in invalid cases,
but that seems difficult to enforce given the numnber of paths
through the type-checker.  It's better to simply be more
conservative here.

huonw/swift,CodaFi/swift,rudkx/swift,austinzheng/swift,bitjammer/swift,JGiola/swift,modocache/swift,roambotics/swift,frootloops/swift,xedin/swift,xedin/swift,uasys/swift,apple/swift,kstaring/swift,harlanhaskins/swift,nathawes/swift,IngmarStein/swift,codestergit/swift,johnno1962d/swift,parkera/swift,modocache/swift,austinzheng/swift,zisko/swift,xedin/swift,austinzheng/swift,arvedviehweger/swift,djwbrown/swift,sschiau/swift,danielmartin/swift,SwiftAndroid/swift,ken0nek/swift,frootloops/swift,practicalswift/swift,practicalswift/swift,lorentey/swift,apple/swift,natecook1000/swift,calebd/swift,karwa/swift,JGiola/swift,parkera/swift,huonw/swift,tardieu/swift,gottesmm/swift,jckarter/swift,apple/swift,jmgc/swift,djwbrown/swift,deyton/swift,amraboelela/swift,bitjammer/swift,karwa/swift,return/swift,harlanhaskins/swift,milseman/swift,rudkx/swift,ahoppen/swift,gribozavr/swift,austinzheng/swift,return/swift,jtbandes/swift,gribozavr/swift,jckarter/swift,JaSpa/swift,ben-ng/swift,devincoughlin/swift,jopamer/swift,ahoppen/swift,return/swift,russbishop/swift,amraboelela/swift,aschwaighofer/swift,russbishop/swift,alblue/swift,apple/swift,jtbandes/swift,IngmarStein/swift,ben-ng/swift,arvedviehweger/swift,tkremenek/swift,tkremenek/swift,kstaring/swift,karwa/swift,arvedviehweger/swift,gribozavr/swift,JaSpa/swift,jmgc/swift,therealbnut/swift,ahoppen/swift,shahmishal/swift,russbishop/swift,JGiola/swift,djwbrown/swift,xedin/swift,ahoppen/swift,gregomni/swift,uasys/swift,kperryua/swift,shahmishal/swift,deyton/swift,jopamer/swift,roambotics/swift,tardieu/swift,uasys/swift,Jnosh/swift,parkera/swift,sschiau/swift,jopamer/swift,therealbnut/swift,CodaFi/swift,manavgabhawala/swift,gmilos/swift,brentdax/swift,harlanhaskins/swift,tjw/swift,aschwaighofer/swift,djwbrown/swift,codestergit/swift,rudkx/swift,nathawes/swift,shajrawi/swift,OscarSwanros/swift,johnno1962d/swift,deyton/swift,stephentyrone/swift,manavgabhawala/swift,gribozavr/swift,kstaring/swift,gregomni/swift,apple/swift,ben-ng/swift,gregomni/swift,therealbnut/swift,codestergit/swift,natecook1000/swift,Jnosh/swift,arvedviehweger/swift,karwa/swift,jtbandes/swift,benlangmuir/swift,glessard/swift,CodaFi/swift,danielmartin/swift,stephentyrone/swift,zisko/swift,shahmishal/swift,kstaring/swift,bitjammer/swift,roambotics/swift,practicalswift/swift,kstaring/swift,nathawes/swift,natecook1000/swift,uasys/swift,parkera/swift,airspeedswift/swift,tinysun212/swift-windows,glessard/swift,practicalswift/swift,SwiftAndroid/swift,devincoughlin/swift,jtbandes/swift,xwu/swift,shahmishal/swift,djwbrown/swift,jckarter/swift,tkremenek/swift,danielmartin/swift,sschiau/swift,jmgc/swift,calebd/swift,kstaring/swift,russbishop/swift,jopamer/swift,gottesmm/swift,felix91gr/swift,modocache/swift,tkremenek/swift,johnno1962d/swift,shajrawi/swift,huonw/swift,karwa/swift,practicalswift/swift,CodaFi/swift,gottesmm/swift,milseman/swift,hughbe/swift,xwu/swift,xedin/swift,arvedviehweger/swift,lorentey/swift,modocache/swift,gregomni/swift,allevato/swift,jckarter/swift,hooman/swift,swiftix/swift,milseman/swift,hooman/swift,zisko/swift,natecook1000/swift,deyton/swift,xwu/swift,russbishop/swift,OscarSwanros/swift,amraboelela/swift,jmgc/swift,amraboelela/swift,gribozavr/swift,shajrawi/swift,swiftix/swift,OscarSwanros/swift,gmilos/swift,hughbe/swift,arvedviehweger/swift,jopamer/swift,tinysun212/swift-windows,frootloops/swift,benlangmuir/swift,kperryua/swift,swiftix/swift,frootloops/swift,tjw/swift,jtbandes/swift,return/swift,manavgabhawala/swift,codestergit/swift,OscarSwanros/swift,alblue/swift,lorentey/swift,brentdax/swift,swiftix/swift,jtbandes/swift,russbishop/swift,hughbe/swift,xedin/swift,tinysun212/swift-windows,felix91gr/swift,parkera/swift,sschiau/swift,roambotics/swift,sschiau/swift,xwu/swift,gribozavr/swift,deyton/swift,therealbnut/swift,tardieu/swift,nathawes/swift,calebd/swift,austinzheng/swift,atrick/swift,harlanhaskins/swift,glessard/swift,kperryua/swift,xwu/swift,calebd/swift,JGiola/swift,austinzheng/swift,natecook1000/swift,brentdax/swift,hooman/swift,CodaFi/swift,Jnosh/swift,airspeedswift/swift,jckarter/swift,johnno1962d/swift,OscarSwanros/swift,deyton/swift,felix91gr/swift,milseman/swift,hooman/swift,rudkx/swift,allevato/swift,manavgabhawala/swift,felix91gr/swift,SwiftAndroid/swift,rudkx/swift,tjw/swift,bitjammer/swift,russbishop/swift,devincoughlin/swift,airspeedswift/swift,uasys/swift,JaSpa/swift,tjw/swift,devincoughlin/swift,tkremenek/swift,jckarter/swift,IngmarStein/swift,CodaFi/swift,bitjammer/swift,gottesmm/swift,calebd/swift,xedin/swift,bitjammer/swift,aschwaighofer/swift,gmilos/swift,jmgc/swift,johnno1962d/swift,amraboelela/swift,ben-ng/swift,gregomni/swift,tardieu/swift,jopamer/swift,practicalswift/swift,glessard/swift,ahoppen/swift,SwiftAndroid/swift,kstaring/swift,ben-ng/swift,jckarter/swift,allevato/swift,roambotics/swift,JaSpa/swift,tinysun212/swift-windows,harlanhaskins/swift,lorentey/swift,airspeedswift/swift,swiftix/swift,brentdax/swift,shahmishal/swift,stephentyrone/swift,ken0nek/swift,ken0nek/swift,djwbrown/swift,tjw/swift,hughbe/swift,huonw/swift,danielmartin/swift,alblue/swift,shahmishal/swift,gmilos/swift,karwa/swift,atrick/swift,hughbe/swift,practicalswift/swift,tinysun212/swift-windows,IngmarStein/swift,hughbe/swift,swiftix/swift,allevato/swift,manavgabhawala/swift,huonw/swift,Jnosh/swift,uasys/swift,karwa/swift,harlanhaskins/swift,therealbnut/swift,sschiau/swift,xedin/swift,SwiftAndroid/swift,tinysun212/swift-windows,CodaFi/swift,tkremenek/swift,alblue/swift,lorentey/swift,felix91gr/swift,shajrawi/swift,johnno1962d/swift,ken0nek/swift,brentdax/swift,devincoughlin/swift,Jnosh/swift,parkera/swift,SwiftAndroid/swift,alblue/swift,IngmarStein/swift,bitjammer/swift,modocache/swift,therealbnut/swift,parkera/swift,gmilos/swift,return/swift,lorentey/swift,austinzheng/swift,atrick/swift,arvedviehweger/swift,zisko/swift,calebd/swift,codestergit/swift,nathawes/swift,return/swift,tinysun212/swift-windows,brentdax/swift,dreamsxin/swift,SwiftAndroid/swift,tkremenek/swift,manavgabhawala/swift,codestergit/swift,gottesmm/swift,huonw/swift,Jnosh/swift,codestergit/swift,shajrawi/swift,tardieu/swift,danielmartin/swift,nathawes/swift,allevato/swift,johnno1962d/swift,sschiau/swift,jopamer/swift,jmgc/swift,nathawes/swift,glessard/swift,IngmarStein/swift,danielmartin/swift,JGiola/swift,djwbrown/swift,frootloops/swift,JGiola/swift,airspeedswift/swift,atrick/swift,karwa/swift,benlangmuir/swift,return/swift,shajrawi/swift,JaSpa/swift,kperryua/swift,zisko/swift,alblue/swift,uasys/swift,IngmarStein/swift,zisko/swift,OscarSwanros/swift,ken0nek/swift,natecook1000/swift,lorentey/swift,ken0nek/swift,gribozavr/swift,xwu/swift,ken0nek/swift,alblue/swift,devincoughlin/swift,frootloops/swift,kperryua/swift,jmgc/swift,ben-ng/swift,gottesmm/swift,therealbnut/swift,calebd/swift,ahoppen/swift,milseman/swift,devincoughlin/swift,stephentyrone/swift,harlanhaskins/swift,hooman/swift,atrick/swift,kperryua/swift,rudkx/swift,shajrawi/swift,natecook1000/swift,Jnosh/swift,tardieu/swift,OscarSwanros/swift,dreamsxin/swift,milseman/swift,devincoughlin/swift,aschwaighofer/swift,stephentyrone/swift,shajrawi/swift,apple/swift,huonw/swift,airspeedswift/swift,practicalswift/swift,modocache/swift,tjw/swift,JaSpa/swift,roambotics/swift,xwu/swift,hooman/swift,benlangmuir/swift,manavgabhawala/swift,tardieu/swift,benlangmuir/swift,felix91gr/swift,swiftix/swift,modocache/swift,amraboelela/swift,parkera/swift,gmilos/swift,aschwaighofer/swift,gribozavr/swift,hughbe/swift,amraboelela/swift,stephentyrone/swift,jtbandes/swift,deyton/swift,gregomni/swift,lorentey/swift,shahmishal/swift,glessard/swift,kperryua/swift,milseman/swift,allevato/swift,hooman/swift,JaSpa/swift,allevato/swift,stephentyrone/swift,sschiau/swift,danielmartin/swift,zisko/swift,tjw/swift,aschwaighofer/swift,shahmishal/swift,frootloops/swift,aschwaighofer/swift,brentdax/swift,gmilos/swift,gottesmm/swift,felix91gr/swift,ben-ng/swift,benlangmuir/swift,atrick/swift,airspeedswift/swift