[SR-11373] Don't perform X-to-pointer conversions for generic params inferred to be pointer types

[hamishknight]

Previous ID	SR-11373
Radar	None
Original Reporter	@hamishknight
Type	Bug

Environment

Swift version 5.1-dev (LLVM c5340df2d1, Swift 98a54a1)
Target: x86_64-apple-darwin18.5.0

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	None
Priority	Medium

md5: 13080b3b09be239733bd1a724a240c24

Issue Description:

Came up with the following bit of cursed Swift this morning:

print(min("b", "a")) // a
print(String(validatingUTF8: min("b", "a"))!) // b

This happens because min's generic parameter T gets inferred to be an UnsafePointer, resulting in string-to-pointer conversions for its arguments (and the first arg just happens to have a lower address). It then leads to undefined behaviour from min returning a now dangling pointer.

Arguably we should ban X-to-pointer conversions for generic parameters that happen to be inferred as pointer types, such as the following:

func id<T>(_ x: T) -> T { x }

let ptr: UnsafePointer<CChar> = id("hello")

I don't know how source breaking such a change would be though. The two examples given above are undefined behaviour, but it could be possible that somebody is using this mechanic in a well-defined manner (I can't think of any particularly motivating examples off the top of my head though). Would such a change require an SE proposal?