[SR-6062] Compiler crash when accessing Dictionary.subscript(_:default:) via key-path expression

[natecook1000]

Previous ID	SR-6062
Radar	rdar://problem/34835322
Original Reporter	@natecook1000
Type	Bug

Environment

swift/master @ aed8c33

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, CompilerCrash
Assignee	None
Priority	Medium

md5: 21a6322780a9e431f9d545abfc86f173

Issue Description:

The following code tries to use a key-path expressions to access the key/default subscript on a dictionary, but results in a compiler crash:

var d = [1: 1, 2: 2]
print(d[keyPath: \[Int: Int].[1, default: 0]])
----
TYPE MISMATCH IN ARGUMENT 1 OF APPLY AT expression at [/Users/nate/Desktop/kp1.swift:2:18 - line:2:44] RangeText="\[Int: Int].[1, default: 0]"   argument value:   %8 = load [trivial] %7 : $*Int   parameter type: $@callee_owned () -> @out Int

I'm not sure if that's the right syntax—omitting the default: label results in an an "expression is ambiguous" error:

var d = [1: 1, 2: 2]
print(d[keyPath: \[Int: Int].[1, 0]])
----
/Users/nate/Desktop/kp1.swift:2:18: error: type of expression is ambiguous without more context
print(d[keyPath: \[Int: Int].[1, 0]])
                 ^~~~~~~~~~~~~~~~~~

[belkadan]

cc @jckarter

[jckarter]

@swift-ci create

[jckarter]

PR for master: #12301

PR for 4.0: #12303

This fixes the compiler crash, though it reveals the underlying problem that the default: argument is an @autoclosure and therefore not Hashable, which is probably not the answer you were looking for. I think it'd be reasonable to look through autoclosures in key path indexes and evaluate, capture, and hash the underlying expression, though I'd say that requires s-e discussion, and a bit more implementation, to achieve.

[natecook1000]

No worries—was honestly just trying out two-argument subscripts, and this was the only built-in one I knew of. I think it's totally reasonable that autoclosures don't do this yet.