[SR-2022] Better fixit for type checking in switch statements

[karwa]

Previous ID	SR-2022
Radar	None
Original Reporter	@karwa
Type	Improvement
Status	In Progress
Resolution

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, DiagnosticsQoI, Parser, StarterBug
Assignee	None
Priority	Medium

md5: 9b53adc1e41e34c46ad044ffe4bfbac9

Issue Description:

switch myVariable {
    case let intValue as? Int: print("Hello, I'm an int: \(intValue)")
    case let stringValue as? String: print("Hello, I'm a string: \(stringValue)")
    default: print("Some other value...")
}

Gives you an error on the casting lines saying "Invalid pattern". It would be better if it told you to make the casts non-conditional, I.e.:

switch myVariable {
    case let intValue *as Int*: print("Hello, I'm an int: \(intValue)")
    case let stringValue *as String*: print("Hello, I'm a string: \(stringValue)")
    default: print("Some other value...")
}

[modocache]

The reported issue still occurs on Swift master as of this morning:

$ cat ~/GitHub/tmp/ParserExamples/BasketCase.swift
protocol IntOrString {}

extension Int: IntOrString {}
extension String: IntOrString {}

func foo(bar: IntOrString) {
  switch bar {
    case let intValue as? Int: print("Hello, I'm an Int: \(intValue)")
    case let stringValue as? String: print("Hello, I'm a String: \(stringValue)")
    default: print("Some other value...")
  }
}

$ swift -frontend -parse ~/tmp/AsQuestionMark.swift
/home/modocache/tmp/AsQuestionMark.swift:3:14: error: pattern variable binding cannot appear in an expression
    case let intValue as? Int: print("Hello, I'm an Int: \(intValue)")
             ^
/home/modocache/tmp/AsQuestionMark.swift:3:23: warning: cast from '<<error type>>' to unrelated type 'Int' always fails
    case let intValue as? Int: print("Hello, I'm an Int: \(intValue)")
             ~~~~~~~~ ^   ~~~
/home/modocache/tmp/AsQuestionMark.swift:3:14: error: pattern variable binding cannot appear in an expression
    case let intValue as? Int: print("Hello, I'm an Int: \(intValue)")
             ^
/home/modocache/tmp/AsQuestionMark.swift:3:23: warning: cast from '<<error type>>' to unrelated type 'Int' always fails
    case let intValue as? Int: print("Hello, I'm an Int: \(intValue)")
             ~~~~~~~~ ^   ~~~
/home/modocache/tmp/AsQuestionMark.swift:3:23: error: expression pattern of type 'Int?' cannot match values of type 'Int'
    case let intValue as? Int: print("Hello, I'm an Int: \(intValue)")
             ~~~~~~~~~^~~~~~~
/home/modocache/tmp/AsQuestionMark.swift:4:14: error: pattern variable binding cannot appear in an expression
    case let stringValue as? String: print("Hello, I'm a String: \(stringValue)")
             ^
/home/modocache/tmp/AsQuestionMark.swift:4:26: warning: cast from '<<error type>>' to unrelated type 'String' always fails
    case let stringValue as? String: print("Hello, I'm a String: \(stringValue)")
             ~~~~~~~~~~~ ^   ~~~~~~
/home/modocache/tmp/AsQuestionMark.swift:4:14: error: pattern variable binding cannot appear in an expression
    case let stringValue as? String: print("Hello, I'm a String: \(stringValue)")
             ^
/home/modocache/tmp/AsQuestionMark.swift:4:26: warning: cast from '<<error type>>' to unrelated type 'String' always fails
    case let stringValue as? String: print("Hello, I'm a String: \(stringValue)")
             ~~~~~~~~~~~ ^   ~~~~~~
/home/modocache/tmp/AsQuestionMark.swift:4:26: error: expression pattern of type 'String?' cannot match values of type 'Int'
    case let stringValue as? String: print("Hello, I'm a String: \(stringValue)")
             ~~~~~~~~~~~~^~~~~~~~~~

$ cat ~/GitHub/tmp/ParserExamples/Case.swift
protocol IntOrString {}

extension Int: IntOrString {}
extension String: IntOrString {}

func foo(bar: IntOrString) {
  switch bar {
    case let intValue as Int: print("Hello, I'm an Int: \(intValue)")
    case let stringValue as String: print("Hello, I'm a String: \(stringValue)")
    default: print("Some other value...")
  }
}

$ ../build/Ninja-ReleaseAssert/swift-linux-x86_64/bin/swift -frontend -parse ~/GitHub/tmp/ParserExamples/Case.swift

The output is pretty confusing. I suspect "pattern variable binding cannot appear in an expression" doesn't make a ton of sense to an end user. The fixit @karwa describes would be very helpful.

[modocache]

I'd like to try my hand at adding a fix-it here. 🙂

[modocache]

On second thought, at a first glance it looks like this logic might be in Sema. I don't know a lot about those yet, so I'll drop this for now. Maybe sometime in late October. Sorry for the noise! 🙂

[swift-ci]

Comment by Christopher Ian Stern (JIRA)

While perhaps this should be caught by the parser, the parser doesn't seem to have the context to do it. Either a bunch of parser methods would need addl. arguments or class Parser needs another InThisKindOfStructure variable.

Sema only needs 20some lines of code to detect and give a clear diagnostic.

[belkadan]

Resetting assignee on all Starter Bugs not modified since 2018.

[swift-ci]

Comment by Christopher Ian Stern (JIRA)

Conflicts have developed between master and PR-20456, which aims to resolve this issue.

Looks like the code that was to be patched in lib/Sema/TypeCheckStmt.cpp moved to a new function `visitSwitchStmt(SwitchStmt *S)`.

[belkadan]

Oops, looks like we missed that PR at the time. Sorry about that.

[belkadan]

#20456

[gregomni]

In the last 5 years the error message has improved here:

 8 |     case let intValue as? Int: print("Hello, I'm an Int: \(intValue)")
   |                           `- error: expression pattern of type 'Int?' cannot match values of type 'any IntOrString'

I think that's clear enough what's happening.

[karwa]

I still don't think it's very good, to be honest. Testing on 5.10:

protocol IntOrString {}

extension Int: IntOrString {}
extension String: IntOrString {}

func foo(bar: IntOrString) {
  switch bar {
    case let intValue as? Int: print("Hello, I'm an Int: \(intValue)")
    case let stringValue as? String: print("Hello, I'm a String: \(stringValue)")
    default: print("Some other value...")
  }
}

<source>:8:14: error: pattern variable binding cannot appear in an expression
    case let intValue as? Int: print("Hello, I'm an Int: \(intValue)")
             ^
<source>:8:27: error: expression pattern of type 'Int?' cannot match values of type 'any IntOrString'
    case let intValue as? Int: print("Hello, I'm an Int: \(intValue)")
             ~~~~~~~~~~~~~^~~
<source>:9:14: error: pattern variable binding cannot appear in an expression
    case let stringValue as? String: print("Hello, I'm a String: \(stringValue)")
             ^
<source>:9:30: error: expression pattern of type 'String?' cannot match values of type 'any IntOrString'
    case let stringValue as? String: print("Hello, I'm a String: \(stringValue)")
             ~~~~~~~~~~~~~~~~^~~~~~

I think that at the very least, there should be an explicit suggestion (e.g. a fix-it) that the developer turn the as? in to an as.

[jameesbrown]

Hi @karwa, I was investigating this and thought of 2 other ways in which it could occur:

let maybeInt: Any = 1

// 1
switch maybeInt {
case let intValue as? Int:
  print(intValue)
default:
  break
}

// 2
if case let intValue as? Int = maybeInt {
  print(intValue)
}

let maybeInts: [Any] = [1, 2, 3]

// 3
for case let intValue as? Int in maybeInts {
  print(intValue)
}

In 1 and 3 I think your suggested fix-it would be ideal. However, in the second case, I think it might be better to suggest changing it to:

if let intValue = maybeInt as? Int {
  print(intValue)
}

Any thoughts on this?