[SR-7562] @discardableResult is disregarded for a required init method

[swift-ci]

Previous ID	SR-7562
Radar	None
Original Reporter	Vandad (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

swift --version

Apple Swift version 4.1 (swiftlang-902.0.48 clang-902.0.37.1)

Target: x86_64-apple-darwin17.5.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, StarterBug
Assignee	@vguerra
Priority	Medium

md5: bddad59f549a8118d77ef9d4bf5040fb

Issue Description:

My problem is sort of related to the following issue: SR-2198

Consider the following example:

protocol HasInitMethod {
    associatedtype Input
    init(input: Input)
}

class Foo<Input>: HasInitMethod {
    @discardableResult required init(input: Input) {
        //empty for now
    }
}

class Bar: Foo<Int> {
}

If I instantiate Foo, with its init method marked with @discardableResult, then the compiler does a good job of not warning me about the unused result.

However, if I instantiate Bar, who inherits its init method from Foo, I will get a warning:

Foo(input :10) //no warnings, nice!
Bar(input: 10) //warning: Result of 'Bar' initializer is unused

I believe this to be a bug and not a feature since Bar can and should inherit its init method from Foo to be able to get the same effect as Foo has.

[belkadan]

I'm inclined to agree! Tagging as a StarterBug. The fix would be changing configureDesignatedInitAttributes in CodeSynthesis.cpp to test if the superclass constructor had a DiscardableResultAttr, and creating one for the new constructor as well if it does.

[vguerra]

hello Vandad (JIRA User) and @belkadan . I took the liberty of assigning the bug to myself. I am new to the swift compiler and was looking at starter bugs I could help with and I think i can tackle this one 🙂.

[belkadan]

Welcome, Victor! Feel free to open a pull request on GitHub as soon as you have something ready for initial review, or even if you just have questions as you go.

[vguerra]

thanks Jordan!

I followed your prev. comment and I got a fix already. I am currently writing a test that makes sure the warning is not emitted but can't manage to execute it locally. Maybe you can give me a hand here 🙂.

I am launching the test as follows:

./llvm/utils/lit/lit.py -svv ./build/Xcode-RelWithDebInfoAssert/swift-macosx-x86_64/test-macosx-x86_64/attr --filter=attr_discardableResult.swift --param swift-version=4 -a

test fails with following stack trace:

Unable to find source-code formatter for language: python. Available languages are: actionscript, html, java, javascript, none, sql, xhtml, xmlTraceback (most recent call last):
  File "/Volumes/VicDrive/ML/swift-tensorflow/llvm/utils/lit/lit/run.py", line 202, in _execute_test_impl
    result = test.config.test_format.execute(test, lit_config)
  File "/Volumes/VicDrive/ML/swift-tensorflow/swift/test/swift_test.py", line 63, in execute
    result = super(SwiftTest, self).execute(test, litConfig)
  File "/Volumes/VicDrive/ML/swift-tensorflow/llvm/utils/lit/lit/formats/shtest.py", line 25, in execute
    self.execute_external)
  File "/Volumes/VicDrive/ML/swift-tensorflow/llvm/utils/lit/lit/TestRunner.py", line 1478, in executeShTest
    script = applySubstitutions(script, substitutions)
  File "/Volumes/VicDrive/ML/swift-tensorflow/llvm/utils/lit/lit/TestRunner.py", line 1178, in applySubstitutions
    return list(map(processLine, script))
  File "/Volumes/VicDrive/ML/swift-tensorflow/llvm/utils/lit/lit/TestRunner.py", line 1172, in processLine
    ln = re.sub(a, b, ln)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/re.py", line 155, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/re.py", line 286, in _subx
    template = _compile_repl(template, pattern)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/re.py", line 271, in _compile_repl
    p = sre_parse.parse_template(repl, pattern)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sre_parse.py", line 737, in parse_template
    s = Tokenizer(source)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sre_parse.py", line 192, in __init__
    self.__next()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sre_parse.py", line 194, in __next
    if self.index >= len(self.string):
TypeError: object of type 'NoneType' has no len()

i added some print statements in `TestRunner.py` to understand a bit more what is going on and I see that failure happens when applying substitution for : `%target-swift-reflection-test`

am I forgetting to set an environment variable? or maybe passing a parameter to the lit.py script?

Thank you for your help in advance!

[belkadan]

Ah, for an Xcode build I believe you have to pass --param build_mode=RelWithDebInfo as well, since it's valid to use the generated Xcode project to do both Debug and RelWithDebInfo builds.

[belkadan]

(Bug-to-file: have lit check if you're in an Xcode build and complain directly if so.)

[vguerra]

thank you Jordan! That made it work.

about your last comment, do you want me to file a bug for that?

[belkadan]

Heh, you're welcome to. I'll get to it if you don't.

[belkadan]

SR-7714

[vguerra]

thanks for creating SR-7714 🙂 .. it would be indeed a nice improvement.

[vguerra]

@belkadan I created a PR for this (#16699 Could you please start smoke tests on it? Thanks!

[vguerra]

PR that solves this issue has been merged.

[belkadan]

Pulling into the 4.2 branch, since it's a small, safe change: #16796