[SR-4082] Compiler Crash on multi-statement if with shadowed variable

[saagarjha]

Previous ID	SR-4082
Radar	None
Original Reporter	@saagarjha
Type	Bug
Status	Resolved
Resolution	Done

Environment

macOS Sierra 10.12.4 Beta (16E163f)
Xcode Version 8.3 beta 2 (8W120l)
Toolchain: Xcode 8.3, Swift Development Snapshot 2/24/17

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug, 3.1Regression, CompilerCrash
Assignee	@gregomni
Priority	Medium

md5: 51eb6d12c1409f11f2a249e9cf9539df

relates to:

• SR-4387 Compiler Segmentation Fault 11 on wrongly ordered lets

Issue Description:

I'm getting a compiler crash with code that looks like this:

import Foundation

struct Percent {
    var percent: String
    
    init(percent: String) {
        self.percent = percent
    }
    
    var percentValue: Double? {
        get {
            if percent.hasSuffix("%"),
                let percent = double(for: percent) {
                return percent / 100
            }
            return nil
        }
    }
    
    func double(for percent: String) -> Double? {
        return Double(percent.substring(to: percent.index(before: percent.endIndex)))
    }
}

print(Percent(percent: "42%").percentValue ?? "This isn't a percent.")

The issue appears to be with the if statement:

if percent.hasSuffix("%"),
    let percent = double(for: percent) {
    return percent / 100
}

If I remove the first statement, or change the if-let to use some other variable name instead of shadowing, the issue goes away. Here's a log with the latest Development Snapshot (2/24/17):

Assertion failed: ((HadError || !M.is<SourceFile*>() || M.get<SourceFile*>()->ASTStage < SourceFile::TypeChecked) && "UnresolvedDot" "in wrong phase"), function walkToExprPre, file /Users/buildnode/jenkins/workspace/oss-swift-package-osx/swift/include/swift/AST/ExprNodes.def, line 87.
0  swift                    0x000000010baee0b8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  swift                    0x000000010baed2d6 llvm::sys::RunSignalHandlers() + 86
2  swift                    0x000000010baee709 SignalHandler(int) + 361
3  libsystem_platform.dylib 0x00007fffbe67cb3a _sigtramp + 26
4  swift                    0x000000010996f044 swift::TypeBase::getDesugaredType() + 20
5  libsystem_c.dylib        0x00007fffbe501420 abort + 129
6  libsystem_c.dylib        0x00007fffbe4c8893 basename_r + 0
7  swift                    0x00000001098b8327 (anonymous namespace)::Verifier::walkToExprPre(swift::Expr*) + 263
8  swift                    0x00000001098ca804 (anonymous namespace)::Traversal::visitApplyExpr(swift::ApplyExpr*) + 36
9  swift                    0x00000001098c6b4b (anonymous namespace)::Traversal::doIt(llvm::MutableArrayRef<swift::StmtConditionElement> const&) + 363
10 swift                    0x00000001098caf32 swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Stmt*) + 1074
11 swift                    0x00000001098cacbd swift::ASTVisitor<(anonymous namespace)::Traversal, swift::Expr*, swift::Stmt*, bool, swift::Pattern*, bool, void>::visit(swift::Stmt*) + 445
12 swift                    0x00000001098cd53a (anonymous namespace)::Traversal::visitAbstractFunctionDecl(swift::AbstractFunctionDecl*) + 1434
13 swift                    0x00000001098c6dfe (anonymous namespace)::Traversal::doIt(swift::Decl*) + 366
14 swift                    0x00000001098ccf24 (anonymous namespace)::Traversal::visitNominalTypeDecl(swift::NominalTypeDecl*) + 1076
15 swift                    0x00000001098c6ded (anonymous namespace)::Traversal::doIt(swift::Decl*) + 349
16 swift                    0x00000001098c6c7b swift::Decl::walk(swift::ASTWalker&) + 27
17 swift                    0x000000010994499a swift::SourceFile::walk(swift::ASTWalker&) + 170
18 swift                    0x00000001098b7cbb swift::verify(swift::SourceFile&) + 59
19 swift                    0x000000010983fcb3 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int) + 2067
20 swift                    0x00000001094433d9 swift::CompilerInstance::performSema() + 3897
21 swift                    0x00000001089fd38c swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 4380
22 swift                    0x00000001089bbd70 main + 3312
23 libdyld.dylib            0x00007fffbe46d235 start + 1

Stack dump:
0.  While walking into decl 'Percent' at /var/folders/x3/wncmj8p52t732rrxyz6h424m0000gn/T/./lldb/56770/playground1.swift:3:1
1.  While walking into body of getter for percentValue at /var/folders/x3/wncmj8p52t732rrxyz6h424m0000gn/T/./lldb/56770/playground1.swift:10:6

[swift-ci]

Comment by Pawel Szot (JIRA)

Minimal code:

let x = 5
if x > 0, let x = Optional(1) { }

[belkadan]

In 3.0 this gave an arguably bogus error:

<stdin>:2:4: error: use of local variable 'x' before its declaration
if x > 0, let x = Optional(1) { }
   ^
<stdin>:2:15: note: 'x' declared here
if x > 0, let x = Optional(1) { }
              ^

[saagarjha]

pszot (JIRA User) I was too tired last night dealing with iTunes Connect to pare it down. Thanks for the minimal reproduction.

[gregomni]

Fixed in <https://github.com/apple/swift/pull/12438\>. The result now is:

4082.swift:2:15: warning: immutable value 'x' was never used; consider replacing with '_' or removing it
if x > 0, let x = Optional(1) { }
              ^
              _

I.e. the x in "x > 0" is the one whose value is 5, and then the new x is unused. Should there be some additional shadowing warning of some kind here, do you think?