Sema: Implicit conversion for single-expression closures of Never type (3.0) #4951
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slavapestov
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This fixes a usability regression with the removal of @NoReturn in Swift 3. Previously, it was legal to write this: let callback: () -> Int = { fatalError() } Now that the special @NoReturn attribute has been replaced with a Never type, the above fails to typecheck, because the expression now has type 'Never', and we expect a value of type 'Int'. Getting around this behavior requires ugly workarounds to force the parser to treat the body as a statement rather than an expression; for example, let callback: () -> Int = { _ = (); fatalError() } This patch generalized single-expression closures to allow the 'Never to T' conversion. Note that this is rather narrow in scope -- it only applies to closure *literals*, single-expression ones at that, not arbitrary function *values*. In fact, it is not really a conversion at all, but more of a desugaring rule for single-expression closures. They can now be summarized as follows: - If the closure literal has contextual return type T and the expression has Never type, the closure desugars as { _ = <expr> }, with no ReturnStmt. - If the closure literal has contextual return type T for some non-void type T, the closure desugars as { return <expr> }; the expression type must be convertible to T. - If the closure literal has contextual return type Void, and the expression has some non-Void type T, the closure desugars as { _ = <expr>; return () }. Fixes <rdar://problem/28269358> and <https://bugs.swift.org/browse/SR-2661>.
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DougGregor
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Sep 23, 2016
| SmallVector<ASTNode, 1> elements; | ||
| elements.push_back(singleExpr); | ||
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| auto braceStmt = BraceStmt::create(tc.Context, |
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This kind of AST hackery feels gross, but... okay. I don't see a better localized fix.
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It's just copy and pasted from the old code (and I know, that's a HORRIBLE excuse)
MaxDesiatov
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[pull] swiftwasm from main
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Neverreturn type, we lost a bit of functionality. Since Never is just an ordinary type, the type checker did not allow a single-expression closure to contain a Never-returning expression unless the closure type itself returned Never. This precludes using single expression closures for callbacks in certain situations. This patch adds a new rule, similar to the existing rule for dropping the return type of a single-expression closure.