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hamishknight
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Look through implicit nodes to allow e.g implicitly synthesized init(erasing:) calls to wrap if/switch expressions, and move the diagnostic logic into performSyntacticExprDiagnostics to ensure we don't miss expressions in property initializers, subscript default arguments, and custom attrs.

rdar://113435870

Previously we would only look through a handful of
AST node types when determining if an if/switch
expression is in a valid position. However this
doesn't handle cases where we synthesize code
around an if/switch expression, such as
`init(erasing:)` calls. As such, relax the logic
such that it can look through any implicit
expression.

rdar://113435870
Move out-of-place SingleValueStmtExpr checking into
`performSyntacticExprDiagnostics`, to ensure we
catch all expressions. Previously we did the walk
as a part of Decl-based MiscDiagnostics, but it
turns out that can miss expressions in property
initializers, subscript default arguments, and
custom attrs.

This does mean that we'll now no longer diagnose
out-of-place if/switch exprs if the expression
didn't type-check, but that's consistent with the
rest of MiscDiagnostics, and I don't think it will
be a major issue in practice. We probably ought to
consider moving this checking into PreCheckExpr,
but that would require first separating out
SequenceExpr folding, which has other consequences,
and so I'm leaving as future work for now.
@hamishknight
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@swift-ci please test

@hamishknight
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@swift-ci please test source compatibility

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2 participants