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Fix #444: be more conservative on when Block and If types should be recomputed #5958

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Feb 21, 2019
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Fix #444: be more conservative on when Block and If types should be recomputed #5958

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14 changes: 12 additions & 2 deletions compiler/src/dotty/tools/dotc/ast/tpd.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -572,15 +572,25 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
override def Block(tree: Tree)(stats: List[Tree], expr: Tree)(implicit ctx: Context): Block = {
val tree1 = untpdCpy.Block(tree)(stats, expr)
tree match {
case tree: Block if expr.tpe eq tree.expr.tpe => tree1.withTypeUnchecked(tree.tpe)
case tree: Block if (expr.tpe eq tree.expr.tpe) && (expr.tpe eq tree.tpe) =>
// the second guard is needed in case avoid somehow widened the type.
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More detailed comment:

          // The last guard is a conservative check: if `tree.tpe` is different from `expr.tpe`, then
          // it was computed from widening `expr.tpe`, and tree transforms might cause `expr.tpe.widen`
          // to change even if `expr.tpe` itself didn't change, e.g:
          //     { val s = ...;  s }
          // If the type of `s` changed, then the type of the block might have changed, even though `expr.tpe`
          // will still be `TermRef(NoPrefix, s)`

// if it did it could potentially need to rewiden it
// eg {val s = ...; s}
// changing type of s should change type of block, though type of expr is unchanged - TermRef(s)
tree1.withTypeUnchecked(tree.tpe)
case _ => ta.assignType(tree1, stats, expr)
}
}

override def If(tree: Tree)(cond: Tree, thenp: Tree, elsep: Tree)(implicit ctx: Context): If = {
val tree1 = untpdCpy.If(tree)(cond, thenp, elsep)
tree match {
case tree: If if (thenp.tpe eq tree.thenp.tpe) && (elsep.tpe eq tree.elsep.tpe) => tree1.withTypeUnchecked(tree.tpe)
case tree: If if (thenp.tpe eq tree.thenp.tpe) && (elsep.tpe eq tree.elsep.tpe) &&
((tree.tpe eq thenp.tpe) || (tree.tpe eq elsep.tpe)) =>
// last guard is needed in case previous if had computed a widened ORType that needs to be recomputed
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New comment:

          // The last guard is a conservative check similar to the one done in `Block` above,
          // if `tree.tpe` is not identical to the type of one of its branch, it might have been
          // computed from the widened type of the branches, so the same reasoning than
          // in `Block` applies.

// eg {val a = ...; val b = ...; if(...) a else b}
// changing type of a or b should change type of if, though types of both trees remain unchanged
tree1.withTypeUnchecked(tree.tpe)
case _ => ta.assignType(tree1, thenp, elsep)
}
}
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