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Improve assertion error message for Apply and TypeApply #18700

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Oct 20, 2023
Merged

Improve assertion error message for Apply and TypeApply #18700

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14 changes: 8 additions & 6 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 @@ -45,19 +45,21 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
def Apply(fn: Tree, args: List[Tree])(using Context): Apply = fn match
case Block(Nil, expr) =>
Apply(expr, args)
case _: RefTree | _: GenericApply | _: Inlined | _: Hole =>
ta.assignType(untpd.Apply(fn, args), fn, args)
case _ =>
assert(ctx.reporter.errorsReported)
assert(
fn.isInstanceOf[RefTree | GenericApply | Inlined | Hole] || ctx.reporter.errorsReported,
s"Illegal Apply function prefix: $fn"
)
ta.assignType(untpd.Apply(fn, args), fn, args)

def TypeApply(fn: Tree, args: List[Tree])(using Context): TypeApply = fn match
case Block(Nil, expr) =>
TypeApply(expr, args)
case _: RefTree | _: GenericApply =>
ta.assignType(untpd.TypeApply(fn, args), fn, args)
case _ =>
assert(ctx.reporter.errorsReported)
assert(
fn.isInstanceOf[RefTree | GenericApply] || ctx.reporter.errorsReported,
s"Illegal TypeApply function prefix: $fn"
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We should use the i interpolator, so it's pretty-printed.

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No. We need to see the exact AST. Otherwise we would not see the difference between a TypeApply and Inlined; or some similar cases where show is equivalent.

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Another way to see this is as a match error. Maybe I could encode it in a simpler way and make it fail with an equivalently useful output. I will try that.

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The alternative is

  def TypeApply(fn: Tree, args: List[Tree])(using Context): TypeApply = fn match
    case Block(Nil, expr) =>
      TypeApply(expr, args)
    case _: RefTree | _: GenericApply =>
      ta.assignType(untpd.TypeApply(fn, args), fn, args)
    case _ if ctx.reporter.errorsReported => // We expect the error. Otherwise, we want to throw a match error.
      ta.assignType(untpd.TypeApply(fn, args), fn, args)

But this one does duplicate ta.assignType(untpd.TypeApply(fn, args), fn, args). I find the current one clearer.

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How about i"Illegal TypeApply function prefix: $fn (${fn.className})" then? Don't need the verbose nested trees, I would think.

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Given the history of the failure, I would argue otherwise. Knowing which tree caused the issue in one of those large projects can be helpful in pinpointing the likely source of the problem.

)
ta.assignType(untpd.TypeApply(fn, args), fn, args)

def Literal(const: Constant)(using Context): Literal =
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