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Match type trace improvements #20178

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Match type trace improvements #20178

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6daf419
Normalize before attempting to record a failed match type reduction
EugeneFlesselle Apr 11, 2024
a5064e4
Dealias for MatchTypeTrace
EugeneFlesselle Apr 13, 2024
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2 changes: 1 addition & 1 deletion compiler/src/dotty/tools/dotc/core/MatchTypeTrace.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ object MatchTypeTrace:

private val MatchTrace = new Property.Key[MatchTrace]

/** Execute `op` and if it involves a failed match type reduction
/** Execute `op` and if it is a failed match type reduction
* return the trace of that reduction. Otherwise return the empty string.
*/
def record(op: Context ?=> Any)(using Context): String =
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3 changes: 2 additions & 1 deletion compiler/src/dotty/tools/dotc/core/Types.scala
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Expand Up @@ -1536,7 +1536,8 @@ object Types extends TypeUtils {

/** If this type can be normalized at the top-level by rewriting match types
* of S[n] types, the result after applying all toplevel normalizations,
* otherwise NoType
* otherwise NoType. Note applied match aliases are not beta reduced
* if the underlying MatchType is stuck.
*/
def tryNormalize(using Context): Type = NoType

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9 changes: 5 additions & 4 deletions compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala
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Expand Up @@ -65,10 +65,11 @@ object ErrorReporting {
val collectMatchTrace = new TypeAccumulator[String]:
def apply(s: String, tp: Type): String =
if s.nonEmpty then s
else tp match
case tp: AppliedType if tp.isMatchAlias => MatchTypeTrace.record(tp.tryNormalize)
case tp: MatchType => MatchTypeTrace.record(tp.tryNormalize)
case _ => foldOver(s, tp)
else
val normed = tp.normalized
val toTrace = if normed.isMatchAlias then normed else normed.dealias
val r = MatchTypeTrace.record(toTrace.tryNormalize)
if r.nonEmpty then r else foldOver(s, tp)
tps.foldLeft("")(collectMatchTrace)

/** A mixin trait that can produce added elements for an error message */
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14 changes: 7 additions & 7 deletions tests/neg/i12049.check → tests/neg/i12049a.check
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@@ -1,4 +1,4 @@
-- [E007] Type Mismatch Error: tests/neg/i12049.scala:6:16 -------------------------------------------------------------
-- [E007] Type Mismatch Error: tests/neg/i12049a.scala:6:16 ------------------------------------------------------------
6 |val x: String = ??? : M[B] // error
| ^^^^^^^^^^
| Found: M[B]
Expand All @@ -15,7 +15,7 @@
| case B => String
|
| longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: tests/neg/i12049.scala:14:17 ------------------------------------------------------------
-- [E007] Type Mismatch Error: tests/neg/i12049a.scala:14:17 -----------------------------------------------------------
14 |val y3: String = ??? : Last[Int *: Int *: Boolean *: String *: EmptyTuple] // error
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| Found: Last[EmptyTuple]
Expand All @@ -31,7 +31,7 @@
| case t *: EmptyTuple => t
|
| longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: tests/neg/i12049.scala:22:20 ------------------------------------------------------------
-- [E007] Type Mismatch Error: tests/neg/i12049a.scala:22:20 -----------------------------------------------------------
22 |val z3: (A, B, A) = ??? : Reverse[(A, B, A)] // error
| ^^^^^^^^^^^^^^^^^^^^^^^^
| Found: Tuple.Concat[Reverse[A *: EmptyTuple.type], (B, A)]
Expand All @@ -48,7 +48,7 @@
| case EmptyTuple => EmptyTuple
|
| longer explanation available when compiling with `-explain`
-- [E172] Type Error: tests/neg/i12049.scala:24:20 ---------------------------------------------------------------------
-- [E172] Type Error: tests/neg/i12049a.scala:24:20 --------------------------------------------------------------------
24 |val _ = summon[M[B]] // error
| ^
| No given instance of type M[B] was found for parameter x of method summon in object Predef
Expand All @@ -62,7 +62,7 @@
| Therefore, reduction cannot advance to the remaining case
|
| case B => String
-- [E172] Type Error: tests/neg/i12049.scala:25:78 ---------------------------------------------------------------------
-- [E172] Type Error: tests/neg/i12049a.scala:25:78 --------------------------------------------------------------------
25 |val _ = summon[String =:= Last[Int *: Int *: Boolean *: String *: EmptyTuple]] // error
| ^
| Cannot prove that String =:= Last[EmptyTuple].
Expand All @@ -75,7 +75,7 @@
|
| case _ *: _ *: t => Last[t]
| case t *: EmptyTuple => t
-- [E172] Type Error: tests/neg/i12049.scala:26:48 ---------------------------------------------------------------------
-- [E172] Type Error: tests/neg/i12049a.scala:26:48 --------------------------------------------------------------------
26 |val _ = summon[(A, B, A) =:= Reverse[(A, B, A)]] // error
| ^
| Cannot prove that (A, B, A) =:= Tuple.Concat[Reverse[A *: EmptyTuple.type], (B, A)].
Expand All @@ -89,7 +89,7 @@
|
| case t1 *: t2 *: ts => Tuple.Concat[Reverse[ts], (t2, t1)]
| case EmptyTuple => EmptyTuple
-- [E008] Not Found Error: tests/neg/i12049.scala:28:21 ----------------------------------------------------------------
-- [E008] Not Found Error: tests/neg/i12049a.scala:28:21 ---------------------------------------------------------------
28 |val _ = (??? : M[B]).length // error
| ^^^^^^^^^^^^^^^^^^^
| value length is not a member of M[B]
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0 tests/neg/i12049.scala → tests/neg/i12049a.scala
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17 changes: 17 additions & 0 deletions tests/neg/i12049b.check
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@@ -0,0 +1,17 @@

-- [E007] Type Mismatch Error: tests/neg/i12049b/B_2.scala:2:17 --------------------------------------------------------
2 |val y1: String = foo // error
| ^^^
| Found: Last[(Int, Int, Boolean, String)]
| Required: String
|
| Note: a match type could not be fully reduced:
|
| trying to reduce Last[EmptyTuple]
| failed since selector EmptyTuple
| matches none of the cases
|
| case _ *: _ *: t => Last[t]
| case t *: EmptyTuple => t
|
| longer explanation available when compiling with `-explain`
6 changes: 6 additions & 0 deletions tests/neg/i12049b/A_1.scala
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@@ -0,0 +1,6 @@

type Last[X <: Tuple] = X match
case _ *: _ *: t => Last[t]
case t *: EmptyTuple => t

def foo: Last[Int *: Int *: Boolean *: String *: EmptyTuple] = ???
2 changes: 2 additions & 0 deletions tests/neg/i12049b/B_2.scala
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@@ -0,0 +1,2 @@

val y1: String = foo // error
34 changes: 34 additions & 0 deletions tests/neg/i12049c.check
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@@ -0,0 +1,34 @@
-- [E007] Type Mismatch Error: tests/neg/i12049c.scala:9:16 ------------------------------------------------------------
9 |val x: String = ??? : M[B] // error
| ^^^^^^^^^^
| Found: M[B]
| Required: String
|
| Note: a match type could not be fully reduced:
|
| trying to reduce M[B]
| failed since selector B
| does not match case A => Int
| and cannot be shown to be disjoint from it either.
| Therefore, reduction cannot advance to the remaining case
|
| case B => String
|
| longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: tests/neg/i12049c.scala:12:17 -----------------------------------------------------------
12 |val x2: String = ??? : MB // error
| ^^^^^^^^
| Found: MB
| Required: String
|
| Note: a match type could not be fully reduced:
|
| trying to reduce M[B]
| failed since selector B
| does not match case A => Int
| and cannot be shown to be disjoint from it either.
| Therefore, reduction cannot advance to the remaining case
|
| case B => String
|
| longer explanation available when compiling with `-explain`
12 changes: 12 additions & 0 deletions tests/neg/i12049c.scala
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@@ -0,0 +1,12 @@

trait A
trait B

type M[X] = X match
case A => Int
case B => String

val x: String = ??? : M[B] // error

type MB = M[B]
val x2: String = ??? : MB // error
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