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Make erased capability-safe #23419

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Make erased capability-safe #23419

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@odersky odersky commented Jun 25, 2025

The purpose of erased is to give compile-time evidence without requiring a runtime value. The compile time evidence is typically for a type class instance or a capability. The previous design of erased is not safe in this respect, as any value can be summoned by various means, including

  • an erasedValue expression of the required type,
  • a null value cast to the required type,
  • An erased def or lazy val that is defined as itself using a a loop.

This PR implements a set of changes to make erased a trustable evidence.

  • Drop erased def.
  • Require erased arguments and initializers to be pure expressions.
  • Don't treat erased vals as late initialized.
  • Don't convert erased vals to getters.
  • Add internal and unsafe variants of erasedValue, which, unlike erasedValue itself,
    are treated as pure expressions.

odersky added 10 commits June 24, 2025 16:09
@odersky
Disallow erased def
a6b0c0f
@odersky
Don't treat erased as late initialized
803b0ce 
Since we will force erased expressions to be pure values, they are always
initialized.
@odersky
Rename erasedParams to paramErasureStatuses
22372e2 
# Conflicts:
#	compiler/src/dotty/tools/dotc/ast/TreeInfo.scala
@odersky
Don't convert erased vals to getters
f55d830
@odersky
Fix handling of compiletime.erasedValue
17fe44f 
Fixes scala#23406
@odersky
Fixes to isPureExpr
5b2ad02 
 - erasedValue[<ConstantType>] is now considered to be pure
 - calls of synthetic case class apply are considered pure if the case
   class is NoInits
 - Companions of Scala-2 classes Tuple and Some are assumed to be NoInits
@odersky
Add variants of compiletime.erasedValue
ea8f4ee
@odersky
Require erased values to be pure expressions
69ee4d9
@odersky
Some fixes to doc page
8c64e69
@odersky
Update SemanticDB and scripting tests
a3247a7
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