MatchError: NoPrefix() when printing code of the NoPrefix() type

[TomasMikula]

Compiler version

3.3.3
3.4.0
3.4.1-RC1
3.4.2-RC1-bin-20240308-18645ee-NIGHTLY

Minimized code

Macro definitions

import scala.quoted.*

inline def noPrefixShortCode: String =
  ${ noPrefixShortCodeImpl }

inline def noPrefixCode: String =
  ${ noPrefixCodeImpl }

inline def noPrefixStructure: String =
  ${ noPrefixStructure }

def noPrefix(using Quotes): quotes.reflect.TypeRepr =
  import quotes.reflect.*
  TypeRepr.of[Unit] match
    case TypeRef(ThisType(TypeRef(prefix, _)), _) => prefix

def noPrefixShortCodeImpl(using Quotes): Expr[String] =
  import quotes.reflect.*
  Expr(Printer.TypeReprShortCode.show(noPrefix))

def noPrefixCodeImpl(using Quotes): Expr[String] =
  import quotes.reflect.*
  Expr(Printer.TypeReprCode.show(noPrefix))

def noPrefixStructure(using Quotes): Expr[String] =
  import quotes.reflect.*
  Expr(Printer.TypeReprStructure.show(noPrefix))

Macro use

@main
def main: Unit =
  println(noPrefixShortCode) // MatchError: NoPrefix() during macro expansion
  println(noPrefixCode)      // MatchError: NoPrefix() during macro expansion
  println(noPrefixStructure) // OK

Output

[info] compiling 2 Scala sources to /Users/tomas/tmp/type-printer-match-error-NoPrefix/target/scala-3.4.2-RC1-bin-20240308-18645ee-NIGHTLY/classes ...
[error] -- Error: /Users/tomas/tmp/type-printer-match-error-NoPrefix/use.scala:3:10 ----
[error] 3 |  println(noPrefixShortCode) // MatchError: NoPrefix() during macro expansion
[error]   |          ^^^^^^^^^^^^^^^^^
[error]   |Exception occurred while executing macro expansion.
[error]   |scala.MatchError: NoPrefix() (of class java.lang.String)
[error]   |	at scala.quoted.runtime.impl.printers.SourceCode$SourceCodePrinter.printType(SourceCode.scala:1251)
[error]   |	at scala.quoted.runtime.impl.printers.SourceCode$.showType(SourceCode.scala:13)
[error]   |	at scala.quoted.runtime.impl.QuotesImpl$$anon$16.show(QuotesImpl.scala:3184)
[error]   |	at scala.quoted.runtime.impl.QuotesImpl$$anon$16.show(QuotesImpl.scala:3183)
[error]   |	at defns$package$.noPrefixShortCodeImpl(defns.scala:19)
[error]   |
[error]   |-----------------------------------------------------------------------------
[error]   |Inline stack trace
[error]   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]   |This location contains code that was inlined from defns.scala:4
[error] 4 |  ${ noPrefixShortCodeImpl }
[error]   |  ^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]    -----------------------------------------------------------------------------
[error] -- Error: /Users/tomas/tmp/type-printer-match-error-NoPrefix/use.scala:4:10 ----
[error] 4 |  println(noPrefixCode)      // MatchError: NoPrefix() during macro expansion
[error]   |          ^^^^^^^^^^^^
[error]   |Exception occurred while executing macro expansion.
[error]   |scala.MatchError: NoPrefix() (of class java.lang.String)
[error]   |	at scala.quoted.runtime.impl.printers.SourceCode$SourceCodePrinter.printType(SourceCode.scala:1251)
[error]   |	at scala.quoted.runtime.impl.printers.SourceCode$.showType(SourceCode.scala:13)
[error]   |	at scala.quoted.runtime.impl.QuotesImpl$$anon$15.show(QuotesImpl.scala:3180)
[error]   |	at scala.quoted.runtime.impl.QuotesImpl$$anon$15.show(QuotesImpl.scala:3179)
[error]   |	at defns$package$.noPrefixCodeImpl(defns.scala:23)
[error]   |
[error]   |-----------------------------------------------------------------------------
[error]   |Inline stack trace
[error]   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]   |This location contains code that was inlined from defns.scala:7
[error] 7 |  ${ noPrefixCodeImpl }
[error]   |  ^^^^^^^^^^^^^^^^^^^^^
[error]    -----------------------------------------------------------------------------
[error] two errors found
[error] (Compile / compileIncremental) Compilation failed

Expectation

Printing NoPrefix should either

• return some stringy representation (like "<no-prefix>"); or
• abort compilation with a useful error message, including the position in user code where the user is trying to print code of NoPrefix.

[nicolasstucki]

I would go against printing "<no-prefix>" because that would hide bugs for the user implementation and our library implementation.

A more useful error message seems to be the solution.

[dwijnand]

I dunno, making a method take a Type and return a string, then typecheck and blow up at runtime because it's the "wrong kind of Type" also sounds like a good way to hide bugs. If we, hypothetically, end up with some library or user code that decomposes a type and constructs a string incorrectly, such as passing a NoPrefix prefix, then they'll realise if they ever care to look at the result, I would think.