Regression in scalamock/scalamock for accessing nested types

[WojciechMazur]

Based on OpenCB failure scalamock/scalamock, version 7.1.0

Compiler version

Fails in 3.7.0-RC1-bin-20250120-db23c08-NIGHTL
Works in 3.6.3

Minimized code

Not minimized yet

Output

info] compiling 71 Scala sources and 7 Java sources to /Users/wmazur/projects/scala/community-build3/repo/core/jvm/target/scala-3.7.0-RC1-bin-20250120-db23c08-NIGHTLY/test-classes ...
[error] -- Error: /Users/wmazur/projects/scala/community-build3/repo/core/shared/src/test/scala/com/paulbutcher/test/mock/MockTest.scala:318:16 
[error] 318 |        val e = mock[m.Embedded[Double]]
[error]     |                ^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |Exception occurred while executing macro expansion.
[error]     |java.lang.AssertionError: assertion failed: asTerm called on not-a-Term val <none>
[error]     |   at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
[error]     |   at dotty.tools.dotc.core.Symbols$Symbol.asTerm(Symbols.scala:186)
[error]     |   at scala.quoted.runtime.impl.QuotesImpl$reflect$DefDef$.apply$$anonfun$3(QuotesImpl.scala:306)
[error]     |   at scala.quoted.runtime.impl.QuotesImpl$reflect$.scala$quoted$runtime$impl$QuotesImpl$reflect$$$withDefaultPos(QuotesImpl.scala:3144)
[error]     |   at scala.quoted.runtime.impl.QuotesImpl$reflect$DefDef$.apply(QuotesImpl.scala:306)
[error]     |   at scala.quoted.runtime.impl.QuotesImpl$reflect$DefDef$.apply(QuotesImpl.scala:303)
[error]     |   at org.scalamock.clazz.MockMaker$.$anonfun$5(MockMaker.scala:141)
[error]     |   at scala.collection.immutable.List.flatMap(List.scala:294)
[error]     |   at org.scalamock.clazz.MockMaker$.instance(MockMaker.scala:98)
[error]     |   at org.scalamock.clazz.MockImpl$.mock(MockImpl.scala:34)
[error]     |
[error]     |---------------------------------------------------------------------------
[error]     |Inline stack trace
[error]     |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]     |This location contains code that was inlined from Mock.scala:30
[error]  30 |  inline def mock[T](implicit mockContext: MockContext): T = ${MockImpl.mock[T]('{mockContext})}
[error]     |                                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]      ---------------------------------------------------------------------------
[error] -- Error: /Users/wmazur/projects/scala/community-build3/repo/core/shared/src/test/scala/newapi/NewApiSpec.scala:274:16 
[error] 274 |    val e = stub[m.Embedded[Double]]
[error]     |            ^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |Exception occurred while executing macro expansion.
[error]     |java.lang.AssertionError: assertion failed: asTerm called on not-a-Term val <none>
[error]     |   at scala.runtime.Scala3RunTime$.assertFailed(Scala3RunTime.scala:8)
[error]     |   at dotty.tools.dotc.core.Symbols$Symbol.asTerm(Symbols.scala:186)
[error]     |   at scala.quoted.runtime.impl.QuotesImpl$reflect$DefDef$.apply$$anonfun$3(QuotesImpl.scala:306)
[error]     |   at scala.quoted.runtime.impl.QuotesImpl$reflect$.scala$quoted$runtime$impl$QuotesImpl$reflect$$$withDefaultPos(QuotesImpl.scala:3144)
[error]     |   at scala.quoted.runtime.impl.QuotesImpl$reflect$DefDef$.apply(QuotesImpl.scala:306)
[error]     |   at scala.quoted.runtime.impl.QuotesImpl$reflect$DefDef$.apply(QuotesImpl.scala:303)
[error]     |   at org.scalamock.stubs.internal.StubMaker.$anonfun$4(StubMaker.scala:131)
[error]     |   at scala.collection.immutable.List.flatMap(List.scala:294)
[error]     |   at org.scalamock.stubs.internal.StubMaker.newInstance(StubMaker.scala:112)
[error]     |   at org.scalamock.stubs.Stubs$package$.stubMacro(Stubs.scala:210)
[error]     |   at org.scalamock.stubs.Stubs$package$.inline$stubMacro(Stubs.scala:209)
[error]     |
[error]     |---------------------------------------------------------------------------
[error]     |Inline stack trace
[error]     |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]     |This location contains code that was inlined from Stubs.scala:183
[error] 183 |  ${ stubMacro[T]('{ collector }) }
[error]     |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[error]     |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
[error]     |This location contains code that was inlined from Stubs.scala:183
[error]  42 |  inline def stub[T]: Stub[T] = stubImpl[T]
[error]     |                                ^^^^^^^^^^^
[error]      ---------------------------------------------------------------------------
[error] two errors found
[error] (scalamockJVM / Test / compileIncremental) Compilation failed

Expectation

[WojciechMazur]

@jchyb Can you take a look at it? You're having the most expertise with both scalamock codebase and quotes-reflection

[jchyb]

Minimised:
Macro.scala

import scala.quoted.*

object MockMaker:
  inline def mock[T]: T = ${instance[T]}

  def instance[T: Type](using quotes: Quotes): Expr[T] =
    import quotes.reflect._
    val tpe = TypeRepr.of[T]
    val symbol = tpe.typeSymbol.methodMember("innerTraitInOptions").head
    val a = tpe.memberType(symbol)
    println(a.show)

    '{???}


trait PolymorphicTrait {
  
  trait Embedded {
    trait ATrait[A, B]
    def innerTraitInOptions(x: ATrait[String, Int]): ATrait[String, Int]
  }
}

Test.scala

@main def Test = 
  val m = new PolymorphicTrait {}
  val e = MockMaker.mock[m.Embedded]

In 3.6.3 the shown method type is:

(x: m.Embedded#ATrait[scala.Predef.String, scala.Int])m.Embedded#ATrait[scala.Predef.String, scala.Int]

In 3.7.0-RC1-bin-20250120-db23c08-NIGHTLY:

(x: scala.Nothing)m.Embedded#ATrait[scala.Predef.String, scala.Int]

For some reason we lose the information about the method argument, which later causes issues in ScalaMock.

[jchyb]

Bisect script points to 26ecda5 (I adjusted the minimisation slightly to error on arguments of type Nothing)

[jchyb]

The issue lies with asSeenFrom used by the typeMember method. Since it is usually used for type checker purposes, it tends to approximate the types if the prefixing type is unstable (like in the m.Embedded here) - in the commit introducing the regression the rule for isLegalPrefix was changed to not require being in the Typer phase, which means that indeed, now the m.Embedded#ATrait[scala.Predef.String, scala.Int] is approximated to Nothing if non-transparent macro is used. It also means that the issue shown here was also a problem if the macro was transparent, even before 26ecda5 introduced the changes. I imagine that ideally, we should have an asSeenFrom-like method that does not approximate the types, for use in macros.