我正在尝试诊断由不同的隐式扩展错误引起的编译器错误。
这是我所看到的：
❯ scala
Welcome to Scala 2.13.1 (OpenJDK 64-Bit Server VM, Java 1.8.0_252).
Type in expressions for evaluation. Or try :help.
scala> :settings -Xlog-implicits
scala> :paste
// Entering paste mode (ctrl-D to finish)
trait Invariant[A]
trait Covariant[+A]
trait Foo
implicit def invariantTuple1[T1: Invariant]: Invariant[Tuple1[T1]] = ???
implicit def invariantFoo: Invariant[Foo] = ???
def a = implicitly[Invariant[Foo]]
implicit def covariant[A: Invariant]: Covariant[A] = ???
def b = implicitly[Covariant[Foo]]
// Exiting paste mode, now interpreting.
def b = implicitly[Covariant[Foo]]
^
<pastie>:13: invariantTuple1 is not a valid implicit value for Invariant[T1] because:
hasMatchingSymbol reported error: diverging implicit expansion for type Invariant[T1]
starting with method invariantFoo
def b = implicitly[Covariant[Foo]]
^
<pastie>:13: invariantFoo is not a valid implicit value for Invariant[T1] because:
diverging implicit expansion for type Invariant[T1]
starting with method invariantTuple1
def b = implicitly[Covariant[Foo]]
^
<pastie>:13: this.invariantTuple1 is not a valid implicit value for Invariant[T1] because:
hasMatchingSymbol reported error: diverging implicit expansion for type Invariant[T1]
starting with method invariantFoo
def b = implicitly[Covariant[Foo]]
^
<pastie>:13: covariant is not a valid implicit value for Covariant[Foo] because:
hasMatchingSymbol reported error: ambiguous implicit values:
both method invariantFoo of type => Invariant[Foo]
and method invariantTuple1 of type [T1](implicit evidence$1: Invariant[T1])Invariant[(T1,)]
match expected type Invariant[A]
def b = implicitly[Covariant[Foo]]
^
<pastie>:13: this.covariant is not a valid implicit value for Covariant[Foo] because:
hasMatchingSymbol reported error: ambiguous implicit values:
both method invariantFoo of type => Invariant[Foo]
and method invariantTuple1 of type [T1](implicit evidence$1: Invariant[T1])Invariant[(T1,)]
match expected type Invariant[A]
def b = implicitly[Covariant[Foo]]
^
<pastie>:13: error: could not find implicit value for parameter e: Covariant[Foo]
我在这里将前两条消息作为一个单独的问题询问为什么 scalac 似乎将隐式扩展得太远了？ https://stackoverflow.com/questions/62335023/why-does-scalac-seem-to-expand-implicits-too-far但我已经包含了输出，因为它可能与这个问题相关。
对于这个问题我想知道的是为什么它这么说：
covariant is not a valid implicit value for Covariant[Foo] because:
hasMatchingSymbol reported error: ambiguous implicit values:
both method invariantFoo of type => Invariant[Foo]
and method invariantTuple1 of type [T1](implicit evidence$1: Invariant[T1])Invariant[(T1,)]
match expected type Invariant[A]
为什么invariantTuple1匹配预期类型Invariant[A]？难道它不知道吗A is Foo那么这不是一个有效的匹配？
None
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