Fix incorrect caching with dependent method parameters (#21699)

The added test case used to fail Ycheck:typer with the seemingly
identicals:

Found: (a: (aa : A{type B = Int}), b: a.B): CCPoly[(aa : A{type B =
Int})]
Required: (a: (aa : A{type B = Int}), b: a.B): CCPoly[(aa : A{type B =
Int})]

In fact one of the `aa` is a a TypeVar instantiated to `A {type B = Int
}`. The MethodType comparison failed the signature check because the
`a.B` where `a` is backed by a type variable had a stale signature
cached.

Fixed by changing `isProvisional` to traverse ParamRefs.

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -144,6 +144,9 @@ object Types extends TypeUtils {
               !t.isPermanentlyInstantiated || test(t.permanentInst, theAcc)
             case t: LazyRef =>
               !t.completed || test(t.ref, theAcc)
+            case t: ParamRef =>
+              (t: Type).mightBeProvisional = false // break cycles
+              test(t.underlying, theAcc)
             case _ =>
               (if theAcc != null then theAcc else ProAcc()).foldOver(false, t)
         end if

tests/neg/i16842.check

@@ -1,4 +1,16 @@
--- Error: tests/neg/i16842.scala:24:7 ----------------------------------------------------------------------------------
-24 |  Liter(SemanticArray[SemanticInt.type], x) // error
-   |       ^
-   |       invalid new prefix (dim: Int): SemanticArray[SemanticInt.type] cannot replace ty.type in type ty.T
+-- [E007] Type Mismatch Error: tests/neg/i16842.scala:24:8 -------------------------------------------------------------
+24 |  Liter(SemanticArray[SemanticInt.type], x) // error // error
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |        Found:    Int => SemanticArray[SemanticInt.type]
+   |        Required: SemanticArray[SemanticType]
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg/i16842.scala:24:41 ------------------------------------------------------------
+24 |  Liter(SemanticArray[SemanticInt.type], x) // error // error
+   |                                         ^
+   |                      Found:    (x : List[Expr2[SemanticInt.type]])
+   |                      Required: ty.T
+   |                      Note that implicit conversions were not tried because the result of an implicit conversion
+   |                      must be more specific than ty.T
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i16842.scala

@@ -21,5 +21,5 @@ def typecheckArrayLiter(
     a: ArrayLiter
 ): Liter[SemanticArray[SemanticType]] = {
   val x: List[Expr2[SemanticInt.type]] = List()
-  Liter(SemanticArray[SemanticInt.type], x) // error
+  Liter(SemanticArray[SemanticInt.type], x) // error // error
 }

tests/pos/dep-poly-class.scala

@@ -0,0 +1,9 @@
+trait A:
+  type B
+
+class CCPoly[T <: A](a: T, b: a.B)
+
+object Test:
+  def test(): Unit =
+    val aa: A { type B = Int } = new A { type B = Int }
+    val x: CCPoly[aa.type] = CCPoly(aa, 1)