Add FIXMEs regarding TypeVar#resetInst breaking our caching logic

I'm afraid we'll have to get rid of it if we want our caches to be correct, but
I don't have the time to look into it right now.

[Cherry-picked 7de3663]

Author: smarter

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

@@ -109,6 +109,11 @@ object Types {
     /** Is this type still provisional? This is the case if the type contains, or depends on,
      *  uninstantiated type variables or type symbols that have the Provisional flag set.
      *  This is an antimonotonic property - once a type is not provisional, it stays so forever.
+     *
+     *  FIXME: The semantics of this flag are broken by the existence of `TypeVar#resetInst`,
+     *         a non-provisional type could go back to being provisional after
+     *         a call to `resetInst`. This means all caches that rely on `isProvisional`
+     *         can likely end up returning stale results.
      */
     def isProvisional(using Context): Boolean = mightBeProvisional && testProvisional
 
@@ -4743,6 +4748,10 @@ object Types {
    *  is different from the variable's creation state (meaning unrolls are possible)
    *  in the current typer state.
    *
+   *  FIXME: the "once" in the statement above is not true anymore now that `resetInst`
+   *         exists, this is problematic for caching (see `Type#isProvisional`),
+   *         we should try getting rid of this method.
+   *
    *  @param  origin           the parameter that's tracked by the type variable.
    *  @param  creatorState     the typer state in which the variable was created.
    *  @param  initNestingLevel the initial nesting level of the type variable. (c.f. nestingLevel)