Handle assertion error in TyperState

#23609 triggers an assertion error in TyperState. The relevant explanation seems to be
in ProtoTypes.scala:
```scala
            // To respect the pre-condition of `mergeConstraintWith` and keep
            // `protoTyperState` committable we must ensure that it does not
            // contain any type variable which don't already exist in the passed
            // TyperState. This is achieved by instantiating any such type
            // variable. NOTE: this does not suffice to discard type variables
            // in ancestors of `protoTyperState`, if this situation ever
            // comes up, an assertion in TyperState will trigger and this code
            // will need to be generalized.
```
We should go to the bottom of it and fix the assertion. But before that's done this PR
offers a temporary hack to catch the exception when it is triggered from a new code path
created by PR #23532. This should fix the regression reported in #23609. We should leave
the issue open as a reminder that we still need a better fix.

Also: handle crash to missing span in a migration helper.

Author: odersky

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

@@ -28,6 +28,8 @@ object TyperState {
 
   opaque type Snapshot = (Constraint, TypeVars, LevelMap)
 
+  class BadTyperStateAssertion(msg: String) extends AssertionError(msg)
+
   extension (ts: TyperState)
     def snapshot()(using Context): Snapshot =
       (ts.constraint, ts.ownedVars, ts.upLevels)
@@ -43,7 +45,7 @@ object TyperState {
 }
 
 class TyperState() {
-  import TyperState.LevelMap
+  import TyperState.{LevelMap, BadTyperStateAssertion}
 
   private var myId: Int = uninitialized
   def id: Int = myId
@@ -269,8 +271,10 @@ class TyperState() {
    */
   private def includeVar(tvar: TypeVar)(using Context): Unit =
     val oldState = tvar.owningState.nn.get
-    assert(oldState == null || !oldState.isCommittable,
-      i"$this attempted to take ownership of $tvar which is already owned by committable $oldState")
+
+    if oldState != null && oldState.isCommittable then
+      throw BadTyperStateAssertion(
+        i"$this attempted to take ownership of $tvar which is already owned by committable $oldState")
     tvar.owningState = new WeakReference(this)
     ownedVars += tvar
 

compiler/src/dotty/tools/dotc/typer/Migrations.scala

@@ -162,6 +162,7 @@ trait Migrations:
   private def checkParentheses(tree: Tree, pt: FunProto)(using Context): Boolean =
     val ptSpan = pt.args.head.span
     ptSpan.exists
+    && tree.span.exists
     && ctx.source.content
       .slice(tree.span.end, ptSpan.start)
       .exists(_ == '(')

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -4368,11 +4368,20 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
               || !isFullyDefined(formal, ForceDegree.none)
                     // more context might constrain type variables which could make implicit scope larger
 
+            // Try to constrain the result using `pt1`, but back out if a BadTyperStateAssertion
+            // is thrown. TODO Find out why the bad typer state arises and prevent it. The try-catch
+            // is a temporary hack to keep projects compiling that would fail otherwise due to
+            // searching more arguments to instantiate implicits (PR #23532). A failing project
+            // is described in issue #23609.
+            def tryConstrainResult(pt: Type): Boolean =
+              try constrainResult(tree.symbol, wtp, pt)
+              catch case ex: TyperState.BadTyperStateAssertion => false
+
             arg.tpe match
               case failed: SearchFailureType if canProfitFromMoreConstraints =>
                 val pt1 = pt.deepenProtoTrans
-                if (pt1 `ne` pt) && (pt1 ne sharpenedPt) && constrainResult(tree.symbol, wtp, pt1)
-                then return implicitArgs(formals, argIndex, pt1)
+                if (pt1 `ne` pt) && (pt1 ne sharpenedPt) && tryConstrainResult(pt1) then
+                  return implicitArgs(formals, argIndex, pt1)
               case _ =>
 
             arg.tpe match