Fix substituters

There's something problematic going on here. TypeMaps usually take a context as a
parameter, which becomes a private parameter acessor field. That parameter shadows
the mapCtx field inherited from TypeMap. `mapCtx` is a variable that is temporarily
mutated when mapping LazyRefs. But since it is shadowed, such mutations do not
affect code that is defined in the subclass of TypeMap.

One fix is to explicitly pass mapCtx to all code that is called from the maps.
That's what's done here for substituters. There might be other solutions as well,
which have to be tried out.

Author: odersky

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

@@ -162,39 +162,39 @@ object Substituters:
     }
 
   final class SubstBindingMap(from: BindingType, to: BindingType)(using Context) extends DeepTypeMap {
-    def apply(tp: Type): Type = subst(tp, from, to, this)
+    def apply(tp: Type): Type = subst(tp, from, to, this)(using mapCtx)
   }
 
   final class Subst1Map(from: Symbol, to: Type)(using Context) extends DeepTypeMap {
-    def apply(tp: Type): Type = subst1(tp, from, to, this)
+    def apply(tp: Type): Type = subst1(tp, from, to, this)(using mapCtx)
   }
 
   final class Subst2Map(from1: Symbol, to1: Type, from2: Symbol, to2: Type)(using Context) extends DeepTypeMap {
-    def apply(tp: Type): Type = subst2(tp, from1, to1, from2, to2, this)
+    def apply(tp: Type): Type = subst2(tp, from1, to1, from2, to2, this)(using mapCtx)
   }
 
   final class SubstMap(from: List[Symbol], to: List[Type])(using Context) extends DeepTypeMap {
-    def apply(tp: Type): Type = subst(tp, from, to, this)
+    def apply(tp: Type): Type = subst(tp, from, to, this)(using mapCtx)
   }
 
   final class SubstSymMap(from: List[Symbol], to: List[Symbol])(using Context) extends DeepTypeMap {
-    def apply(tp: Type): Type = substSym(tp, from, to, this)
+    def apply(tp: Type): Type = substSym(tp, from, to, this)(using mapCtx)
   }
 
   final class SubstThisMap(from: ClassSymbol, to: Type)(using Context) extends DeepTypeMap {
-    def apply(tp: Type): Type = substThis(tp, from, to, this)
+    def apply(tp: Type): Type = substThis(tp, from, to, this)(using mapCtx)
   }
 
   final class SubstRecThisMap(from: Type, to: Type)(using Context) extends DeepTypeMap {
-    def apply(tp: Type): Type = substRecThis(tp, from, to, this)
+    def apply(tp: Type): Type = substRecThis(tp, from, to, this)(using mapCtx)
   }
 
   final class SubstParamMap(from: ParamRef, to: Type)(using Context) extends DeepTypeMap {
-    def apply(tp: Type): Type = substParam(tp, from, to, this)
+    def apply(tp: Type): Type = substParam(tp, from, to, this)(using mapCtx)
   }
 
   final class SubstParamsMap(from: BindingType, to: List[Type])(using Context) extends DeepTypeMap {
-    def apply(tp: Type): Type = substParams(tp, from, to, this)
+    def apply(tp: Type): Type = substParams(tp, from, to, this)(using mapCtx)
   }
 
   /** An approximating substitution that can handle wildcards in the `to` list */