[AST] Fold away nested ErrorTypes

Remove the now-unnecessary hack where we could build recursive
ErrorTypes.

Author: hamishknight

lib/AST/ASTContext.cpp

@@ -3618,15 +3618,11 @@ static Type replacingTypeVariablesAndPlaceholders(Type ty) {
 Type ErrorType::get(Type originalType) {
   // The original type is only used for printing/debugging, and we don't support
   // solver-allocated ErrorTypes. As such, fold any type variables and
-  // placeholders into bare ErrorTypes, which print as placeholders.
-  //
-  // FIXME: If the originalType is itself an ErrorType we ought to be flattening
-  // it, but that's currently load-bearing as it avoids crashing for recursive
-  // generic signatures such as in `0120-rdar34184392.swift`. To fix this we
-  // ought to change the evaluator to ensure the outer step of a request cycle
-  // returns the same default value as the inner step such that we don't end up
-  // with conflicting generic signatures on encountering a cycle.
+  // placeholders into ErrorTypes. If we have a top-level one, we can return
+  // that directly.
   originalType = replacingTypeVariablesAndPlaceholders(originalType);
+  if (isa<ErrorType>(originalType.getPointer()))
+    return originalType;
 
   ASSERT(originalType);
   ASSERT(!originalType->getRecursiveProperties().isSolverAllocated() &&

lib/Sema/ConstraintSystem.cpp

@@ -1917,9 +1917,6 @@ static Type replacePlaceholderType(PlaceholderType *placeholder,
 
     return Type(gp);
   });
-  if (isa<TypeVariableType>(replacement.getPointer()))
-    return ErrorType::get(ctx);
-
   // For completion, we want to produce an archetype instead of an ErrorType
   // for a top-level generic parameter.
   // FIXME: This is pretty weird, we're producing a contextual type outside of
@@ -1930,8 +1927,8 @@ static Type replacePlaceholderType(PlaceholderType *placeholder,
       return GP->getDecl()->getInnermostDeclContext()->mapTypeIntoContext(GP);
   }
   // Return an ErrorType with the replacement as the original type. Note that
-  // if we failed to replace a type variable with a generic parameter in a
-  // dependent member, `ErrorType::get` will fold it away.
+  // if we failed to replace a type variable with a generic parameter,
+  // `ErrorType::get` will fold it away.
   return ErrorType::get(replacement);
 }