[CS] NFC: Remove FreeTypeVariableBinding::UnresolvedType

This is now unused.

Author: hamishknight

include/swift/Sema/ConstraintSystem.h

@@ -102,9 +102,7 @@ enum class FreeTypeVariableBinding {
   /// Disallow any binding of such free type variables.
   Disallow,
   /// Allow the free type variables to persist in the solution.
-  Allow,
-  /// Bind the type variables to UnresolvedType to represent the ambiguity.
-  UnresolvedType
+  Allow
 };
 
 /// Describes whether or not a result builder method is supported.

lib/Sema/CSGen.cpp

@@ -4872,29 +4872,16 @@ bool ConstraintSystem::generateConstraints(
         return convertTypeLocator;
       };
 
-      // Substitute type variables in for placeholder types (and unresolved
-      // types, if allowed).
-      if (allowFreeTypeVariables == FreeTypeVariableBinding::UnresolvedType) {
-        convertType = convertType.transformRec([&](Type type) -> std::optional<Type> {
-          if (type->is<UnresolvedType>() || type->is<PlaceholderType>()) {
-            return Type(createTypeVariable(getLocator(type),
-                                           TVO_CanBindToNoEscape |
-                                               TVO_PrefersSubtypeBinding |
-                                               TVO_CanBindToHole));
-          }
-          return std::nullopt;
-        });
-      } else {
-        convertType = convertType.transformRec([&](Type type) -> std::optional<Type> {
-          if (type->is<PlaceholderType>()) {
-            return Type(createTypeVariable(getLocator(type),
-                                           TVO_CanBindToNoEscape |
-                                               TVO_PrefersSubtypeBinding |
-                                               TVO_CanBindToHole));
-          }
-          return std::nullopt;
-        });
-      }
+      // Substitute type variables in for placeholder types.
+      convertType = convertType.transformRec([&](Type type) -> std::optional<Type> {
+        if (type->is<PlaceholderType>()) {
+          return Type(createTypeVariable(getLocator(type),
+                                         TVO_CanBindToNoEscape |
+                                             TVO_PrefersSubtypeBinding |
+                                             TVO_CanBindToHole));
+        }
+        return std::nullopt;
+      });
 
       addContextualConversionConstraint(expr, convertType, ctp,
                                         convertTypeLocator);

lib/Sema/CSSolver.cpp

@@ -91,10 +91,6 @@ Solution ConstraintSystem::finalize() {
 
     case FreeTypeVariableBinding::Allow:
       break;
-
-    case FreeTypeVariableBinding::UnresolvedType:
-      assignFixedType(tv, ctx.TheUnresolvedType);
-      break;
     }
   }