[CSFix] Add a skeleton fix to require explicit existential coercion

Author: xedin

include/swift/Sema/CSFix.h

@@ -394,6 +394,10 @@ enum class FixKind : uint8_t {
   /// Ignore a type mismatch while trying to infer generic parameter type
   /// from default expression.
   IgnoreDefaultExprTypeMismatch,
+
+  /// Coerce a result type of a call to a particular existential type
+  /// by adding `as any <#Type#>`.
+  AddExplicitExistentialCoercion,
 };
 
 class ConstraintFix {
@@ -2981,6 +2985,26 @@ class IgnoreDefaultExprTypeMismatch : public AllowArgumentMismatch {
   }
 };
 
+class AddExplicitExistentialCoercion final : public ConstraintFix {
+  Type ErasedResultType;
+
+  AddExplicitExistentialCoercion(ConstraintSystem &cs, Type erasedResultTy,
+                                 ConstraintLocator *locator)
+      : ConstraintFix(cs, FixKind::AddExplicitExistentialCoercion, locator),
+        ErasedResultType(erasedResultTy) {}
+
+public:
+  std::string getName() const override {
+    return "add explicit existential type coercion";
+  }
+
+  bool diagnose(const Solution &solution, bool asNote = false) const override;
+
+  static AddExplicitExistentialCoercion *create(ConstraintSystem &cs,
+                                                Type resultTy,
+                                                ConstraintLocator *locator);
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -2183,3 +2183,15 @@ IgnoreDefaultExprTypeMismatch::create(ConstraintSystem &cs, Type argType,
   return new (cs.getAllocator())
       IgnoreDefaultExprTypeMismatch(cs, argType, paramType, locator);
 }
+
+bool AddExplicitExistentialCoercion::diagnose(const Solution &solution,
+                                              bool asNote) const {
+  return false;
+}
+
+AddExplicitExistentialCoercion *
+AddExplicitExistentialCoercion::create(ConstraintSystem &cs, Type resultTy,
+                                       ConstraintLocator *locator) {
+  return new (cs.getAllocator())
+      AddExplicitExistentialCoercion(cs, resultTy, locator);
+}

lib/Sema/CSSimplify.cpp

@@ -13078,6 +13078,7 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::DropAsyncAttribute:
   case FixKind::AllowSwiftToCPointerConversion:
   case FixKind::AllowTupleLabelMismatch:
+  case FixKind::AddExplicitExistentialCoercion:
     llvm_unreachable("handled elsewhere");
   }