[CSFix] Add a fix to detect/diagnose references to invalid declarations

Upon attempt to generate constraints for invalid declaration reference
let's turn that into a hole and record this new fix to diagnose it
once solution has been reached.

Author: xedin

include/swift/Sema/CSFix.h

@@ -281,6 +281,10 @@ enum class FixKind : uint8_t {
 
   /// Resolve type of `nil` by providing a contextual type.
   SpecifyContextualTypeForNil,
+
+  /// Allow expressions to reference invalid declarations by turning
+  /// them into holes.
+  AllowRefToInvalidDecl,
 };
 
 class ConstraintFix {
@@ -2055,6 +2059,21 @@ class SpecifyContextualTypeForNil final : public ConstraintFix {
                                              ConstraintLocator * locator);
 };
 
+class AllowRefToInvalidDecl final : public ConstraintFix {
+  AllowRefToInvalidDecl(ConstraintSystem &cs, ConstraintLocator *locator)
+      : ConstraintFix(cs, FixKind::AllowRefToInvalidDecl, locator) {}
+
+public:
+  std::string getName() const override {
+    return "ignore invalid declaration reference";
+  }
+
+  bool diagnose(const Solution &solution, bool asNote = false) const override;
+
+  static AllowRefToInvalidDecl *create(ConstraintSystem &cs,
+                                       ConstraintLocator *locator);
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -1615,3 +1615,14 @@ SpecifyContextualTypeForNil::create(ConstraintSystem &cs,
                                     ConstraintLocator *locator) {
   return new (cs.getAllocator()) SpecifyContextualTypeForNil(cs, locator);
 }
+
+bool AllowRefToInvalidDecl::diagnose(const Solution &solution,
+                                     bool asNote) const {
+  return false;
+}
+
+AllowRefToInvalidDecl *
+AllowRefToInvalidDecl::create(ConstraintSystem &cs,
+                              ConstraintLocator *locator) {
+  return new (cs.getAllocator()) AllowRefToInvalidDecl(cs, locator);
+}

lib/Sema/CSSimplify.cpp

@@ -10144,7 +10144,8 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyFixConstraint(
   case FixKind::SpecifyLabelToAssociateTrailingClosure:
   case FixKind::AllowKeyPathWithoutComponents:
   case FixKind::IgnoreInvalidResultBuilderBody:
-  case FixKind::SpecifyContextualTypeForNil: {
+  case FixKind::SpecifyContextualTypeForNil:
+  case FixKind::AllowRefToInvalidDecl: {
     return recordFix(fix) ? SolutionKind::Error : SolutionKind::Solved;
   }