[CSFix] Add a fix to detect when type of couldn't be inferred

Author: xedin

lib/Sema/CSFix.cpp

@@ -1603,3 +1603,14 @@ IgnoreInvalidFunctionBuilderBody *IgnoreInvalidFunctionBuilderBody::create(
   return new (cs.getAllocator())
       IgnoreInvalidFunctionBuilderBody(cs, phase, locator);
 }
+
+bool SpecifyContextualTypeForNil::diagnose(const Solution &solution,
+                                           bool asNote) const {
+  return false;
+}
+
+SpecifyContextualTypeForNil *
+SpecifyContextualTypeForNil::create(ConstraintSystem &cs,
+                                    ConstraintLocator *locator) {
+  return new (cs.getAllocator()) SpecifyContextualTypeForNil(cs, locator);
+}

lib/Sema/CSFix.h

@@ -279,6 +279,9 @@ enum class FixKind : uint8_t {
 
   /// Ignore function builder body which fails `pre-check` call.
   IgnoreInvalidFunctionBuilderBody,
+
+  /// Resolve type of `nil` by providing a contextual type.
+  SpecifyContextualTypeForNil,
 };
 
 class ConstraintFix {
@@ -2026,6 +2029,26 @@ class IgnoreInvalidFunctionBuilderBody final : public ConstraintFix {
   create(ConstraintSystem &cs, ErrorInPhase phase, ConstraintLocator *locator);
 };
 
+class SpecifyContextualTypeForNil final : public ConstraintFix {
+  SpecifyContextualTypeForNil(ConstraintSystem &cs,
+                              ConstraintLocator *locator)
+    : ConstraintFix(cs, FixKind::SpecifyContextualTypeForNil, locator) {}
+
+public:
+  std::string getName() const override {
+    return "specify contextual type to resolve `nil` literal";
+  }
+
+  bool diagnose(const Solution &solution, bool asNote = false) const override;
+
+  bool diagnoseForAmbiguity(CommonFixesArray commonFixes) const override {
+    return diagnose(*commonFixes.front().first);
+  }
+
+  static SpecifyContextualTypeForNil *create(ConstraintSystem & cs,
+                                             ConstraintLocator * locator);
+};
+
 } // end namespace constraints
 } // end namespace swift