[CSFix] Fix inability to infer contextual base type for member ref

Author: xedin

lib/Sema/CSFix.cpp

@@ -1048,3 +1048,13 @@ std::string TreatEphemeralAsNonEphemeral::getName() const {
   name += ::getName(ConversionKind);
   return name;
 }
+
+bool SpecifyBaseTypeForContextualMember::diagnose(bool asNote) const {
+  return false;
+}
+
+SpecifyBaseTypeForContextualMember *SpecifyBaseTypeForContextualMember::create(
+    ConstraintSystem &cs, DeclName member, ConstraintLocator *locator) {
+  return new (cs.getAllocator())
+      SpecifyBaseTypeForContextualMember(cs, member, locator);
+}

lib/Sema/CSFix.h

@@ -223,6 +223,10 @@ enum class FixKind : uint8_t {
   /// Allow an ephemeral argument conversion for a parameter marked as being
   /// non-ephemeral.
   TreatEphemeralAsNonEphemeral,
+
+  /// Base type in reference to the contextual member e.g. `.foo` couldn't be
+  /// inferred and has to be specified explicitly.
+  SpecifyBaseTypeForContextualMember,
 };
 
 class ConstraintFix {
@@ -1535,6 +1539,27 @@ class TreatEphemeralAsNonEphemeral final : public AllowArgumentMismatch {
          bool downgradeToWarning);
 };
 
+class SpecifyBaseTypeForContextualMember final : public ConstraintFix {
+  DeclName MemberName;
+
+  SpecifyBaseTypeForContextualMember(ConstraintSystem &cs, DeclName member,
+                                     ConstraintLocator *locator)
+      : ConstraintFix(cs, FixKind::SpecifyBaseTypeForContextualMember, locator),
+        MemberName(member) {}
+
+public:
+  std::string getName() const {
+    const auto baseName = MemberName.getBaseName();
+    return "specify base type in reference to member '" +
+           baseName.userFacingName().str() + "'";
+  }
+
+  bool diagnose(bool asNote = false) const;
+
+  static SpecifyBaseTypeForContextualMember *
+  create(ConstraintSystem &cs, DeclName member, ConstraintLocator *locator);
+};
+
 } // end namespace constraints
 } // end namespace swift