[Diagnostics] Diagnose inability to infer contextual base type for member ref

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -3044,6 +3044,8 @@ ERROR(collection_literal_empty,none,
 ERROR(unresolved_member_no_inference,none,
       "reference to member %0 cannot be resolved without a contextual type",
       (DeclName))
+ERROR(cannot_infer_base_of_unresolved_member,none,
+      "cannot infer contextual base in reference to member %0", (DeclName))
 ERROR(unresolved_collection_literal,none,
       "cannot infer type for empty collection literal without a "
       "contextual type", ())

lib/Sema/CSDiagnostics.cpp

@@ -5976,3 +5976,26 @@ bool AssignmentTypeMismatchFailure::diagnoseAsNote() {
   return false;
 }
 
+bool MissingContextualBaseInMemberRefFailure::diagnoseAsError() {
+  auto *anchor = getAnchor();
+  auto &cs = getConstraintSystem();
+
+  // Member reference could be wrapped into a number of parens
+  // e.g. `((.foo))`.
+  auto *parentExpr = findParentExpr(anchor);
+  do {
+    // If we have found something which isn't a paren let's stop,
+    // otherwise let's keep unwrapping until there are either no
+    // more parens or no more parents...
+    if (!parentExpr || !isa<ParenExpr>(parentExpr))
+      break;
+  } while ((parentExpr = findParentExpr(parentExpr)));
+
+  auto diagnostic = parentExpr || cs.getContextualType(anchor)
+                        ? diag::cannot_infer_base_of_unresolved_member
+                        : diag::unresolved_member_no_inference;
+
+  emitDiagnostic(anchor->getLoc(), diagnostic, MemberName)
+      .highlight(anchor->getSourceRange());
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -2010,7 +2010,7 @@ class NonEphemeralConversionFailure final : public ArgumentMismatchFailure {
   /// valid for the duration of the call, and suggests an alternative to use.
   void emitSuggestionNotes() const;
 };
-	
+
 class AssignmentTypeMismatchFailure final : public ContextualFailure {
 public:
   AssignmentTypeMismatchFailure(ConstraintSystem &cs,
@@ -2025,6 +2025,17 @@ class AssignmentTypeMismatchFailure final : public ContextualFailure {
   bool diagnoseMissingConformance() const;
 };
 
+class MissingContextualBaseInMemberRefFailure final : public FailureDiagnostic {
+  DeclName MemberName;
+
+public:
+  MissingContextualBaseInMemberRefFailure(ConstraintSystem &cs, DeclName member,
+                                          ConstraintLocator *locator)
+      : FailureDiagnostic(cs, locator), MemberName(member) {}
+
+  bool diagnoseAsError();
+};
+
 } // end namespace constraints
 } // end namespace swift
 

lib/Sema/CSFix.cpp

@@ -1050,7 +1050,9 @@ std::string TreatEphemeralAsNonEphemeral::getName() const {
 }
 
 bool SpecifyBaseTypeForContextualMember::diagnose(bool asNote) const {
-  return false;
+  auto &cs = getConstraintSystem();
+  MissingContextualBaseInMemberRefFailure failure(cs, MemberName, getLocator());
+  return failure.diagnose(asNote);
 }
 
 SpecifyBaseTypeForContextualMember *SpecifyBaseTypeForContextualMember::create(