[Fixit] Add a fixit to remove type alias self-referring declaration. (#3418)

* [Fixit] Add a fixit to remote type alias self-referring definition like typealias A = A. rdar://24869708

* [test] Update existing test.

Author: nkcsgexi

include/swift/AST/DiagnosticsSema.def

@@ -3013,6 +3013,10 @@ WARNING(extraneous_default_args_in_call, none,
 //------------------------------------------------------------------------------
 ERROR(circular_reference, none,
       "circular reference", ())
+
+ERROR(redundant_type_alias_define, none,
+"redundant type alias declaration", ())
+
 NOTE(circular_reference_through, none,
      "through reference here", ())
 

lib/Sema/IterativeTypeChecker.cpp

@@ -112,16 +112,35 @@ void IterativeTypeChecker::satisfy(TypeCheckRequest request) {
   }
 }
 
+static bool isSelfRedefinedTypeAliasDecl(const TypeCheckRequest &Request) {
+  if (Request.getKind() == TypeCheckRequest::Kind::ResolveTypeDecl) {
+    if (auto TAD = dyn_cast<TypeAliasDecl>(Request.getAnchor())) {
+      SourceRange SR = TAD->getUnderlyingTypeLoc().getSourceRange();
+      SourceManager &SM = TAD->getASTContext().SourceMgr;
+      CharSourceRange CR = CharSourceRange(SM, SR.Start,
+                                           Lexer::getLocForEndOfToken(SM,
+                                                                      SR.End));
+      return CR.str() == TAD->getNameStr();
+    }
+  }
+  return false;
+}
+
 //===----------------------------------------------------------------------===//
 // Diagnostics
 //===----------------------------------------------------------------------===//
 void IterativeTypeChecker::diagnoseCircularReference(
        ArrayRef<TypeCheckRequest> requests) {
   bool isFirst = true;
   for (const auto &request : requests) {
-    diagnose(request.getLoc(),
-             isFirst ? diag::circular_reference
-                     : diag::circular_reference_through);
+    if (isSelfRedefinedTypeAliasDecl(request)) {
+      diagnose(request.getLoc(), diag::redundant_type_alias_define).
+        fixItRemove(request.getAnchor()->getSourceRange());
+    } else {
+      diagnose(request.getLoc(),
+               isFirst ? diag::circular_reference :
+                         diag::circular_reference_through);
+    }
 
     isFirst = false;
   }

test/Sema/diag_typealias.swift

@@ -0,0 +1,6 @@
+// RUN: %target-parse-verify-swift
+
+struct S {}
+
+typealias S = S // expected-error {{redundant type alias declaration}}{{1-17=}}
+

test/decl/init/basic_init.swift

@@ -10,7 +10,7 @@ class C {
 	init() {}
 }
 
-typealias t = t // expected-error {{circular reference}}
+typealias t = t // expected-error {{redundant type alias declaration}}{{1-17=}}