migrator: avoid adding conversion functions when expression types change as well.

lib/Migrator/APIDiffMigratorPass.cpp

@@ -363,6 +363,24 @@ struct APIDiffMigratorPass : public ASTMigratorPass, public SourceEntityWalker {
   std::vector<ConversionFunctionInfo> HelperFuncInfo;
   SourceLoc FileEndLoc;
 
+  /// For a given expression, check whether the type of this expression is
+  /// name alias type, and the name alias type is known to change to raw
+  /// representable type.
+  bool isRecognizedTypeAliasChange(Expr *E) {
+    if (auto Ty = E->getType()) {
+      if (auto *NT = dyn_cast<NameAliasType>(Ty.getPointer())) {
+        for (auto Item: getRelatedDiffItems(NT->getDecl())) {
+          if (auto CI = dyn_cast<CommonDiffItem>(Item)) {
+            if (CI->DiffKind == NodeAnnotation::TypeAliasDeclToRawRepresentable) {
+              return true;
+            }
+          }
+        }
+      }
+    }
+    return false;
+  }
+
   APIDiffMigratorPass(EditorAdapter &Editor, SourceFile *SF,
                       const MigratorOptions &Opts):
     ASTMigratorPass(Editor, SF, Opts), DiffStore(Diags),
@@ -395,6 +413,11 @@ struct APIDiffMigratorPass : public ASTMigratorPass, public SourceEntityWalker {
       if (count > 1)
         continue;
       assert(count == 1);
+
+      // A conversion function will be redundant if the expression will change
+      // from type alias to raw-value representable.
+      if (isRecognizedTypeAliasChange(Cur.ExpressionToWrap))
+        continue;
       auto FuncName = Cur.getFuncName();
 
       // Avoid inserting the helper function if it's already present.

test/Migrator/Inputs/Cities.swift

@@ -87,4 +87,6 @@ public class Wrapper {
     public var rawValue: String
     public typealias RawValue = String
   }
-}
+}
+
+public typealias AliasAttribute = String

test/Migrator/Inputs/string-representable.json

@@ -249,4 +249,15 @@
     "RightComment": "String",
     "ModuleName": "Cities"
   },
+  {
+    "DiffItemKind": "CommonDiffItem",
+    "NodeKind": "TypeAlias",
+    "NodeAnnotation": "TypeAliasDeclToRawRepresentable",
+    "ChildIndex": "0",
+    "LeftUsr": "s:6Cities14AliasAttributea",
+    "LeftComment": "String",
+    "RightUsr": "",
+    "RightComment": "String",
+    "ModuleName": "Cities"
+  },
 ]

test/Migrator/string-representable.swift

@@ -54,3 +54,9 @@ func revert(_ a: AwesomeCityAttribute, b: Wrapper.Attribute) {
   _ = Wrapper.Attribute.init(rawValue: "somevalue")
   _ = b.rawValue
 }
+
+
+func bar(_ c: Container) {
+  let attr: AliasAttribute = ""
+  c.add(single: attr)
+}

test/Migrator/string-representable.swift.expected

@@ -55,6 +55,12 @@ func revert(_ a: AwesomeCityAttribute, b: Wrapper.Attribute) {
   _ = b
 }
 
+
+func bar(_ c: Container) {
+  let attr: AliasAttribute = ""
+  c.add(single: attr)
+}
+
 // Helper function inserted by Swift 4.2 migrator.
 fileprivate func convertToNewAttribute(_ input: String) -> NewAttribute {
 	return NewAttribute(rawValue: input)