AST: Use TypeTransform::transformSubMap() to transform TypeAliasType

Author: slavapestov

include/swift/AST/TypeTransform.h

@@ -433,23 +433,28 @@ case TypeKind::Id:
         if (!newParentType) return newUnderlyingTy;
       }
 
-      auto subMap = alias->getSubstitutionMap();
-      for (Type oldReplacementType : subMap.getReplacementTypes()) {
-        Type newReplacementType = doIt(oldReplacementType, TypePosition::Invariant);
-        if (!newReplacementType)
-          return newUnderlyingTy;
-
-        // If anything changed with the replacement type, we lose the sugar.
-        // FIXME: This is really unfortunate.
-        if (newReplacementType.getPointer() != oldReplacementType.getPointer())
-          return newUnderlyingTy;
-      }
+      if (newParentType && newParentType->isExistentialType())
+        return newUnderlyingTy;
+
+      auto oldSubMap = alias->getSubstitutionMap();
+      auto newSubMap = asDerived().transformSubMap(oldSubMap);
+      if (oldSubMap && !newSubMap)
+        return Type();
 
       if (oldParentType.getPointer() == newParentType.getPointer() &&
-          oldUnderlyingTy.getPointer() == newUnderlyingTy.getPointer())
+          oldUnderlyingTy.getPointer() == newUnderlyingTy.getPointer() &&
+          oldSubMap == newSubMap)
         return t;
 
-      return TypeAliasType::get(alias->getDecl(), newParentType, subMap,
+      // Don't leave local archetypes and type variables behind in sugar
+      // if they don't appear in the underlying type, to avoid confusion.
+      auto props = newSubMap.getRecursiveProperties();
+      if (props.hasLocalArchetype() && !newUnderlyingTy->hasLocalArchetype())
+        return newUnderlyingTy;
+      if (props.hasTypeVariable() && !newUnderlyingTy->hasTypeVariable())
+        return newUnderlyingTy;
+
+      return TypeAliasType::get(alias->getDecl(), newParentType, newSubMap,
                                 newUnderlyingTy);
     }
 

lib/AST/TypeSubstitution.cpp

@@ -536,20 +536,6 @@ TypeSubstituter::transform(TypeBase *type, TypePosition position) {
     return std::nullopt;
   }
 
-  // Special-case TypeAliasType; we need to substitute conformances.
-  if (auto aliasTy = dyn_cast<TypeAliasType>(type)) {
-    Type parentTy;
-    if (auto origParentTy = aliasTy->getParent())
-      parentTy = doIt(origParentTy, TypePosition::Invariant);
-    auto underlyingTy = doIt(aliasTy->getSinglyDesugaredType(),
-                             TypePosition::Invariant);
-    if (parentTy && parentTy->isExistentialType())
-      return underlyingTy;
-    auto subMap = aliasTy->getSubstitutionMap().subst(IFS);
-    return Type(TypeAliasType::get(aliasTy->getDecl(), parentTy,
-                                   subMap, underlyingTy));
-  }
-
   auto oldLevel = level;
   SWIFT_DEFER { level = oldLevel; };
 

test/Constraints/closures.swift

@@ -389,7 +389,7 @@ func rdar21078316() {
 
 // <rdar://problem/20978044> QoI: Poor diagnostic when using an incorrect tuple element in a closure
 var numbers = [1, 2, 3]
-zip(numbers, numbers).filter { $0.2 > 1 }  // expected-error {{value of tuple type '(Int, Int)' has no member '2'}}
+zip(numbers, numbers).filter { $0.2 > 1 }  // expected-error {{value of tuple type 'Zip2Sequence<[Int], [Int]>.Element' (aka '(Int, Int)') has no member '2'}}
 
 
 

test/Constraints/phantom_existential_typealias.swift

@@ -0,0 +1,41 @@
+// RUN: %target-typecheck-verify-swift
+
+// This is an elaborate reduction for a problem where we create a TypeAliasType
+// with a concrete underlying type, but type variables in its substitutions.
+//
+// Specifically, we get something like `G<$1, $2>.B` below, but the underlying
+// type of B is just S, it does not depend on its generic arguments.
+//
+// The solver doesn't simplify the type variables away in this case, so to
+// avoid problems the TypeTransform desugars the typealias in this case.
+
+public protocol P {
+    associatedtype A
+}
+
+public struct Horse<A>: P {}
+
+public struct S {}
+
+@resultBuilder
+public enum E<A, B> {
+    public static func buildExpression<T: P>(_ t: T) -> T where T.A == A {
+        return t
+    }
+
+    public static func buildBlock<T: P>(_ t: T) -> T {
+        return t
+    }
+}
+
+public struct G<A, C: P> where C.A == A {
+    public typealias B = S
+
+    public init(_: A.Type, @E<A, B> _: () -> C) {}
+}
+
+func testHorse() {
+    _ = G(S.self) {
+        Horse()
+    }
+}

test/IDE/complete_call_arg.swift

@@ -1119,7 +1119,7 @@ func testLValueBaseTyOfSubscript() {
   var cache: [String: Codable] = [:]
   if let cached = cache[#^LVALUEBASETY^#
 
-  // LVALUEBASETY-DAG: Decl[Subscript]/CurrNominal/Flair[ArgLabels]/IsSystem: ['[']{#(position): Dictionary<String, any Codable>.Index#}[']'][#(key: String, value: any Codable)#];
+  // LVALUEBASETY-DAG: Decl[Subscript]/CurrNominal/Flair[ArgLabels]/IsSystem: ['[']{#(position): Dictionary<String, any Codable>.Index#}[']'][#Dictionary<String, any Codable>.Element#];
   // LVALUEBASETY-DAG: Decl[Subscript]/CurrNominal/Flair[ArgLabels]/IsSystem: ['[']{#(key): String#}[']'][#@lvalue (any Codable)?#];
 }
 

test/Macros/macro_default_argument_diagnostics.swift

@@ -55,5 +55,5 @@ func testIdentifier(notOkay: Stringified<String> = #stringify(myString)) {}
 // expected-error@+1{{only literals are permitted}}
 func testString(interpolated: Stringified<String> = #stringify("Hello \(0b10001)")) {}
 
-// expected-error@+1{{default argument value of type '(Int, String)' cannot be converted to type 'Int'}}
+// expected-error@+1{{default argument value of type 'Stringified<Int>' (aka '(Int, String)') cannot be converted to type 'Int'}}
 func testReturn(wrongType: Int = #stringify(0)) {}