Merge pull request #79061 from slavapestov/fix-rdar143707820

Sema: Fix generic type alias resolution edge case

Author: slavapestov

lib/Sema/TypeCheckType.cpp

@@ -6136,6 +6136,8 @@ Type TypeChecker::substMemberTypeWithBase(TypeDecl *member,
     // Cope with the presence of unbound generic types, which are ill-formed
     // at this point but break the invariants of getContextSubstitutionMap().
     if (baseTy->hasUnboundGenericType()) {
+      memberType = memberType->getReducedType(aliasDecl->getGenericSignature());
+
       if (memberType->hasTypeParameter())
         return ErrorType::get(memberType);
 

test/decl/typealias/fully_constrained.swift

@@ -3,22 +3,23 @@
 struct OtherGeneric<U> {}
 
 struct Generic<T> {
-  // FIXME: Should work with 'T' as well
   typealias NonGeneric = Int where T == Int
+  typealias FakeGeneric = T where T == Int
 
   typealias Unbound = OtherGeneric where T == Int
   typealias Generic = OtherGeneric where T == Int
 }
 
 extension Generic where T == Int {
-  // FIXME: Should work with 'T' as well
   typealias NonGenericInExtension = Int
+  typealias FakeGenericInExtension = T
 
   typealias UnboundInExtension = OtherGeneric
   typealias GenericInExtension = OtherGeneric
 }
 
 func use(_: Generic.NonGeneric,
+         _: Generic.FakeGeneric,
          _: Generic.Unbound<String>,
          _: Generic.Generic<String>,
          _: Generic.NonGenericInExtension,
@@ -28,16 +29,20 @@ func use(_: Generic.NonGeneric,
   // FIXME: Get these working too
 #if false
   let _ = Generic.NonGeneric.self
+  let _ = Generic.FakeGeneric.self
   let _ = Generic.Unbound<String>.self
   let _ = Generic.Generic<String>.self
 
   let _ = Generic.NonGenericInExtension.self
+  let _ = Generic.FakeGenericInExtension.self
   let _ = Generic.UnboundInExtension<String>.self
   let _ = Generic.GenericInExtension<String>.self
+#endif
 
   let _: Generic.NonGeneric = 123
+  let _: Generic.FakeGeneric = 123
   let _: Generic.NonGenericInExtension = 123
-#endif
+  let _: Generic.FakeGenericInExtension = 123
 
   let _: Generic.Unbound = OtherGeneric<String>()
   let _: Generic.Generic = OtherGeneric<String>()
@@ -48,11 +53,13 @@ func use(_: Generic.NonGeneric,
 
 struct Use {
   let a1: Generic.NonGeneric
-  let b1: Generic.Unbound<String>
-  let c1: Generic.Generic<String>
+  let b1: Generic.FakeGeneric
+  let c1: Generic.Unbound<String>
+  let d1: Generic.Generic<String>
   let a2: Generic.NonGenericInExtension
-  let b2: Generic.UnboundInExtension<String>
-  let c2: Generic.GenericInExtension<String>
+  let b2: Generic.FakeGenericInExtension
+  let c2: Generic.UnboundInExtension<String>
+  let d2: Generic.GenericInExtension<String>
 }
 
 extension Generic.NonGeneric {}