Merge pull request swiftlang#35420 from slavapestov/relax-typealias-redeclaration-check

Sema: Allow re-declarations of typealiases in constrained extensions with different generic signatures

Author: slavapestov

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -565,20 +565,22 @@ CheckRedeclarationRequest::evaluate(Evaluator &eval, ValueDecl *current) const {
     if (current == other || (other->hasInterfaceType() && other->isInvalid()))
       continue;
 
+    auto *otherDC = other->getDeclContext();
+
     // Skip declarations in other modules.
-    if (currentModule != other->getModuleContext())
+    if (currentModule != otherDC->getParentModule())
       continue;
 
     // If both declarations are in the same file, only diagnose the second one.
-    if (currentFile == other->getDeclContext()->getParentSourceFile())
+    if (currentFile == otherDC->getParentSourceFile())
       if (current->getLoc().isValid() &&
           ctx.SourceMgr.isBeforeInBuffer(
             current->getLoc(), other->getLoc()))
         continue;
 
     // Don't compare methods vs. non-methods (which only happens with
     // operators).
-    if (currentDC->isTypeContext() != other->getDeclContext()->isTypeContext())
+    if (currentDC->isTypeContext() != otherDC->isTypeContext())
       continue;
 
     // In local context, only consider exact name matches.
@@ -592,7 +594,7 @@ CheckRedeclarationRequest::evaluate(Evaluator &eval, ValueDecl *current) const {
     if (!conflicting(currentSig, otherSig))
       continue;
 
-    // Skip declarations in other files.
+    // Skip inaccessible declarations in other files.
     // In practice, this means we will warn on a private declaration that
     // shadows a non-private one, but only in the file where the shadowing
     // happens. We will warn on conflicting non-private declarations in both
@@ -605,6 +607,15 @@ CheckRedeclarationRequest::evaluate(Evaluator &eval, ValueDecl *current) const {
     if (other->isInvalid())
       continue;
 
+    // Allow redeclarations of typealiases in different constrained
+    // extensions.
+    if (isa<TypeAliasDecl>(current) &&
+        isa<TypeAliasDecl>(other) &&
+        currentDC != otherDC &&
+        currentDC->getGenericSignatureOfContext().getCanonicalSignature() !=
+        otherDC->getGenericSignatureOfContext().getCanonicalSignature())
+      continue;
+
     // Thwart attempts to override the same declaration more than once.
     const auto *currentOverride = current->getOverriddenDecl();
     const auto *otherOverride = other->getOverriddenDecl();

test/decl/protocol/req/missing_conformance.swift

@@ -132,7 +132,7 @@ extension CountSteps1 // expected-error {{type 'CountSteps1<T>' does not conform
      where T : Equatable
 {
   typealias Index = Int
-  // expected-error@-1 {{invalid redeclaration of synthesized implementation for protocol requirement 'Index'}}
+
   func index(_ i: Index, offsetBy d: Int) -> Index {
     return i + d
   }

test/decl/typealias/redeclaration.swift

@@ -0,0 +1,45 @@
+// RUN: %target-typecheck-verify-swift
+
+struct NonGenericStruct {
+  typealias Horse = Int // expected-note {{'Horse' previously declared here}}
+  typealias Horse = String // expected-error {{invalid redeclaration of 'Horse'}}
+}
+
+struct NonGenericExtendedStruct {}
+
+extension NonGenericExtendedStruct {
+  typealias Horse = Int // expected-note {{'Horse' previously declared here}}
+}
+
+extension NonGenericExtendedStruct {
+  typealias Horse = String // expected-error {{invalid redeclaration of 'Horse'}}
+}
+
+struct GenericStruct<T> {
+  typealias Horse = Int // expected-note {{'Horse' previously declared here}}
+  typealias Horse = String // expected-error {{invalid redeclaration of 'Horse'}}
+}
+
+struct GenericExtendedStruct<T> {}
+
+extension GenericExtendedStruct {
+  typealias Horse = Int // expected-note {{'Horse' previously declared here}}
+}
+
+extension GenericExtendedStruct {
+  typealias Horse = String // expected-error {{invalid redeclaration of 'Horse'}}
+}
+
+struct GenericConstrainedExtendedStruct<T> {}
+
+protocol SomeProtocol {}
+
+extension GenericConstrainedExtendedStruct where T : SomeProtocol {
+  typealias Horse = Int
+}
+
+protocol OtherProtocol {}
+
+extension GenericConstrainedExtendedStruct where T : OtherProtocol {
+  typealias Horse = String // This is OK!
+}