Sema: Allow duplicate internal parameter names on protocol requirements

This fixes a recent source compatibility regression.

Fixes <rdar://problem/70445751>.

Author: slavapestov

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1318,13 +1318,21 @@ static void maybeDiagnoseClassWithoutInitializers(ClassDecl *classDecl) {
   diagnoseClassWithoutInitializers(classDecl);
 }
 
-void TypeChecker::checkParameterList(ParameterList *params) {
+void TypeChecker::checkParameterList(ParameterList *params,
+                                     DeclContext *owner) {
   for (auto param: *params) {
     checkDeclAttributes(param);
   }
 
-  // Check for duplicate parameter names.
-  diagnoseDuplicateDecls(*params);
+  // For source compatibilty, allow duplicate internal parameter names
+  // on protocol requirements.
+  //
+  // FIXME: Consider turning this into a warning or error if we do
+  // another -swift-version.
+  if (!isa<ProtocolDecl>(owner->getParent())) {
+    // Check for duplicate parameter names.
+    diagnoseDuplicateDecls(*params);
+  }
 }
 
 void TypeChecker::diagnoseDuplicateBoundVars(Pattern *pattern) {
@@ -1769,7 +1777,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     (void) SD->isSetterMutating();
     (void) SD->getImplInfo();
 
-    TypeChecker::checkParameterList(SD->getIndices());
+    TypeChecker::checkParameterList(SD->getIndices(), SD);
 
     checkDefaultArguments(SD->getIndices());
 
@@ -2329,7 +2337,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
       checkAccessControl(FD);
 
-      TypeChecker::checkParameterList(FD->getParameters());
+      TypeChecker::checkParameterList(FD->getParameters(), FD);
     }
 
     TypeChecker::checkDeclAttributes(FD);
@@ -2440,7 +2448,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     TypeChecker::checkDeclAttributes(EED);
 
     if (auto *PL = EED->getParameterList()) {
-      TypeChecker::checkParameterList(PL);
+      TypeChecker::checkParameterList(PL, EED);
 
       checkDefaultArguments(PL);
     }
@@ -2596,7 +2604,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     }
 
     TypeChecker::checkDeclAttributes(CD);
-    TypeChecker::checkParameterList(CD->getParameters());
+    TypeChecker::checkParameterList(CD->getParameters(), CD);
 
     // Check whether this initializer overrides an initializer in its
     // superclass.

lib/Sema/TypeCheckStmt.cpp

@@ -2061,7 +2061,7 @@ TypeCheckFunctionBodyRequest::evaluate(Evaluator &evaluator,
 }
 
 bool TypeChecker::typeCheckClosureBody(ClosureExpr *closure) {
-  TypeChecker::checkParameterList(closure->getParameters());
+  TypeChecker::checkParameterList(closure->getParameters(), closure);
 
   BraceStmt *body = closure->getBody();
 

lib/Sema/TypeChecker.h

@@ -448,7 +448,7 @@ void typeCheckDecl(Decl *D);
 
 void addImplicitDynamicAttribute(Decl *D);
 void checkDeclAttributes(Decl *D);
-void checkParameterList(ParameterList *params);
+void checkParameterList(ParameterList *params, DeclContext *owner);
 
 void diagnoseDuplicateBoundVars(Pattern *pattern);
 

test/Sema/redeclaration-checking.swift

@@ -98,4 +98,17 @@ func stmtTest() {
 func fullNameTest() {
   let x = 123 // expected-warning {{never used}}
   func x() {}
+}
+
+// For source compatibility, allow duplicate parameter labels on
+// protocol requirements.
+protocol SillyProtocol {
+  init(x: Int, x: Int)
+  init(a x: Int, b x: Int)
+
+  func foo(x: Int, x: Int)
+  func foo(a x: Int, b x: Int)
+
+  subscript(x: Int, x: Int) -> Int { get }
+  subscript(a x: Int, b x: Int) -> Int { get }
 }