[ModuleInterface] Don't print typealiases that match generic params

These are synthesized to satisfy associated type requirements, but
they're not needed in source, and they look like self-referential
definitions (`typealias X = X`).

lib/AST/ASTPrinter.cpp

@@ -72,8 +72,9 @@ PrintOptions PrintOptions::printTextualInterfaceFile() {
   result.FullyQualifiedTypes = true;
   result.SkipImports = true;
 
-  class UsableFromInlineOnly : public ShouldPrintChecker {
+  class ShouldPrintForTextualInterface : public ShouldPrintChecker {
     bool shouldPrint(const Decl *D, const PrintOptions &options) override {
+      // Skip anything that isn't 'public' or '@usableFromInline'.
       if (auto *VD = dyn_cast<ValueDecl>(D)) {
         AccessScope accessScope =
             VD->getFormalAccessScope(/*useDC*/nullptr,
@@ -81,10 +82,29 @@ PrintOptions PrintOptions::printTextualInterfaceFile() {
         if (!accessScope.isPublic())
           return false;
       }
+
+      // Skip typealiases that just redeclare generic parameters.
+      if (auto *alias = dyn_cast<TypeAliasDecl>(D)) {
+        if (alias->isImplicit()) {
+          const Decl *parent =
+              D->getDeclContext()->getAsDeclOrDeclExtensionContext();
+          if (auto *genericCtx = parent->getAsGenericContext()) {
+            bool matchesGenericParam =
+                llvm::any_of(genericCtx->getInnermostGenericParamTypes(),
+                             [alias](const GenericTypeParamType *param) {
+              return param->getName() == alias->getName();
+            });
+            if (matchesGenericParam)
+              return false;
+          }
+        }
+      }
+
       return ShouldPrintChecker::shouldPrint(D, options);
     }
   };
-  result.CurrentPrintabilityChecker = std::make_shared<UsableFromInlineOnly>();
+  result.CurrentPrintabilityChecker =
+      std::make_shared<ShouldPrintForTextualInterface>();
 
   // FIXME: We don't really need 'public' on everything; we could just change
   // the default to 'public' and mark the 'internal' things.

test/ModuleInterface/conformances.swift

@@ -0,0 +1,20 @@
+// RUN: %target-swift-frontend -emit-interface-path %t.swiftinterface -emit-module -o /dev/null %s
+// RUN: %FileCheck %s < %t.swiftinterface
+// RUN: %FileCheck -check-prefix NEGATIVE %s < %t.swiftinterface
+
+// CHECK-LABEL: public protocol SimpleProto {
+public protocol SimpleProto {
+  // CHECK: associatedtype Element
+  associatedtype Element
+  // CHECK: associatedtype Inferred
+  associatedtype Inferred
+  func inference(_: Inferred)
+} // CHECK: {{^}$}}
+
+// CHECK-LABEL: public struct SimplImpl<Element> : SimpleProto {
+public struct SimplImpl<Element>: SimpleProto {
+  // NEGATIVE-NOT: typealias Element =
+  // CHECK: public func inference(_: Int){{$}}
+  public func inference(_: Int) {}
+  // CHECK: public typealias Inferred = Swift.Int
+} // CHECK: {{^}$}}