[TBDGen] Ensure that we emit default argument generators for subscripts.

Fixes rdar://89258902.

Author: DougGregor

lib/TBDGen/TBDGen.cpp

@@ -848,6 +848,11 @@ void TBDGenVisitor::visitVarDecl(VarDecl *VD) {
   visitAbstractStorageDecl(VD);
 }
 
+void TBDGenVisitor::visitSubscriptDecl(SubscriptDecl *SD) {
+  visitDefaultArguments(SD, SD->getIndices());
+  visitAbstractStorageDecl(SD);
+}
+
 void TBDGenVisitor::visitNominalTypeDecl(NominalTypeDecl *NTD) {
   auto declaredType = NTD->getDeclaredType()->getCanonicalType();
 

lib/TBDGen/TBDGenVisitor.h

@@ -214,6 +214,8 @@ class TBDGenVisitor : public ASTVisitor<TBDGenVisitor> {
 
   void visitVarDecl(VarDecl *VD);
 
+  void visitSubscriptDecl(SubscriptDecl *SD);
+
   void visitEnumDecl(EnumDecl *ED);
 
   void visitEnumElementDecl(EnumElementDecl *EED);

test/TBD/subscript.swift

@@ -0,0 +1,25 @@
+// REQUIRES: VENDOR=apple 
+// RUN: %target-swift-frontend -emit-ir -o/dev/null -parse-as-library -module-name test -validate-tbd-against-ir=all %s
+// RUN: %target-swift-frontend -emit-ir -o/dev/null -parse-as-library -module-name test -validate-tbd-against-ir=all %s -enable-testing
+// RUN: %target-swift-frontend -emit-ir -o/dev/null -parse-as-library -module-name test -validate-tbd-against-ir=all %s -O
+// RUN: %target-swift-frontend -emit-ir -o/dev/null -parse-as-library -module-name test -validate-tbd-against-ir=all %s -enable-testing -O
+
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -typecheck -parse-as-library -module-name test %s -emit-tbd -emit-tbd-path %t/typecheck.tbd
+// RUN: %target-swift-frontend -emit-ir -parse-as-library -module-name test %s -emit-tbd -emit-tbd-path %t/emit-ir.tbd
+// RUN: diff -u %t/typecheck.tbd %t/emit-ir.tbd
+
+public protocol DefaultInit {
+  init()
+}
+
+@available(macOS 10.15, *)
+struct X {
+  subscript<T: DefaultInit>(_ value: Int = 0, other: T = T()) -> Int { value + 1 }
+
+  static subscript<T>(_ value: String = "hello", other: T) -> String {
+    get { value }
+    set { }
+  }
+}
+