[AST] Consider storage signature when computing opaque params implicit generic params

Author: LucianoPAlmeida

lib/AST/NameLookup.cpp

@@ -2845,6 +2845,11 @@ createOpaqueParameterGenericParams(GenericContext *genericContext, GenericParamL
   if (!value)
     return { };
 
+  // Consider storage signature opaque parameters for accessor declaration. 
+  if (auto *AD = dyn_cast<AccessorDecl>(value)) {
+    value = AD->getStorage();
+  }
+
   // Functions, initializers, and subscripts can contain opaque parameters.
   ParameterList *params = nullptr;
   if (auto func = dyn_cast<AbstractFunctionDecl>(value))

test/type/opaque_parameters.swift

@@ -115,3 +115,19 @@ typealias FnType<T> = (T) -> Void
 
 func consumingA(fn: (some P) -> Void) { } // expected-error{{'some' cannot appear in parameter position in parameter type '(some P) -> Void'}}
 func consumingB(fn: FnType<some P>) { } // expected-error{{'some' cannot appear in parameter position in parameter type '(some P) -> Void'}}
+
+// https://github.com/apple/swift/issues/61387
+struct I61387 {
+  subscript(s: some StringProtocol) -> String {
+    fatalError()
+  }
+}
+
+struct I61387_1 {
+  subscript(s: some StringProtocol) -> String {
+    get { fatalError() }
+    set {
+      fatalError()
+    }
+  }
+}