[CodeComplete] Typecheck SubscriptDecl context prior to completion

Completing signature depends on its GenericEnvironment which is set
during typechecking. Previously, completing signature using generic type
used to cause assertion failure. e.g.

  extension Collection {
    subscript(some index: Int) -> Iterator.<COMPLETE HERE>

rdar://problem/41227754

Author: rintaro

lib/IDE/CodeCompletion.cpp

@@ -1370,8 +1370,7 @@ class CodeCompletionCallbacksImpl : public CodeCompletionCallbacks {
       llvm_unreachable("module scope context handled above");
 
     case DeclContextKind::SubscriptDecl:
-      // FIXME: what do we need to check here?
-      return true;
+      return typeCheckCompletionDecl(cast<SubscriptDecl>(DC));
 
     case DeclContextKind::TopLevelCodeDecl:
       return typeCheckTopLevelCodeDecl(cast<TopLevelCodeDecl>(DC));

test/IDE/complete_type_subscript.swift

@@ -1,3 +1,7 @@
+protocol It {
+  associatedtype Assoc
+}
+
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=PARAM_0 | %FileCheck %s -check-prefix=TOP_LEVEL_0
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=RETURN_0 | %FileCheck %s -check-prefix=TOP_LEVEL_0
 
@@ -86,4 +90,35 @@ struct G5 {
   subscript<T>(x: T) -> T.#^GEN_RETURN_5^# { return 0 }
 }
 // GEN_PARAM_5: Keyword/None:                       Type[#T.Type#];
-// GEN_PARAM_5: Keyword/CurrNominal:                self[#T#];
+// GEN_PARAM_5: Keyword/CurrNominal:                self[#T#];
+
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GEN_PARAM_6 | %FileCheck %s -check-prefix=GEN_PARAM_6
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GEN_RETURN_6 | %FileCheck %s -check-prefix=GEN_PARAM_6
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GEN_EXT_PARAM_6 | %FileCheck %s -check-prefix=GEN_PARAM_6
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GEN_EXT_RETURN_6 | %FileCheck %s -check-prefix=GEN_PARAM_6
+struct G6<T: It> {
+  subscript(a x: T.#^GEN_PARAM_6^#) -> Int { return 0 }
+  subscript(a x: Int) -> T.#^GEN_RETURN_6^# { return 0 }
+}
+extension G6 {
+  subscript(b x: T.#^GEN_EXT_PARAM_6^#) -> Int { return 0 }
+  subscript(b x: Int) -> T.#^GEN_EXT_RETURN_6^# { return 0 }
+}
+// GEN_PARAM_6-DAG: Decl[AssociatedType]/Super:         Assoc;
+// GEN_PARAM_6-DAG: Keyword/None:                       Type[#T.Type#];
+
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GENPROTO_PARAM_1 | %FileCheck %s -check-prefix=GENPROTO_1
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GENPROTO_RETURN_1 | %FileCheck %s -check-prefix=GENPROTO_1
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GENPROTO_EXT_PARAM_1 | %FileCheck %s -check-prefix=GENPROTO_1
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GENPROTO_EXT_RETURN_1 | %FileCheck %s -check-prefix=GENPROTO_1
+protocol GP1 {
+  associatedtype I: It
+  subscript(a x: I.#^GENPROTO_PARAM_1^#) -> Int
+  subscript(ax: Int) -> I.#^GENPROTO_RETURN_1^#
+}
+extension GP1 {
+  subscript(b x: I.#^GENPROTO_EXT_PARAM_1^#) -> Int { return 1 }
+  subscript(b x: Int) -> I.#^GENPROTO_EXT_RETURN_1^# { return 1 }
+}
+// GENPROTO_1-DAG: Decl[AssociatedType]/Super:         Assoc;
+// GENPROTO_1-DAG: Keyword/None:                       Type[#Self.I.Type#];