[Completion] Tweak assert for subscript call pattern

We may not have a semantic context kind if the
subscript decl is missing, for cases such as
the implicit `keyPath` subscript.

rdar://139333904

Author: hamishknight

lib/IDE/ArgumentCompletion.cpp

@@ -374,12 +374,15 @@ void ArgumentTypeCheckCompletionCallback::collectResults(
       }
       if (Result.FuncTy) {
         if (auto FuncTy = Result.FuncTy) {
+          // Only show call pattern completions if the function isn't
+          // overridden.
           if (ShadowedDecls.count(Result.FuncD) == 0) {
-            // Don't show call pattern completions if the function is
-            // overridden.
             if (Result.IsSubscript) {
-              assert(SemanticContext != SemanticContextKind::None);
+              // The subscript decl may not be preset for e.g the implicit
+              // `keyPath:` subscript. Such a subscript is allowed on any
+              // non-nominal type, so the semantic context may be none.
               auto *SD = dyn_cast_or_null<SubscriptDecl>(Result.FuncD);
+              assert(!SD || SemanticContext != SemanticContextKind::None);
               Lookup.addSubscriptCallPattern(FuncTy, SD, SemanticContext);
             } else {
               auto *FD = dyn_cast_or_null<AbstractFunctionDecl>(Result.FuncD);

test/IDE/complete_subscript.swift

@@ -133,3 +133,10 @@ func testSettableSub(x: inout HasSettableSub) {
 // SETTABLE_SUBSCRIPT-DAG: Pattern/CurrNominal/Flair[ArgLabels]: ['[']{#keyPath: KeyPath<HasSettableSub, Value>#}[']'][#Value#];
 // SETTABLE_SUBSCRIPT-DAG: Decl[Subscript]/CurrNominal/Flair[ArgLabels]: ['[']{#(a): String#}[']'][#@lvalue Int#];
 // SETTABLE_SUBSCRIPT-DAG: Decl[LocalVar]/Local/TypeRelation[Convertible]: local[#String#]; name=local
+
+// rdar://139333904 - Make sure we don't hit an assertion.
+func testFnKeyPathSubscript() {
+  // The keyPath: subscript is supported on all non-nominal types, including functions.
+  test1[#^FN_KEYPATH_SUBSCRIPT^#]
+  // FN_KEYPATH_SUBSCRIPT: Pattern/None/Flair[ArgLabels]: ['[']{#keyPath: KeyPath<() -> (), Value>#}[']'][#Value#]; name=keyPath:
+}