[CodeComplete] Fix a nullptr crash

If `FuncDeclRef` is `null` we shouldn’t be trying to get any parameters from it.

Author: ahoppen

lib/IDE/ArgumentCompletion.cpp

@@ -53,7 +53,10 @@ bool ArgumentTypeCheckCompletionCallback::addPossibleParams(
     // Since not all function types are backed by declarations (e.g. closure
     // paramters), `DeclParam` might be `nullptr`.
     const AnyFunctionType::Param *TypeParam = &ParamsToPass[Idx];
-    const ParamDecl *DeclParam = getParameterAt(Res.FuncDeclRef, Idx);
+    const ParamDecl *DeclParam = nullptr;
+    if (Res.FuncDeclRef) {
+      DeclParam = getParameterAt(Res.FuncDeclRef, Idx);
+    }
 
     bool Required = true;
     if (DeclParam && DeclParam->isDefaultArgument()) {

test/IDE/complete_call_pattern_heuristics.swift

@@ -34,3 +34,7 @@ func testArg2Name3() {
 // LABELED_FIRSTARG-DAG: Pattern/Local/Flair[ArgLabels]: {#arg1: Int#}[#Int#];
 // LABELED_FIRSTARG-NOT: ['(']{#arg1: Int#}, {#arg2: Int#}[')'][#Void#];
 
+func subscriptAccess(info: [String: Int]) {
+  info[#^SUBSCRIPT_ACCESS^#]
+// SUBSCRIPT_ACCESS: Pattern/Local/Flair[ArgLabels]:     {#keyPath: KeyPath<[String : Int], Value>#}[#KeyPath<[String : Int], Value>#]; name=keyPath:
+}