[CodeCompletion] Guard for null interface type in calculateTypeRelationForDecl (#18591)

Fixes a crash.

https://bugs.swift.org/browse/SR-8470
rdar://problem/43057054

Author: rintaro

lib/IDE/CodeCompletion.cpp

@@ -961,7 +961,7 @@ calculateTypeRelationForDecl(const Decl *D, Type ExpectedType,
                              bool UseFuncResultType = true) {
   auto VD = dyn_cast<ValueDecl>(D);
   auto DC = D->getDeclContext();
-  if (!VD)
+  if (!VD || !VD->hasInterfaceType())
     return CodeCompletionResult::ExpectedTypeRelation::Unrelated;
 
   if (auto FD = dyn_cast<AbstractFunctionDecl>(VD)) {

test/IDE/complete_call_arg.swift

@@ -46,6 +46,7 @@
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=FORCED_IUO | %FileCheck %s -check-prefix=MEMBEROF_IUO
 
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GENERIC_TO_GENERIC | %FileCheck %s -check-prefix=GENERIC_TO_GENERIC
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=NESTED_CLOSURE | %FileCheck %s -check-prefix=NESTED_CLOSURE
 
 var i1 = 1
 var i2 = 2
@@ -416,3 +417,11 @@ class Bar {
   // MEMBEROF_IUO: Decl[InstanceVar]/CurrNominal: x[#Int#]; name=x
   // MEMBEROF_IUO: End completions
 }
+
+func curry<T1, T2, R>(_ f: @escaping (T1, T2) -> R) -> (T1) -> (T2) -> R {
+  return { t1 in { t2 in f(#^NESTED_CLOSURE^#, t2) } }
+  // NESTED_CLOSURE: Begin completions
+  // FIXME: Should be '/TypeRelation[Invalid]: t2[#T2#]'
+  // NESTED_CLOSURE: Decl[LocalVar]/Local:               t2; name=t2
+  // NESTED_CLOSURE: Decl[LocalVar]/Local:               t1[#T1#]; name=t1
+}