[CodeCompletion] Add dyn_cast check to prevent crash

rdar://problem/53507773

Author: rintaro

lib/IDE/CodeCompletionResultBuilder.h

@@ -377,8 +377,11 @@ class CodeCompletionResultBuilder {
     // If the parameter is of the type @autoclosure ()->output, then the
     // code completion should show the parameter of the output type
     // instead of the function type ()->output.
-    if (isAutoClosure)
-      Ty = Ty->castTo<FunctionType>()->getResult();
+    if (isAutoClosure) {
+      // 'Ty' may be ErrorType.
+      if (auto funcTy = Ty->getAs<FunctionType>())
+        Ty = funcTy->getResult();
+    }
 
     PrintOptions PO;
     PO.SkipAttributes = true;

test/IDE/complete_call_arg.swift

@@ -92,6 +92,7 @@
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=IMPLICIT_MEMBER_ARRAY_2_SKIPPED | %FileCheck %s -check-prefix=IMPLICIT_MEMBER_SKIPPED
 
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=ARCHETYPE_GENERIC_1 | %FileCheck %s -check-prefix=ARCHETYPE_GENERIC_1
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=PARAM_WITH_ERROR_AUTOCLOSURE| %FileCheck %s -check-prefix=PARAM_WITH_ERROR_AUTOCLOSURE
 
 var i1 = 1
 var i2 = 2
@@ -738,3 +739,15 @@ func testGenricMethodOnGenericOfArchetype<Wrapped>(value: Wrap<Wrapped>) {
 // ARCHETYPE_GENERIC_1: Begin completions
 // ARCHETYPE_GENERIC_1: Decl[InstanceMethod]/CurrNominal:   ['(']{#(fn): (Wrapped) -> _##(Wrapped) -> _#}[')'][#Wrap<_>#];
 }
+
+struct TestHasErrorAutoclosureParam {
+  func hasErrorAutoclosureParam(value: @autoclosure () -> Value) {
+    fatalError()
+  }
+  func test() {
+    hasErrorAutoclosureParam(#^PARAM_WITH_ERROR_AUTOCLOSURE^#
+// PARAM_WITH_ERROR_AUTOCLOSURE: Begin completions, 1 items
+// PARAM_WITH_ERROR_AUTOCLOSURE: Decl[InstanceMethod]/CurrNominal:   ['(']{#value: <<error type>>#}[')'][#Void#];
+// PARAM_WITH_ERROR_AUTOCLOSURE: End completions
+  }
+}