[CodeCompletion] Don't provide 'init(nilLiteral:)' et al in optional context

For unresolved member completion, member decls in `Optional`
referenceable by implicit member expression are useless in most cases.
- `init(_: <#Wrapped#>)`
- `init(nilLiteral: ())`
- `.some(<#Wrapped#>)`
- `.none`

Instead, provide `nil` with erasing `.` instruction.

rdar://problem/47806831

Author: rintaro

lib/IDE/CodeCompletion.cpp

@@ -2737,7 +2737,8 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
   void addKeyword(StringRef Name, Type TypeAnnotation = Type(),
                   SemanticContextKind SK = SemanticContextKind::None,
                   CodeCompletionKeywordKind KeyKind
-                    = CodeCompletionKeywordKind::None) {
+                    = CodeCompletionKeywordKind::None,
+                  unsigned NumBytesToErase = 0) {
     CodeCompletionResultBuilder Builder(
         Sink, CodeCompletionResult::ResultKind::Keyword, SK,
         expectedTypeContext);
@@ -2746,6 +2747,8 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
     Builder.setKeywordKind(KeyKind);
     if (TypeAnnotation)
       addTypeAnnotation(Builder, TypeAnnotation);
+    if (NumBytesToErase > 0)
+      Builder.setNumBytesToErase(NumBytesToErase);
   }
 
   void addKeyword(StringRef Name, StringRef TypeAnnotation,
@@ -3634,6 +3637,14 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
           return false;
       }
 
+      // In optional context, ignore '.init(<some>)', 'init(nilLiteral:)',
+      // '.some(<some>)' and '.none'. They are useless in most cases.
+      if (T->getOptionalObjectType() &&
+          VD->getModuleContext()->isStdlibModule() &&
+          (isa<EnumElementDecl>(VD) || isa<ConstructorDecl>(VD))) {
+        return false;
+      }
+
       // Enum element decls can always be referenced by implicit member
       // expression.
       if (isa<EnumElementDecl>(VD))
@@ -3704,6 +3715,14 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
         // If this is optional type, perform completion for the object type.
         // i.e. 'let _: Enum??? = .enumMember' is legal.
         getUnresolvedMemberCompletions(objT->lookThroughAllOptionalTypes());
+
+        // Add 'nil' keyword with erasing '.' instruction.
+        unsigned bytesToErase = 0;
+        auto &SM = CurrDeclContext->getASTContext().SourceMgr;
+        if (DotLoc.isValid())
+          bytesToErase = SM.getByteDistance(DotLoc, SM.getCodeCompletionLoc());
+        addKeyword("nil", T, SemanticContextKind::ExpressionSpecific,
+                   CodeCompletionKeywordKind::kw_nil, bytesToErase);
       }
       getUnresolvedMemberCompletions(T);
     }

test/IDE/complete_unresolved_members.swift

@@ -14,6 +14,7 @@
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=UNRESOLVED_OPT_1 | %FileCheck %s -check-prefix=UNRESOLVED_3_OPT
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=UNRESOLVED_OPT_2 | %FileCheck %s -check-prefix=UNRESOLVED_3_OPT
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=UNRESOLVED_OPT_3 | %FileCheck %s -check-prefix=UNRESOLVED_3_OPTOPTOPT
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=UNRESOLVED_OPT_4 | %FileCheck %s -check-prefix=UNRESOLVED_OPT_4
 
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=UNRESOLVED_12 | %FileCheck %s -check-prefix=UNRESOLVED_3
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=UNRESOLVED_13 | %FileCheck %s -check-prefix=UNRESOLVED_3
@@ -252,18 +253,42 @@ class C4 {
 // UNRESOLVED_3_OPT: Begin completions
 // UNRESOLVED_3_OPT-DAG: Decl[EnumElement]/ExprSpecific:     North[#SomeEnum1#];
 // UNRESOLVED_3_OPT-DAG: Decl[EnumElement]/ExprSpecific:     South[#SomeEnum1#];
-// UNRESOLVED_3_OPT-DAG: Decl[EnumElement]/ExprSpecific:     none[#Optional<SomeEnum1>#]; name=none
-// UNRESOLVED_3_OPT-DAG: Decl[EnumElement]/ExprSpecific:     some({#SomeEnum1#})[#Optional<SomeEnum1>#];
-// UNRESOLVED_3_OPT-DAG: Decl[Constructor]/CurrNominal:      init({#(some): SomeEnum1#})[#Optional<SomeEnum1>#];
-// UNRESOLVED_3_OPT-DAG: Decl[Constructor]/CurrNominal:      init({#nilLiteral: ()#})[#Optional<SomeEnum1>#];
+// UNRESOLVED_3_OPT-DAG: Keyword[nil]/ExprSpecific/Erase[1]: nil[#SomeEnum1?#]; name=nil
+// UNRESOLVED_3_OPT-NOT: none
+// UNRESOLVED_3_OPT-NOT: some
+// UNRESOLVED_3_OPT-NOT: init({#(some):
+// UNRESOLVED_3_OPT-NOT: init({#nilLiteral:
 
 // UNRESOLVED_3_OPTOPTOPT: Begin completions
 // UNRESOLVED_3_OPTOPTOPT-DAG: Decl[EnumElement]/ExprSpecific:     North[#SomeEnum1#];
 // UNRESOLVED_3_OPTOPTOPT-DAG: Decl[EnumElement]/ExprSpecific:     South[#SomeEnum1#];
-// UNRESOLVED_3_OPTOPTOPT-DAG: Decl[EnumElement]/ExprSpecific:     none[#Optional<SomeEnum1??>#]; name=none
-// UNRESOLVED_3_OPTOPTOPT-DAG: Decl[EnumElement]/ExprSpecific:     some({#SomeEnum1??#})[#Optional<SomeEnum1??>#];
-// UNRESOLVED_3_OPTOPTOPT-DAG: Decl[Constructor]/CurrNominal:      init({#(some): SomeEnum1??#})[#Optional<SomeEnum1??>#];
-// UNRESOLVED_3_OPTOPTOPT-DAG: Decl[Constructor]/CurrNominal:      init({#nilLiteral: ()#})[#Optional<SomeEnum1??>#];
+// UNRESOLVED_3_OPTOPTOPT-DAG: Keyword[nil]/ExprSpecific/Erase[1]: nil[#SomeEnum1???#]; name=nil
+// UNRESOLVED_3_OPTOPTOPT-NOT: none
+// UNRESOLVED_3_OPTOPTOPT-NOT: some
+// UNRESOLVED_3_OPTOPTOPT-NOT: init({#(some):
+// UNRESOLVED_3_OPTOPTOPT-NOT: init({#nilLiteral:
+
+enum Somewhere {
+  case earth, mars
+}
+extension Optional where Wrapped == Somewhere {
+  init(str: String) { fatalError() }
+  static var nowhere: Self { return nil }
+}
+func testOptionalWithCustomExtension() {
+  var _: Somewhere? = .#^UNRESOLVED_OPT_4^#
+// UNRESOLVED_OPT_4: Begin completions
+// UNRESOLVED_OPT_4-DAG: Decl[EnumElement]/ExprSpecific:     earth[#Somewhere#];
+// UNRESOLVED_OPT_4-DAG: Decl[EnumElement]/ExprSpecific:     mars[#Somewhere#];
+// UNRESOLVED_OPT_4-DAG: Keyword[nil]/ExprSpecific/Erase[1]: nil[#Somewhere?#]; name=nil
+// UNRESOLVED_OPT_4-DAG: Decl[Constructor]/CurrNominal:      init({#str: String#})[#Optional<Somewhere>#]; name=init(str: String)
+// UNRESOLVED_OPT_4-DAG: Decl[StaticVar]/CurrNominal/TypeRelation[Identical]: nowhere[#Optional<Somewhere>#]; name=nowhere
+// UNRESOLVED_OPT_4-NOT: none
+// UNRESOLVED_OPT_4-NOT: some
+// UNRESOLVED_OPT_4-NOT: init({#(some):
+// UNRESOLVED_OPT_4-NOT: init({#nilLiteral:
+}
+
 
 class C5 {
   func f1() {