[CodeCompletion] Don't offer completion names for closure argument names

Parse a completion token as an empty parameter name (just like `_`) to
prevent the parser to parse it as an expression and offer completions
for it.

rdar://78798718

Author: rintaro

lib/Parse/ParseExpr.cpp

@@ -2460,7 +2460,7 @@ ParserStatus Parser::parseClosureSignatureIfPresent(
   // speculative parse to validate it and look for 'in'.
   if (Tok.isAny(
           tok::at_sign, tok::l_paren, tok::l_square, tok::identifier,
-          tok::kw__)) {
+          tok::kw__, tok::code_complete)) {
     BacktrackingScope backtrack(*this);
 
     // Consume attributes.
@@ -2496,11 +2496,11 @@ ParserStatus Parser::parseClosureSignatureIfPresent(
       }
 
       // Okay, we have a closure signature.
-    } else if (Tok.isIdentifierOrUnderscore()) {
+    } else if (Tok.isIdentifierOrUnderscore() || Tok.is(tok::code_complete)) {
       // Parse identifier (',' identifier)*
       consumeToken();
       while (consumeIf(tok::comma)) {
-        if (Tok.isIdentifierOrUnderscore()) {
+        if (Tok.isIdentifierOrUnderscore() || Tok.is(tok::code_complete)) {
           consumeToken();
           continue;
         }
@@ -2687,7 +2687,7 @@ ParserStatus Parser::parseClosureSignatureIfPresent(
       bool HasNext;
       do {
         SyntaxParsingContext ClParamCtx(SyntaxContext, SyntaxKind::ClosureParam);
-        if (Tok.isNot(tok::identifier, tok::kw__)) {
+        if (Tok.isNot(tok::identifier, tok::kw__, tok::code_complete)) {
           diagnose(Tok, diag::expected_closure_parameter_name);
           status.setIsParseError();
           break;
@@ -2698,7 +2698,10 @@ ParserStatus Parser::parseClosureSignatureIfPresent(
         if (Tok.is(tok::identifier)) {
           nameLoc = consumeIdentifier(name, /*diagnoseDollarPrefix=*/false);
         } else {
-          nameLoc = consumeToken(tok::kw__);
+          assert(Tok.isAny(tok::kw__ , tok::code_complete));
+          // Consume and ignore code_completion token so that completion don't
+          // suggest anything for the parameter name declaration.
+          nameLoc = consumeToken();
         }
         auto var = new (Context)
             ParamDecl(SourceLoc(), SourceLoc(),

test/IDE/complete_in_closures.swift

@@ -1,8 +1,8 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-swift-ide-test -batch-code-completion -source-filename %s -filecheck %raw-FileCheck -completion-output-dir %t
 
-// ERROR_COMMON: found code completion token
-// ERROR_COMMON-NOT: Begin completions
+// EMTPY: Token
+// EMPTY-NOT: Begin completions
 
 //===--- Helper types that are used in this test
 
@@ -194,7 +194,7 @@ acceptsListAndTrailingClosureTVoid(
 
 func getInt() -> Int? { return 0 }
 func testAcceptsTrailingClosureInt1() {
-  acceptsTrailingClosureFooVoid { #^IN_TRAILING_CLOSURE_16?check=WITH_GLOBAL_DECLS^# in
+  acceptsTrailingClosureFooVoid { #^IN_TRAILING_CLOSURE_16?check=EMPTY^# in
     if let myvar = getInt() {
     }
   }
@@ -371,3 +371,18 @@ func testInsideTernaryClosureReturn(test: Bool) -> [String] {
         // SINGLE_TERNARY_EXPR_CLOSURE_CONTEXT: End completions
     }
 }
+
+func testSignature() {
+    func accept<T>(_: () -> T) {}
+
+    accept { #^PARAM_BARE_1?check=EMPTY^# in }
+    accept { #^PARAM_BARE_2?check=EMPTY^#, arg2 in }
+    accept { arg1, #^PARAM_BARE_3?check=EMPTY^# in }
+
+    accept { (#^PARAM_PAREN_1?check=EMPTY^#) in }
+    accept { (#^PARAM_PAREN_2?check=EMPTY^#, arg2) in }
+    accept { (arg1, #^PARAM_PAREN_3?check=EMPTY^#) in }
+
+    accept { (arg1: #^PARAMTYPE_1?check=WITH_GLOBAL_DECLS^#) in }
+    accept { (arg1: Int, arg2: #^PARAMTYPE_2?check=WITH_GLOBAL_DECLS^#) in }
+}