[SyntaxParse] Fix crasher for generic parameter with attributes

It used to crash for:
Attribute without its name (e.g. Foo<@>)
Missing name with attributes (e.g. Foo<@available(*, unavailable)>

- Synthesize identifier token so that the SyntaxParsingContext can
  successfully construct the Attribute syntax.
- Don't throw away the parsed attributes if the parameter name is
  missing. Instead, construct GenericParamater syntax anyway, and handle
  it in ASTGen.

Author: rintaro

lib/Parse/ASTGen.cpp

@@ -976,6 +976,9 @@ GenericParamList *ASTGen::generate(const GenericParameterClauseSyntax &clause,
   params.reserve(clause.getGenericParameterList().getNumChildren());
 
   for (auto elem : clause.getGenericParameterList()) {
+    auto nameTok = elem.getName();
+    if (nameTok.isMissing())
+      break;
 
     DeclAttributes attrs = generateDeclAttributes(elem, Loc, false);
     Identifier name = Context.getIdentifier(elem.getName().getIdentifierText());

lib/Parse/ParseDecl.cpp

@@ -1632,6 +1632,9 @@ ParserStatus Parser::parseDeclAttribute(DeclAttributes &Attributes, SourceLoc At
       }
       consumeToken(tok::code_complete);
       return makeParserCodeCompletionStatus();
+    } else {
+      // Synthesize an r_brace syntax node if the token is absent
+      SyntaxContext->synthesize(tok::identifier, AtLoc.getAdvancedLoc(1));
     }
 
     diagnose(Tok, diag::expected_attribute_name);

lib/Parse/ParseGeneric.cpp

@@ -74,6 +74,7 @@ Parser::parseGenericParameterClauseSyntax() {
 
     // Parse attributes.
     // TODO: Implement syntax attribute parsing.
+    Optional<ParsedAttributeListSyntax> attrs;
     if (Tok.is(tok::at_sign)) {
       SyntaxParsingContext TmpCtxt(SyntaxContext);
       TmpCtxt.setTransparent();
@@ -83,18 +84,23 @@ Parser::parseGenericParameterClauseSyntax() {
       parseDeclAttributeList(attrsAST);
       if (!attrsAST.isEmpty())
         Generator.addDeclAttributes(attrsAST, AttrsLoc);
-      auto attrs = SyntaxContext->popIf<ParsedAttributeListSyntax>();
-      if (attrs)
-        paramBuilder.useAttributes(std::move(*attrs));
+      attrs = SyntaxContext->popIf<ParsedAttributeListSyntax>();
     }
 
     // Parse the name of the parameter.
     auto ident = Context.getIdentifier(Tok.getText());
     auto name = parseIdentifierSyntax(diag::expected_generics_parameter_name);
     if (!name) {
+      if (attrs) {
+        paramBuilder.useAttributes(std::move(*attrs));
+        builder.addGenericParameterListMember(paramBuilder.build());
+      }
       status.setIsParseError();
       break;
     }
+
+    if (attrs)
+      paramBuilder.useAttributes(std::move(*attrs));
     paramBuilder.useName(std::move(*name));
 
     // Parse the ':' followed by a type.

test/Parse/invalid.swift

@@ -145,3 +145,6 @@ enum sr8202_enum<@indirect T> {} // expected-error {{'indirect' is a declaration
 protocol P {
   @available(swift, introduced: 4.2) associatedtype Assoc // expected-error {{'@availability' attribute cannot be applied to this declaration}}
 }
+
+struct genericParamIncomplete1<@> {} // expected-error {{expected an attribute name}} expected-error {{expected an identifier to name generic parameter}}
+struct genericParamIncomplete2<@objc> {} // expected-error {{expected an identifier to name generic parameter}}