[Parse] Updated protocol diagnostic to allow read.

With the `CoroutineAccessors` feature, `read` is allowed along with
`get` and `set`; alter the diagnostic that's issued when a disallowed
introducer is listed in the requirement list to indicate that `read` is
one of those which are valid, but only when the feature is enabled.

Author: nate-chandler

include/swift/AST/DiagnosticsParse.def

@@ -255,6 +255,8 @@ ERROR(static_var_decl_global_scope,none,
       (StaticSpellingKind))
 ERROR(expected_getset_in_protocol,none,
       "expected get or set in a protocol property", ())
+ERROR(expected_getreadset_in_protocol,none,
+      "expected get, read, or set in a protocol property", ())
 ERROR(unexpected_getset_implementation_in_protocol,none,
       "protocol property %0 cannot have a default implementation specified "
       "here; use extension instead", (StringRef))

lib/Parse/ParseDecl.cpp

@@ -8144,7 +8144,10 @@ ParserStatus Parser::parseGetSet(ParseDeclOptions Flags, ParameterList *Indices,
 
       // parsingLimitedSyntax mode cannot have a body.
       if (parsingLimitedSyntax) {
-        diagnose(Tok, diag::expected_getset_in_protocol);
+        auto diag = Context.LangOpts.hasFeature(Feature::CoroutineAccessors)
+                        ? diag::expected_getreadset_in_protocol
+                        : diag::expected_getset_in_protocol;
+        diagnose(Tok, diag);
         Status |= makeParserError();
         break;
       }
@@ -8178,7 +8181,10 @@ ParserStatus Parser::parseGetSet(ParseDeclOptions Flags, ParameterList *Indices,
     // avoid having to deal with them everywhere.
     if (parsingLimitedSyntax && !isAllowedWhenParsingLimitedSyntax(
                                     Kind, SF.Kind == SourceFileKind::SIL)) {
-      diagnose(Loc, diag::expected_getset_in_protocol);
+      auto diag = Context.LangOpts.hasFeature(Feature::CoroutineAccessors)
+                      ? diag::expected_getreadset_in_protocol
+                      : diag::expected_getset_in_protocol;
+      diagnose(Loc, diag);
       continue;
     }
 

test/Parse/coroutine_accessors.swift

@@ -334,10 +334,10 @@ var ir_rm_m: Int {
 protocol P {
   var goodP: Int { read set } //expected-disabled-error{{property in protocol must have explicit { get } or { get set } specifier}}
                               //expected-disabled-error@-1{{expected get or set in a protocol property}}
-  var badP: Int { read modify } //expected-enabled-error{{expected get or set in a protocol property}}
+  var badP: Int { read modify } //expected-enabled-error{{expected get, read, or set in a protocol property}}
                                 //expected-disabled-error@-1{{property in protocol must have explicit { get } or { get set } specifier}}
                                 //expected-disabled-error@-2{{expected get or set in a protocol property}}
   subscript(goodS goodS: Int) -> Int { read set } //expected-disabled-error{{expected get or set in a protocol property}}
-  subscript(badS badS: Int) -> Int { read modify } //expected-enabled-error{{expected get or set in a protocol property}}
+  subscript(badS badS: Int) -> Int { read modify } //expected-enabled-error{{expected get, read, or set in a protocol property}}
                                                    //expected-disabled-error@-1{{expected get or set in a protocol property}}
 }