[Diagnostics] Resolve SR-11677: Offer a better diagnostic when && is used in trailing where clause declaration

Author: RayZhao1998

include/swift/AST/DiagnosticsParse.def

@@ -1524,6 +1524,8 @@ ERROR(expected_generics_type_restriction,none,
       (Identifier))
 ERROR(requires_single_equal,none,
       "use '==' for same-type requirements rather than '='", ())
+ERROR(requires_comma,none,
+      "expected ',' to separate the requirements of this 'where' clause", ())
 ERROR(expected_requirement_delim,none,
       "expected ':' or '==' to indicate a conformance or same-type requirement",
       ())

lib/Parse/ParseGeneric.cpp

@@ -368,6 +368,13 @@ ParserStatus Parser::parseGenericWhereClause(
     BodyContext.reset();
     HasNextReq = consumeIf(tok::comma);
     // If there's a comma, keep parsing the list.
+    // If there's a "&&", diagnose replace with a comma and keep parsing
+    if (Tok.isBinaryOperator() && Tok.getText() == "&&" && !HasNextReq) {
+      diagnose(Tok, diag::requires_comma)
+        .fixItReplace(SourceRange(Tok.getLoc()), ",");
+      consumeToken();
+      HasNextReq = true;
+    }
   } while (HasNextReq);
 
   if (Requirements.empty())

test/Parse/rdar38225184.swift

@@ -0,0 +1,4 @@
+// RUN: %target-typecheck-verify-swift
+
+extension Collection where Element == Int && Index == Int {}
+// expected-error@-1 {{expected ',' to separate the requirements of this 'where' clause}} {{43-45=,}}