Add diagnostic for extraneous case keyword when multiple patterns.

Author: Greg Titus

include/swift/AST/DiagnosticsParse.def

@@ -1222,6 +1222,8 @@ ERROR(expected_case_where_expr,PointsToFirstBadToken,
       "expected expression for 'where' guard of 'case'", ())
 ERROR(expected_case_colon,PointsToFirstBadToken,
       "expected ':' after '%0'", (StringRef))
+ERROR(extra_case_keyword,none,
+      "extraneous 'case' keyword in pattern", ())
 ERROR(default_with_where,none,
       "'default' cannot be used with a 'where' guard expression",
       ())

lib/Parse/ParseStmt.cpp

@@ -2633,6 +2633,12 @@ static ParserStatus parseStmtCase(Parser &P, SourceLoc &CaseLoc,
       isFirst = false;
       if (!P.consumeIf(tok::comma))
         break;
+
+      if (P.Tok.is(tok::kw_case)) {
+        P.diagnose(P.Tok, diag::extra_case_keyword)
+          .fixItRemove(SourceRange(P.Tok.getLoc()));
+        P.consumeToken(tok::kw_case);
+      }
     }
   }
 

test/Parse/switch.swift

@@ -73,7 +73,7 @@ default:
 // Multiple cases per case block
 switch x { // expected-error {{switch must be exhaustive}} expected-note{{add a default clause}}
 case 0: // expected-error {{'case' label in a 'switch' must have at least one executable statement}} {{8-8= break}}
-case 1:
+case 1, case 2: // expected-error {{extraneous 'case' keyword in pattern}} {{9-14=}}
   x = 0
 }