[Parser] Improve diagnostics for generic parameter list in enum cases

include/swift/AST/DiagnosticsParse.def

@@ -226,6 +226,8 @@ ERROR(let_cannot_be_addressed_property,none,
 ERROR(disallowed_var_multiple_getset,none,
       "'var' declarations with multiple variables cannot have explicit"
       " getters/setters", ())
+ERROR(unexpected_generic_in_enum_case,none,
+      "enum cases cannot have generic parameters. did you mean to attach it to enum declaration?", ())
 
 ERROR(disallowed_init,none,
       "initial value is not allowed here", ())

lib/Parse/ParseDecl.cpp

@@ -8902,7 +8902,7 @@ Parser::parseDeclEnumCase(ParseDeclOptions Flags,
 
   // Parse comma-separated enum elements.
   SmallVector<EnumElementDecl*, 4> Elements;
-  
+
   SourceLoc CommaLoc;
   for (;;) {
     Identifier Name;
@@ -8970,6 +8970,12 @@ Parser::parseDeclEnumCase(ParseDeclOptions Flags,
       }
     }
 
+    // See if there's an illegal generic parameter list.
+    if (startsWithLess(Tok)) {
+      diagnose(Tok, diag::unexpected_generic_in_enum_case);
+      skipUntilGreaterInTypeList();
+    }
+
     // See if there's a following argument type.
     ParserResult<ParameterList> ArgParams;
     SmallVector<Identifier, 4> argumentNames;

test/decl/enum/enumtest.swift

@@ -636,3 +636,10 @@ if case nil = foo1 {} // Okay
 if case .none? = foo1 {} // Okay
 if case nil = foo2 {} // Okay
 if case .none?? = foo2 {} // Okay
+
+enum UnsupportedGenericEnumCase { 
+  case one<Type>(param: Type) // expected-error {{enum cases cannot have generic parameters. did you mean to attach it to enum declaration?}} 
+  // expected-error@-1 {{cannot find type 'Type' in scope}}
+  case two<Partial(param: Int) // expected-error {{enum cases cannot have generic parameters. did you mean to attach it to enum declaration?}}
+  case three<(param: Int) // expected-error {{enum cases cannot have generic parameters. did you mean to attach it to enum declaration?}}
+}