Improve diagnostics for generics in enum case

include/swift/AST/DiagnosticsParse.def

@@ -217,6 +217,8 @@ ERROR(number_cant_start_decl_name,none,
       (StringRef))
 ERROR(expected_identifier_after_case_comma, PointsToFirstBadToken,
       "expected identifier after comma in enum 'case' declaration", ())
+ERROR(generic_param_cant_be_used_in_enum_case_decl, PointsToFirstBadToken,
+      "generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?", ())
 ERROR(let_cannot_be_computed_property,none,
       "'let' declarations cannot be computed properties", ())
 ERROR(let_cannot_be_observing_property,none,

lib/Parse/ParseDecl.cpp

@@ -8969,6 +8969,32 @@ Parser::parseDeclEnumCase(ParseDeclOptions Flags,
         diagnose(CaseLoc, diag::expected_identifier_in_decl, "enum 'case'");
       }
     }
+
+    // See if there are generic params.
+    auto genericResults = maybeParseGenericParams()
+      .getPtrOrNull();
+    auto enumDecl = dyn_cast<EnumDecl>(CurDeclContext);
+    if (genericResults && !genericResults->getParams().empty() && enumDecl) {
+      auto genericParamDecl = genericResults->getParams().front();
+
+      std::string fixStr;
+      llvm::raw_string_ostream OS(fixStr);
+      genericResults->print(OS);
+
+      // Check if enum is already generic.
+      auto enumGenericParams = enumDecl->getGenericParams();
+      if (enumGenericParams && !enumGenericParams->getParams().empty()) {
+        diagnose(genericParamDecl->getStartLoc(), diag::generic_param_cant_be_used_in_enum_case_decl)
+          .fixItRemove(genericResults->getSourceRange())
+          .fixItReplace(enumGenericParams->getSourceRange(), fixStr);
+      } else {
+        SourceLoc insertLoc = enumDecl->getNameLoc()
+          .getAdvancedLoc(enumDecl->getName().getLength());
+        diagnose(genericParamDecl->getStartLoc(), diag::generic_param_cant_be_used_in_enum_case_decl)
+          .fixItRemove(genericResults->getSourceRange())
+          .fixItInsert(insertLoc, fixStr);
+      }
+    }
 
     // See if there's a following argument type.
     ParserResult<ParameterList> ArgParams;

test/decl/enum/enumtest.swift

@@ -636,3 +636,29 @@ if case nil = foo1 {} // Okay
 if case .none? = foo1 {} // Okay
 if case nil = foo2 {} // Okay
 if case .none?? = foo2 {} // Okay
+
+enum EnumCaseWithGenericDeclaration {
+  // expected-error@+1 {{cannot find type 'T' in scope}}
+  case foo<T>(T) // expected-error {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{11-14=}} {{-2:36-36=<T>}}
+  // expected-error@+1 {{cannot find type 'T' in scope}}
+  case bar<T>(param: T) // expected-error {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{11-14=}} {{-4:36-36=<T>}}
+  case baz<T> // expected-error {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{11-14=}} {{-5:36-36=<T>}}
+  case one, two<Key> // expected-error {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{16-21=}} {{-6:36-36=<Key>}}
+  case three<Element>, four // expected-error {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{13-22=}} {{-7:36-36=<Element>}}
+  // expected-error@+1 {{cannot find type 'T' in scope}}
+  case five<T>(param: T), six // expected-error {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{12-15=}} {{-9:36-36=<T>}}
+  // expected-error@+2 {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{13-16=}} {{-12:36-36=<T>}}
+  // expected-error@+1 {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{23-26=}} {{-12:36-36=<U>}}
+  case seven<T>, eight<U>
+  case nine // OK
+}
+
+enum EnumWithGenericDeclaration<Element> {
+  case foo<T> // expected-error {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{11-14=}} {{-1:32-41=<T>}}
+  // expected-error@+1 {{cannot find type 'Key' in scope}}
+  case bar<Key>(param: Key) // expected-error {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{11-16=}} {{-3:32-41=<Key>}}
+  // expected-error@+2 {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{11-14=}} {{-6:32-41=<T>}}
+  // expected-error@+1 {{generic signature cannot be declared in enum 'case'. did you mean to attach it to enum declaration?}} {{19-22=}} {{-6:32-41=<U>}}
+  case one<T>, two<U>
+  case nine // OK
+}