[Clang] Avoid crashes when parsing using enum declarations

In Parser::ParseUsingDeclaration(...) when we call ParseEnumSpecifier(...) it is
not calling SetTypeSpecError() on DS when it detects an error. That means that
DS is left set to TST_unspecified. When we then pass DS into
Sema::ActOnUsingEnumDeclaration(...) we hit an llvm_unreachable(...) since it
expects it to be one of three states TST_error, TST_enum or TST_typename.

This fixes https://github.com/llvm/llvm-project/issues/57347

Differential Revision: https://reviews.llvm.org/D132695

clang/lib/Parse/ParseDecl.cpp

@@ -4647,6 +4647,7 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
 
     if (Spec.isSet() && Tok.isNot(tok::identifier)) {
       Diag(Tok, diag::err_expected) << tok::identifier;
+      DS.SetTypeSpecError();
       if (Tok.isNot(tok::l_brace)) {
         // Has no name and is not a definition.
         // Skip the rest of this declarator, up until the comma or semicolon.
@@ -4663,6 +4664,7 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
       Tok.isNot(tok::colon)) {
     Diag(Tok, diag::err_expected_either) << tok::identifier << tok::l_brace;
 
+    DS.SetTypeSpecError();
     // Skip the rest of this declarator, up until the comma or semicolon.
     SkipUntil(tok::comma, StopAtSemi);
     return;
@@ -4838,6 +4840,7 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
   if (!Name && TUK != Sema::TUK_Definition) {
     Diag(Tok, diag::err_enumerator_unnamed_no_def);
 
+    DS.SetTypeSpecError();
     // Skip the rest of this declarator, up until the comma or semicolon.
     SkipUntil(tok::comma, StopAtSemi);
     return;

clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct/p9-0x.cpp

@@ -4,4 +4,4 @@
 // will necessarily be ill-formed as a trailing return type for a function
 // definition), and recover with a "type cannot be defined in a trailing return
 // type" error.
-auto j() -> enum { e3 }; // expected-error{{unnamed enumeration must be a definition}} expected-error {{expected a type}}
+auto j() -> enum { e3 }; // expected-error{{unnamed enumeration must be a definition}}

clang/test/CXX/drs/dr3xx.cpp

@@ -30,7 +30,7 @@ namespace dr301 { // dr301: yes
     typename T::template operator+<int> a; // expected-error {{typename specifier refers to a non-type template}} expected-error +{{}}
     // FIXME: This shouldn't say (null).
     class T::template operator+<int> b; // expected-error {{identifier followed by '<' indicates a class template specialization but (null) refers to a function template}}
-    enum T::template operator+<int> c; // expected-error {{expected identifier}} expected-error {{does not declare anything}}
+    enum T::template operator+<int> c; // expected-error {{expected identifier}}
     enum T::template operator+<int>::E d; // expected-error {{qualified name refers into a specialization of function template 'T::template operator +'}} expected-error {{forward reference}}
     enum T::template X<int>::E e;
     T::template operator+<int>::foobar(); // expected-error {{qualified name refers into a specialization of function template 'T::template operator +'}}

clang/test/Parser/cxx20-using-enum.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+namespace GH57347 {
+namespace A {}
+
+void f() {
+  using enum A::+; // expected-error {{expected identifier}}
+  using enum; // expected-error {{expected identifier or '{'}}
+  using enum class; // expected-error {{expected identifier or '{'}}
+  using enum : blah; // expected-error {{unknown type name 'blah'}} expected-error {{unnamed enumeration must be a definition}}
+}
+}

clang/test/Parser/recovery.cpp

@@ -209,7 +209,7 @@ namespace pr15133 {
 namespace InvalidEmptyNames {
 // These shouldn't crash, the diagnostics aren't important.
 struct ::, struct ::; // expected-error 2 {{expected identifier}} expected-error 2 {{declaration of anonymous struct must be a definition}} expected-warning {{declaration does not declare anything}}
-enum ::, enum ::; // expected-error 2 {{expected identifier}} expected-warning {{declaration does not declare anything}}
+enum ::, enum ::; // expected-error 2 {{expected identifier}}
 struct ::__super, struct ::__super; // expected-error 2 {{expected identifier}} expected-error 2 {{expected '::' after '__super'}}
 struct ::template foo, struct ::template bar; // expected-error 2 {{expected identifier}} expected-error 2 {{declaration of anonymous struct must be a definition}} expected-warning {{declaration does not declare anything}}
 struct ::foo struct::; // expected-error {{no struct named 'foo' in the global namespace}} expected-error {{expected identifier}} expected-error {{declaration of anonymous struct must be a definition}}

clang/test/Sema/enum.c

@@ -160,7 +160,7 @@ struct EnumRedeclStruct {
   } e;
 };
 
-enum struct GH42372_1 { // expected-error {{expected identifier or '{'}} expected-warning {{declaration does not declare anything}}
+enum struct GH42372_1 { // expected-error {{expected identifier or '{'}}
   One
 };
 

clang/test/SemaCXX/cxx1y-variable-templates_top_level.cpp

@@ -432,7 +432,7 @@ namespace nested_name {
   a<int>::b c; // expected-error {{qualified name refers into a specialization of variable template 'a'}}
 
   class a<int> {}; // expected-error {{identifier followed by '<' indicates a class template specialization but 'a' refers to a variable template}}
-  enum a<int> {}; // expected-error {{expected identifier or '{'}} expected-warning {{does not declare anything}}
+  enum a<int> {}; // expected-error {{expected identifier or '{'}}
 }
 
 namespace PR18530 {

clang/test/SemaCXX/enum-scoped.cpp

@@ -114,8 +114,7 @@ enum : long {
   long_enum_val = 10000
 };
 
-enum : long x; // expected-error{{unnamed enumeration must be a definition}} \
-// expected-warning{{declaration does not declare anything}}
+enum : long x; // expected-error{{unnamed enumeration must be a definition}}
 
 void PR9333() {
   enum class scoped_enum { yes, no, maybe };