accept ill-formed typedef with not fully defined enum

[Dushistov]

Bugzilla Link	39426
Version	7.0
OS	All
CC	@DougGregor

Extended Description

clang version 7.0.0 (tags/RELEASE_700/final) compiles such code without
any error:

$ cat > test.cpp

typedef enum FooEnum : int FooEnum;
enum FooEnum : int { A = 1, B };
$ clang++ -c -pedantic -std=c++11 test.cpp
$

while gcc (8.2.1) rejects it.

according to https://stackoverflow.com/questions/52971159/strong-enum-typedef-clang-bug-or-c11-standard-uncertainty/52973516#52973516

[dcl.typedef]/1

The typedef specifier shall not be combined in a decl-specifier-seq with any other kind of specifier except a defining-type-specifier,[...]

[dcl.type]/1

defining-type-specifier:

    type-specifier

    class-specifier

    enum-specifier

[dcl.enum]/1

enum-specifier:

    enum-head { enumerator-listopt }

    enum-head { enumerator-list , }

So the code bellow is legal c++:

typedef enum FooEnum : int { A = 1, B } FooEnum;

but this one is not legal c++:

typedef enum FooEnum : int FooEnum;

Because enum FooEnum:int is not a defining-type-specifier.

[Dushistov]

Looks like fixed starting from clang 11.0.1

error: non-defining declaration of enumeration with a fixed underlying type is only permitted as a standalone declaration; missing list of enumerators? [-Welaborated-enum-base]
typedef enum FooEnum : int FooEnum;
                     ^~~~~

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Evgeniy Dushistov (Dushistov)

| | | | --- | --- | | Bugzilla Link | [39426](https://llvm.org/bz39426) | | Version | 7.0 | | OS | All | | CC | @DougGregor |

Extended Description

clang version 7.0.0 (tags/RELEASE_700/final) compiles such code without
any error:

$ cat > test.cpp

typedef enum FooEnum : int FooEnum;
enum FooEnum : int { A = 1, B };
$ clang++ -c -pedantic -std=c++11 test.cpp
$

while gcc (8.2.1) rejects it.

according to https://stackoverflow.com/questions/52971159/strong-enum-typedef-clang-bug-or-c11-standard-uncertainty/52973516#52973516

[dcl.typedef]/1

The typedef specifier shall not be combined in a decl-specifier-seq with any other kind of specifier except a defining-type-specifier,[...]

[dcl.type]/1

defining-type-specifier:

    type-specifier

    class-specifier

    enum-specifier

[dcl.enum]/1

enum-specifier:

    enum-head { enumerator-listopt }

    enum-head { enumerator-list , }

So the code bellow is legal c++:

typedef enum FooEnum : int { A = 1, B } FooEnum;

but this one is not legal c++:

typedef enum FooEnum : int FooEnum;

Because enum FooEnum:int is not a defining-type-specifier.