Clang handles variable declaration with unknown type poorly

[urazoff]

For a given code in foo.c file

int foo() {
  some_type *bar;
}

clang gives a cascade of confusing diagnostics:

foo.c:2:3: error: use of undeclared identifier 'some_type'
  some_type *bar;
  ^
foo.c:2:12: error: use of undeclared identifier 'bar'
  some_type *bar;

and AST does not have a node with corresponding invalid VarDecl. So clang parser does not handle this code as variable declaration.

gcc gives far better diagnostics:

foo.c: In function 'foo':
foo.c:2:3: error: unknown type name 'some_type'
    2 |   some_type *bar;
      |   ^~~~~~~~~

In case of foo.cpp, clang works pretty well:

foo.c: In function 'foo':
foo.c:2:3: error: unknown type name 'some_type'
    2 |   some_type *bar;
      |   ^~~~~~~~~

So some_type is handled as unknown type and this code is handled as although invalid but variable declaration.

The above behavior remains for variable declaration without * declarator.

Suggestion is to make clang handle this case consistently for C and C++ code. It will give better diagnostics for users and AST will have invalid declaration nodes and declaration reference nodes to this invalid declarations under RecoveryExpr.