[cfe-dev] MS-Incompatibility regarding "typename" in dependent context

[Friedman, Eli via cfe-dev]

On 1/31/2017 3:55 PM, Keane, Erich via cfe-dev wrote:
> I'm currently working on a MS compatibility bug that I am pretty stuck with, that I was hoping someone could give me a hand on how to figure out.  This is currently blocking MySQL from building in MS compatibility mode.  First, the reproducer:
>
> template<class T> struct Struct{
>    typedef int TypeDef;
> };
>
> template<class TMP> void foo() {
>    Struct<TMP>::TypeDef varname;
> }
>
> void foo2()
> {
>    foo<int>();
> }
>
> The result is:
> typename_test.cpp:8:9: error: expected ';' after expression
>    Struct<TMP>::TypeDef varname;
>
> This is due to Parser::ParseStatementOrDeclarationAfterAttributes (ParseStmt.cpp:152) deciding that it is an expression.  I've dug into this, and I see it go into the case tok::identifier's branch "Next.isNot(tok::coloncolon)".  Inside that, TryAnnotateName annotates the Tok token as "annot_primary_expr".
>
> I would expect it to be annot_typename or something similar, such that the call to isCXXDeclarationSpecifier (via isDeclarationStatement()) succeeds.

If you assume the code is well-formed, and see just the tokens 
"Struct<TMP>::TypeDef", it's pretty clearly not a typename according to 
normal C++ parsing rules.  Of course, your testcase isn't well-formed, 
so you want to look ahead, figure out that "Struct<TMP>::TypeDef 
varname" can't be an expression, then print a diagnostic and force 
"Struct<TMP>::TypeDef" to parse as a type.
> So I guess my question is:  Am I better off trying to comprehend this functionality in TryAnnotateName, or just alter isCXXDeclarationSpecifier to give a different answer?  Is there a better option that I'm missing?

If you change "Next.isNot(tok::coloncolon)" to 
"Next.isNot(tok::coloncolon) && Next.isNot(tok::less)", it throws you 
onto the right codepath for your testcase.  I'm not sure that's 
precisely what you want, but maybe a start.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
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