[cfe-dev] Tooling vs -fdelayed-template-parsing WAS: Clang-cl.exe and the VC++ preprocessor

[Reid Kleckner]

On Mon, Aug 4, 2014 at 10:26 AM, Kim Gräsman <kim.grasman at gmail.com> wrote:

> On Mon, Aug 4, 2014 at 7:14 PM, Manuel Klimek <klimek at google.com> wrote:
>
>>
>> why are those not instantiated when a FrontendAction is run?
>>
>
> My choice of words was bad -- the template is not instantiated here,
> merely parsed. We don't have any template arguments to give it.
>
> We just (a few minutes ago, while experimenting with this) found a case
> where late-parsing a template whose body contains undefined symbols would
> generate an incomplete AST, e.g.
>
> template<class T>
> void foo(const T& t) {
>   UndefinedSymbol::bar(t);  // colon-colon
> }
>
> $ clang-check.exe -ast-dump incomplete.cpp -- -fno-delayed-template-parsing
> TranslationUnitDecl 0xebe7e0 <<invalid sloc>> <invalid sloc>
> |-CXXRecordDecl 0xebeaa0 <<invalid sloc>> <invalid sloc> implicit class
> type_info
> |-TypedefDecl 0xebeb20 <<invalid sloc>> <invalid sloc> implicit size_t
> 'unsigned int'
> |-TypedefDecl 0xebeb80 <<invalid sloc>> <invalid sloc> implicit
> __builtin_va_list 'char *'
> `-FunctionTemplateDecl 0xebed80 <incomplete.cpp:2:1, line:5:1> line:3:6 foo
>   |-TemplateTypeParmDecl 0xebebb0 <line:2:10, col:16> col:16 class T
>   `-FunctionDecl 0xebed10 <line:3:1, line:5:1> line:3:6 foo 'void (const T
> &)'
>     |-ParmVarDecl 0xebec80 <col:10, col:19> col:19 referenced t 'const T &'
>     `-CompoundStmt 0xebee40 <col:22, line:5:1>
>       `-CallExpr 0xebee20 <line:4:3, col:25> '<dependent type>'
>         |-DependentScopeDeclRefExpr 0xebede0 <col:3, col:20> '<dependent
> type>' lvalue
>         `-DeclRefExpr 0xebee04 <col:24> 'const T' lvalue ParmVar 0xebec80
> 't' 'const T &'
>
> This runs without error, while this:
>
> template<class T>
> void foo(const T& t) {
>   UndefinedSymbol.bar(t);  // dot
> }
>
> throws an error, as expected:
>
>   incomplete.cpp:4:4: error: use of undeclared identifier 'UndefinedSymbol'
>      UndefinedSymbol.bar(t);
>      ^
>
> I don't know if this is a general bug in the parser, but it'd be nice to
> always get a diagnostic here.
>

This is an -fms-compatibility hack, which is separate from delayed template
parsing.  I'm surprised we don't have a -Wmicrosoft warning here, though.
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