c++ - cast operator function compiles fine in g++ but not in other compilers. Why? -

this question has answer here:

• operator cast, gcc , clang: compiler right? 1 answer

consider following program:

struct s {     using t = float;     operator t() { return 9.9f; } }; int main() {     s m;     s::t t = m;     t = m.operator t(); // correct ? } 

the program compiles fine in g++ ( see live demo here )

but fails in compilation in clang++, msvc++ & intel c++ compiler

clang++ gives following errors ( see live demo here )

main.cpp:8:20: error: unknown type name 't'; did mean 's::t'?     t = m.operator t(); // correct ?                    ^                    s::t main.cpp:2:11: note: 's::t' declared here     using t = float; 

msvc++ gives following errors ( see live demo here )

source_file.cpp(8): error c2833: 'operator t' not recognized operator or type source_file.cpp(8): error c2059: syntax error: 'newline' 

intel c++ compiler rejects code ( see live demo here )

so, question compiler right here ? g++ incorrect here or other 3 compilers incorrect here ? c++ standard says ?

[basic.lookup.classref]/7:

if id-expression conversion-function-id, its conversion-type-id first looked in class of object expression , name, if found, used. otherwise looked in context of entire postfix-expression. in each of these lookups, names denote types or templates specializations types considered. [ example:

struct { }; namespace n {   struct {     void g() { }     template <class t> operator t();   }; }  int main() {   n::a a;   a.operator a();  // calls n::a::operator n::a } 

— end example]

this indicates example fine, although in above example, a has been declared type name, visible main.

this discussed in core issue 156, filed way in 1999:

how about:

struct { typedef int t; operator t(); }; struct b : { operator t(); } b; void foo() {   b.a::operator t(); // 2) error t not found in context                      // of postfix-expression? } 

is interpretation correct? or intent error if t found in both scopes , referred different entities?

erwin unruh: intent in both contexts. if find once, that's symbol. if find in both, both symbols must "the same" in respect. (if don't find it, error).

so i'd clang wrong: intent, expressed in wording extent, find t, if in class.