What is the rationale behind the differences between the compiler diagnostics issued when it sees std::vector<Typo> and std::vector<struct Typo>

Question

Consider these two programs and their attempted compilations.

#include <vector>

int main() {
    std::vector<struct Typo> a; // Errors centered around `Typo` being an incomplete type
}

gcc 6.3, for example, gives error messages centered around Typo being an incomplete type, including

/usr/include/c++/6/bits/stl_vector.h:161:9: error: invalid use of incomplete type ‘struct main()::Typo’

#include <vector>

int main() {
    std::vector<Typo> b; // Errors centred around `Typo` not being defined
}

gcc 6.3, for example, gives errors centered around Typo not being defined, including

prog.cpp:4:14: error: ‘Typo’ was not declared in this scope

Both vector instantiations are errors of course, but what is the rationale for the differing diagnostic messages as cited in the program comments?

Answer 1

struct Typo is an elaborated type specifier. That affects how name lookup works.

[basic.lookup.elab]

2: If the elaborated-type-specifier has no nested-name-specifier, and unless the elaborated-type-specifier appears in a declaration with the following form:

class-key attribute-specifier-seq identifier ;

the identifier is looked up according to [basic.lookup.unqual] but ignoring any non-type names that have been declared. If the elaborated-type-specifier is introduced by the enum keyword and this lookup does not find a previously declared type-name, the elaborated-type-specifier is ill-formed. If the elaborated-type-specifier is introduced by the class-key and this lookup does not find a previously declared type-name, or if the elaborated-type-specifier appears in a declaration with the form:

class-key attribute-specifier-seq identifier ;

the elaborated-type-specifier is a declaration that introduces the class-name as described in [basic.scope.pdecl].

So in std::vector<struct Typo> a;, because struct Typo does not find a previously declared Typo, it serves as a forward declaration for the type. And as such, its the vector instantiation that may¹ complain the type it got is incomplete because it will try to do stuff with it.

While in std::vector<Typo> b;, looking up Typo finds nothing. There is no previous declaration for it, so its at this point a diagnostic should be issued that this identifier is undeclared.

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_{¹ - Depends on what standard you are compiling to, and what members of the vector you are using. Support for incomplete types has been added.}