Sorry, for the last example of what works I meant:

template
int foo(T v) {
return C{4}.a;
}

Confirmed, presumably we are incorrectly concluding that the dependent non-deduced type C cannot possibly be a class type.

A coworker of mine just came across the same issue. I have another set of repros and a few cases that do work as expected. I've tested and reproduced the same error back to Clang 5.0 and through until trunk as of today. Interestingly enough it occurs during parsing and not during template instantiation.

template
struct S
{
S(T t) : t(t) {}
T t;
};

template
void good1(T v)
{
(void)S(v);
}

template
void good2(T v)
{
auto s = S(v);
(void)s.t;
}

void good3()
{
(void)[](int v) { (void)S(v).t; };
}

void good4()
{
(void)S(42).t;
}

void good5()
{
(void) { (void)S(42).t; };
}

template
void bad1(T v)
{
// fails with:
// error: member reference base type 'S' is not a structure or union
(void)S(v).t;
}

void bad2()
{
// fails with:
// error: member reference base type 'S' is not a structure or union
(void)[](auto v) { (void)S(v).t; };
}

Looks like this has been fixed? The repro at least no longer does in Trunk: https://godbolt.org/z/jofz6e

Yes, fixed under duplicate llvm/llvm-bugzilla-archive#47175 .

*** This bug has been marked as a duplicate of bug llvm/llvm-bugzilla-archive#47175 ***

mentioned in issue llvm/llvm-bugzilla-archive#47175