Why doesn't it compile?
> auto foo() {
>     struct Foo {
>         int a, b;
>     }
>     return Foo({
>         a: 10,
>         b: 20
>     });
> }

Valid C++14:
> auto foo() {
>     struct Foo {
>         int a, b;
>     };
>     return Foo({
>         .a = 10,
>         .b = 20
>     });
> }

On Monday, 16 November 2020 at 10:22:49 UTC, Jack Applegame wrote:
> Why doesn't it compile?
>> auto foo() {
>>     struct Foo {
>>         int a, b;
>>     }
>>     return Foo({
>>         a: 10,
>>         b: 20
>>     });
>> }

Because in-place struct initialization has never been implemented. Once the named parameters DIP is implemented, you should be able to do that by removing the {}:
https://github.com/dlang/DIPs/blob/master/DIPs/accepted/DIP1030.md

On Monday, 16 November 2020 at 10:22:49 UTC, Jack Applegame wrote:
>
> Valid C++14:
>> auto foo() {
>>     struct Foo {
>>         int a, b;
>>     };
>>     return Foo({
>>         .a = 10,
>>         .b = 20
>>     });
>> }

Nope:

/tmp % clang++ -c -std=c++14 -pedantic cpp.cpp                                                                                                  1 :(
cpp.cpp:6:13: warning: designated initializers are a C++20 extension [-Wc++20-designator]
            .a = 10,
            ^
1 warning generated.

On Monday, 16 November 2020 at 10:22:49 UTC, Jack Applegame wrote:
> Why doesn't it compile?
>> auto foo() {
>>     struct Foo {
>>         int a, b;
>>     }
>>     return Foo({
>>         a: 10,
>>         b: 20
>>     });
>> }
>
> Valid C++14:
>> auto foo() {
>>     struct Foo {
>>         int a, b;
>>     };
>>     return Foo({
>>         .a = 10,
>>         .b = 20
>>     });
>> }

Foo foo = {
         a: 10,
         b: 20
     }

is working in D at the moment. I can't wait for better initialization, named arguments to come into play too.

It's a shame that in some cases C is more expressive than D: https://github.com/gfx-rs/wgpu-native/blob/master/examples/triangle/main.c , it even allows to return a pointer to a struct literal.