[Issue 20504] New: grammar allows missing function body for AutoFuncDeclaration

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20504

          Issue ID: 20504
           Summary: grammar allows missing function body for
                    AutoFuncDeclaration
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: spec
          Severity: normal
          Priority: P1
         Component: dlang.org
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

This does not compile:
```
void foo()() {
    nothrow bar() {} // allowed
    nothrow bar(); // should parse according to spec
}
```
The spec says that in an AutoFuncDeclaration a MissingFunctionBody is allowed,
yet even before instantiating template foo it raises an error:
Error: function declaration without return type.

I suggest only allowing SpecifiedFunctionBody in an AutoFuncDeclaration since
it would always be a semantic error anyways (you can't infer the return type
without a function body).

https://dlang.org/spec/grammar.html#AutoFuncDeclaration
https://dlang.org/spec/grammar.html#FunctionBody

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