> This seems even more strange; instead of giving a “missing type in composite literal” error, it gives a syntax error.

That's a syntactic limitation (wilful I assume): `{1, 2}` is not a valid expression in general, however it is specifically allowed as an item within an existing composite literal:

    CompositeLit  = LiteralType LiteralValue .
    LiteralType   = StructType | ArrayType | "[" "..." "]" ElementType |
                    SliceType | MapType | TypeName .
    LiteralValue  = "{" [ ElementList [ "," ] ] "}" .
    ElementList   = KeyedElement { "," KeyedElement } .
    KeyedElement  = [ Key ":" ] Element .
    Element       = Expression | LiteralValue .

The "LiteralValue" item occurs only as a sub-item of the CompositeLit rule, which means it's not valid as a function parameter (or as a value to set on a variable, or as a return value).