Change Log: 2.079.0

dmd now shows which argument failed to match a parameter with an explanatory error message.

void fill(ref char[16] buf, char c);

void main()
{
    fill("1234567890123456", '*');

    const char[16] s;
    fill(s, '*');
}

Output:

fillchar.d(5): Error: function fillchar.fill(ref char[16] buf, char c) is not callable using argument types (string, char)
fillchar.d(5):        cannot pass rvalue argument "1234567890123456" of type string to parameter ref char[16] buf
fillchar.d(8): Error: function fillchar.fill(ref char[16] buf, char c) is not callable using argument types (const(char[16]), char)
fillchar.d(8):        cannot pass argument s of type const(char[16]) to parameter ref char[16] buf

Note: Currently this change doesn't apply when the function has overloads.

Comma expressions have proven to be a frequent source of confusion, and bugs. Using their result will now trigger an error message.

The comma operator (,) allows executing multiple expressions and discards the result of them except for the last which is returned.

int a = 1;
int b = 2;
bool ret = a == 2, b == 2; // true

It's also common to use the comma operator in for-loop increment statements to allow multiple expressions.

for (; !a.empty && !b.empty; a.popFront, b.popFront)

Hence, using the comma operator in for-loop increment statements is still allowed.

Corrective Action

If possible, split the comma operator in two statements. Otherwise use lambdas.

auto result = foo(), bar();

// split off in two statements
foo();
auto result = bar();

// or use lambdas
auto result = {foo(); return bar();}();

Rationale

The comma operator leads to unintended behavior (see below for a selection) Moreover it is not commonly used and it blocks the ability to implement tuples as a language feature using commas.

A selection of problems through the accidental use of the comma operator:

writeln( 6, mixin("7,8"), 9 ); // 6, 8, 9

struct Vec
{
    this(T...)(T args) { ... }
}
// missing type name
Vec v = (0, 0, 3); // Vec(3)

int a = 0;
int b = 2;
if (a == 1, b == 2) {
    // will always be reached
}

void foo(int x, int y=0) {}
foo((a, b)); // Oops, foo(b) is called

synchronized (lockA, lockB) {}
// multiple expressions aren't currently implemented, but it compiles due to the comma operator

Function parameters with default values are now allowed after variadic template parameters and when IFTI is used, always take their default values. This allows using special tokens (eg __FILE__) after variadic parameters, which was previously impossible.

For example:

string log(T...)(T a, string file = __FILE__, int line = __LINE__)
{
  return text(file, ":", line, " ", a);
}

assert(log(10, "abc") == text(__FILE__, ":", __LINE__, " 10abc"));

This should be preferred to the previous workaround, which causes a new template instantiation for every invocation:

string log(string file = __FILE__, int line = __LINE__, T...)(T a);

See the Deprecated Features for more information.

Starting with this release, using the delete keyword will result in a deprecation warning.

As a replacement, users are encouraged to use destroy if feasible, or core.memory.__delete as a last resort.

The deprecation period of the -dylib flag on OSX has ended.

dmd -dylib awesome_d_library.d

Use the -shared flag to generate a shared library:

dmd -shared awesome_d_library.d

DIP 1008 has been merged and it can be previewed under the experimental -dip1008 flag:

void main() @nogc
{
    throw new Exception("I'm @nogc now");
}

rdmd -dip1008 app.d

Prior to this release is was impossible to filter out @disable functions without using __trait(compiles), which was less than ideal since false could be returned for other reasons.

Now, in metaprogramming code, @disable functions can be detected accurately, using __traits(isDisabled) and even in overload sets:

module runnable;

struct Foo
{
    import std.stdio;
    @disable static void foo() {__PRETTY_FUNCTION__.writeln;}
    static void foo(int v) {__PRETTY_FUNCTION__.writeln;}
    static void bar() {__PRETTY_FUNCTION__.writeln;}
    @disable static void bar(int v) {__PRETTY_FUNCTION__.writeln;}
}

void test(T)()
{
    foreach (member; __traits(allMembers, T))
        foreach (overload; __traits(getOverloads, T, member))
            static if (!__traits(isDisabled, overload))
    {
        static if (is(typeof(&overload) == void function()))
            overload();
        else static if (is(typeof(&overload) == void function(int)))
            overload(42);
    }
}

void main(){test!Foo;}

prints:

void runnable.Foo.foo(int v)
void runnable.Foo.bar()

Selectively importing a symbol should work only if the symbol imported is defined or publicly imported in the imported module. Due to a compiler bug, selectively importing a symbol works even if the symbol is defined in a privately imported module in the imported module.

//a.d
int bar;

//b.d
import a;

//c.d
import b : bar;

The above code will now result in a deprecation message which states that bar cannot be accessed since it is privately imported in b.

delegates may now be initialised at module scope. This changes the effect of the fix for 13259 (turning the ICE that resulted into an error) making the follow legal:

void delegate() bar = (){};

The function pointer is set to the function of the delegate, the context pointer is set to null.

This is a breaking change (on OSX 64).

Due to the erratic implementation defined behavior of C++ name mangling, it was difficult to get D's long/ulong to portably match up with the corresponding C++ compiler.

By instead relying on how the corresponding C++ compiler mangled int64_t/uint64_t it makes the C++ side of the D<=>C++ interface much simpler.

For the current platforms dmd supports, only the OSX 64 bit mangling changes. In this case from 'm' to 'y'.

Note: int64_t and uint64_t are defined in stdint.h

When implementing a struct which has a local private import the imported symbols are leaked if present in a VarDeclaration statement. For more information and examples see : https://issues.dlang.org/show_bug.cgi?id=18219

Symbols from the private import should not visible outside the struct scope. A deprecation is now issued when such cases are encountered

Currently, ddoc automatically highlights all occurrences of words in running text that coincide with the symbol or module being documented, or a parameter name of a function being documented. While convenient, it often caused unintended highlighting of normal words in text when module, function, or parameter identifiers coincide with normal words. This led to a proliferation of prefixing words with _ in order to suppress this behaviour.

Now a better solution has been implemented to completely opt-out of this feature via the DDOC_AUTO_PSYMBOL, DDOC_AUTO_KEYWORD, and DDOC_AUTO_PARAM macros, which are used for all such automatically-highlighted words in running text. Occurrences of module, function, or parameter names inside code blocks are not included. By default, these macros simply redirect to DDOC_PSYMBOL, DDOC_KEYWORD, and DDOC_PARAM, but the user can now redefine these macros so that they simply expand to the word itself without any highlighting:

DDOC_AUTO_PSYMBOL = $0
DDOC_AUTO_KEYWORD = $0
DDOC_AUTO_PARAM = $0

Furthermore, whenever a word is prefixed with _ to suppress automatic highlighting, it is now wrapped in the DDOC_AUTO_PSYMBOL_SUPPRESS macro. This is to provide users who wish to opt out of automatic highlighting an easy way to find all occurrences of these underscore prefixes so that they can be removed from the text. For example, they can redefine this macro to something highly-visible and easily searched for, such as:

DDOC_AUTO_PSYMBOL_SUPPRESS = FIXME_UNDERSCORE_PREFIX 

and then search the generated documentation for the string FIXME_UNDERSCORE_PREFIX and delete the _ prefix from all corresponding parts of the documentation comment text.

The compiler has been updated to prefix all extern(D) symbols with an extra underscore where the platform expects one on all external symbols. This allows compiled code to work better with binutil programs, such as the ability to list symbols demangled inside a debugger.

This is an ABI breaking change and requires recompiling libraries.

HexString literals are deprecated. Use std.conv.hexString instead.

Added the command line option -i which causes the compiler to treat imported modules as if they were given on the command line. The option also accepts "module patterns" that include/exclude modules based on their name. For example, the following will include all modules whose names start with foo, except for those that start with foo.bar:

dmd -i=foo -i=-foo.bar

The option -i by itself is equivalent to:

dmd -i=-std -i=-core -i=-etc -i=-object

Added -Xi=<name> to include more fields in the JSON output. Currently there are 4 fields that can be included: "compilerInfo", "buildInfo", "modules" and "semantics", i.e.

dmd -Xi=compilerInfo -Xi=buildInfo -Xi=semantics -Xi=modules

will generate a JSON file with the following:

{
  "compilerInfo" : {
    "binary" : "<filename-of-compiler-binary>",
    "version" : "<compiler-version>",
    "supportsIncludeImports" : true,
  },
  "buildInfo" : {
    "config" : "<config-filename>",
    "cwd" : "<cwd-during-build>",
    "importParths" : [
      "<import-path1>",
      "<import-path2>",
      // ...
    ]
  },
  "semantics" : {
    "modules" : [
      {
        "name" : "<module-name>",
        "file" : "<module-filename>",
        "isRoot" : true|false
      },
      // more module objects...
    ]
  },
  "modules" : [
    // an array of the syntax data for all the modules,
    // this is the same array that would be generated
    // for a JSON file with no -Xi=<field> options
  ]
}

If JSON is generated without any -Xi= options then the old format is used. The old format is the same data that would appear in the new "modules" field.

Also note that the compiler can now be invoked with no source files as long as at least one JSON field is provided, i.e.

dmd -Xi=compilerInfo

This is an experimental command-line flag and will be stabilized in the next release.

It is now possible to compare two lambda functions, under certain constraints, using __traits(isSame, lamda1, lambda2). In order to correctly compare two lambdas, the following conditions must be satisfied:

• the lambda function arguments must not have a template instantiation as an explicit argument type. Any other argument types (basic, user-defined, template) are supported.
• the lambda function body must contain a single expression (no return statement) which contains only numeric values, manifest constants, enum values and arguments. If the expression contains local variables, function calls or return statements, the function is considered uncomparable.

These limitations might be lifted in the next release version.

Whenever a lambda is considered uncomparable, the __traits(isSame, ...) expression in which it's used will return false, no matter what other lambda is used in the comparison.

The Windows installer now adds platform libraries built from the MinGW definitions and a wrapper library for the VC2010 shared C runtime. When building COFF object files with -m64 or -m32mscoff and no Visual Studio installation is detected or no platform libraries are found these will be used as replacements. If the Microsoft linker is not found, the LLVM linker LLD will be used.

DMD has been further decoupled from the runtime so it is now easier and more convenient to use D without the runtime in a pay-as-you-go fashion. This will be of interest to users wishing to incrementally or partially port D to new platforms, users targeting bare-metal or resource constrained platforms, and users wishing to use D as a library from other languages without the runtime.

Prior to this release, if one attempted to compile a simple D application that made no use of any runtime features, the compiler would have emitted a number of errors about a missing object.d, a missing Error class, missing TypeInfo, and missing ModuleInfo, among others.

Starting with this release, one can now create a library for use from another language, requiring only the .d source file that implements that library, and an empty object.d file

Example 1

module object

module math;

extern(C) int add(int a, int b)
{
    return a + b;
}

dmd -conf= -lib math.d
size math.a
   text    data     bss     dec     hex filename
      0       0       0       0       0 math.o (ex math.a)
     20       0       0      20      14 math_1_129.o (ex math.a)

Also, starting with this release, one can now create very small executables with a minimal runtime implementation.

Example 2

DMD auto-generates a call to _d_run_main which, in turn, calls the user-defined main function. DMD automatically generates a call to g++ which links in the C runtime.

module object;

private alias extern(C) int function(char[][] args) MainFunc;
private extern (C) int _d_run_main(int argc, char** argv, MainFunc mainFunc)
{
    return mainFunc(null);  // assumes `void main()` for simplicity
}

module main;

void main() { }

dmd -conf= -defaultlib= -fPIC main.d object.d -of=main
size main
   text    data     bss     dec     hex filename
   1403     584      16    2003     7d3 main

Example 3

Manually generated call to main. No C runtime.

module object;

extern(C) void __d_sys_exit(long arg1)
{
    asm
    {
        mov RAX, 60;
        mov RDI, arg1;
        syscall;
    }
}

extern void main();
private extern(C) void _start()
{
    main();
    __d_sys_exit(0);
}

module main;

void main() { }

dmd -c -lib main.d object.d -of=main.o
ld main.o -o main
size main
   text    data     bss     dec     hex filename
     56       0       0      56      38 main

Usage of more advanced D features (e.g. classes, exceptions, etc...) will require runtime implementation code, but they can be implemented in a pay-as-you-go fashion.

The compiler has been updated to use 10.9 and link with libc++ on OSX. This is due the shared libstdc++ library from older versions of macOS having compatibility issues with the headers included in a modern XCode.

The minimum required version of running the compiler is now Mac OS X Mavericks (10.9).

The specification states that a private member is visible only from within the same module. Prior to this release, due to a bug, private members were visible through selective imports from other modules, violating the specification. Beginning with this release, accessing private members from outside the module in which they are declared will result in a deprecation message.

The deprecation period for using .ptr on arrays in @safe ended. The following now triggers an error instead of a deprecation:

@safe ubyte* oops(ubyte[] arr) {
    return arr.ptr;
}

Use &arr[0] instead:

@safe ubyte* oops(ubyte[] arr) {
    return &arr[0];
}

Note that this only applies to SafeD - in @system code .ptr may still be used:

@system ubyte* oops(ubyte[] arr) {
    return arr.ptr;
}

core.memory.__delete allows easy migration from the deprecated delete. __delete behaves exactly like delete:

bool dtorCalled;
class B
{
    int test;
    ~this()
    {
        dtorCalled = true;
    }
}
B b = new B();
B a = b;
b.test = 10;

__delete(b);
assert(b is null);
assert(dtorCalled);
// but be careful, a still points to it
assert(a !is null);

For example, on a Posix platform you can simply run:

sed "s/delete \(.*\);/__delete(\1);/" -i **/*.d

Users should prefer object.destroy` to explicitly finalize objects, and only resort to core.memory.__delete when object.destroy would not be a feasible option.

The runtime now lazily initializes the GC on first use, thus allowing applications that do not use the GC to skip its initialization.

Giving std.format.format a string as a template parameter allows the type correctness of the parameters to be checked at compile time:

import std.format : format;

auto s1 = format!"%d"(4); // works fine
auto s2 = format("%d"); // runtime exception
auto s3 = format!"%d"(); // compile time error

Now, using this overload also allows std.format to make an educated guess at the length of the resulting string, reducing the total number of reallocations made to the output buffer.

import std.format : format;

auto s1 = format!"%02d:%02d:%02d"(10, 30, 50); // known for certain to be 8 chars long
auto s2 = format!"%s %d"("Error Code: ", 42); // Makes an educated guess

Longer format strings benefit the most from this change.

std.conv.hexString now returns an immutable string literal rather than an array of ubytes. This is more in keeping with the documentation that says it is a replacement for the deprecated x"deadbeef" string literal syntax.

The benefits of this change are:

• behavior consistency with the documentation
• the hexString template instantiations no longer appear in the generated object file
• the generated object file no longer contains references to TypeInfo and various druntime functions
• it is now compatible with -betterC mode

In some cases, code did rely on it being an array, and a cast will fix the issue:

// now an error:
enum ubyte[8] input = hexString!"c3 fc 3d 7e fb ea dd aa";

// add cast to fix:
enum ubyte[8] input = cast(ubyte[8]) hexString!"c3 fc 3d 7e fb ea dd aa";

Previously, when .nullify is called on a Nullable!C where C is a class or interface, the underlying object is destructed immediately via the .destroy function. This led to bugs when there are still references to the object outside of the Nullable instance:

class C
{
    int canary = 0xA71FE;
    ~this()
    {
        canary = 0x5050DEAD;
    }
}

auto c = new C;
assert(c.canary == 0xA71FE);

Nullable!C nc = nullable(c);
nc.nullify;
assert(c.canary == 0xA71FE); // This would fail

The .nullify method has been fixed so that it no longer calls .destroy on class or interface instances, and the above code will now work correctly.

std.algorithm.searching.substitute yields a lazy range with all occurrences of substs in r replaced with their substitution:

import std.algorithm.comparison : equal;
import std.algorithm.searching : substitute;

// substitute single elements
assert("do_it".substitute('_', ' ').equal("do it"));

// substitute multiple, single elements
assert("do_it".substitute('_', ' ',
                           'd', 'g',
                           'i', 't',
                           't', 'o')
              .equal("go to"));

// substitute subranges
assert("do_it".substitute("_", " ",
                          "do", "done")
              .equal("done it"));

// substitution works for any ElementType
int[] x = [1, 2, 3];
auto y = x.substitute(1, 0.1);
assert(y.equal([0.1, 2, 3]));
static assert(is(typeof(y.front) == double));

If the substitution parameters are known at compile-time, the faster template overload can be used:

import std.algorithm.comparison : equal;
import std.algorithm.searching : substitute;

// substitute subranges of a range
assert("apple_tree".substitute!("apple", "banana",
                                "tree", "shrub").equal("banana_shrub"));

// substitute elements in a range
assert("apple_tree".substitute!('a', 'b',
                                't', 'f').equal("bpple_free"));

// substitute values
assert('a'.substitute!('a', 'b', 't', 'f') == 'b');

std.bigint.divMod calculates both the quotient and the remainder in one go:

void divMod(const BigInt dividend, const BigInt divisor, out BigInt quotient, out BigInt remainder) pure nothrow
{
auto a = BigInt(123);
auto b = BigInt(25);
BigInt q, r;

divMod(a, b, q, r);

assert(q == 4);
assert(r == 23);
assert(q * b + r == a);
}

std.algorithm.BigInt.getDigit gives the ulongs or uints that make up the underlying representation of the BigInt.

import std.bigint;

auto a = BigInt("1000");
assert(a.getDigit(0) == 1000);

auto b = BigInt("2_000_000_000_000_000_000_000_000_000");
assert(b.getDigit(0) == 4584946418820579328);
assert(b.getDigit(1) == 108420217);

std.exception.enforce now mirrors the behavior of std.exception.enforceEx and can be used as an eponymous template:

import std.conv : ConvException;
alias convEnforce = enforce!ConvException;
assertNotThrown(convEnforce(true));
assertThrown!ConvException(convEnforce(false, "blah"));

With this change, std.exception.enforce is a strict superset of std.exception.enforceEx, which will be deprecated in 2.079.

std.experimental.all allows convenient use of all Phobos modules with one import:

import std.experimental.all;
void main()
{
    10.iota.map!log.sum.writeln;
}

For short scripts a lot of imports are often needed to get all the modules from the standard library. With this release it's possible to use import std.experimental.all for importing the entire standard library at once. This can be used for fast prototyping or REPLs:

import std.experimental.all;
void main()
{
    6.iota
      .filter!(a => a % 2) // 0 2 4
      .map!(a => a * 2) // 0 4 8
      .tee!writeln
      .sum
      .writefln!"Sum: %d"; // 18
}

As before, symbol conflicts will only arise if a symbol with collisions is used. In this case, static imports or renamed imports can be used to uniquely select a specific symbol.

The baseline cost for import std.experimental.all is less than half a second (varying from system to system) and work is in progress to reduce this overhead even further.

Motivation:

Keep track of references to allocators so they don't escape, dangle, and cause undefined behavior.

From now on, RCIAllocator will be used instead of the old IAllocator interface. std.experimental.allocator.allocatorObject can be used to build a RCIAllocator out of a custom allocator.

import std.experimental.allocator.mallocator : Mallocator;

RCIAllocator a = allocatorObject(Mallocator.instance);
auto b = a.allocate(100);
assert(b.length == 100);
assert(a.deallocate(b));

Motivation:

Keep track of references to allocators so they don't escape, dangle, and cause undefined behavior.

From now on, RCISharedAllocator will be used instead of the old ISharedAllocator interface. std.experimental.allocator.sharedAllocatorObject` can be used to build a RCISharedAllocator out of a custom allocator.

import std.experimental.allocator.building_blocks.free_list : SharedFreeList;
import std.experimental.allocator.mallocator : Mallocator;

shared SharedFreeList!(Mallocator, chooseAtRuntime, chooseAtRuntime) sharedFL;
shared RCISharedAllocator sharedFLObj = sharedAllocatorObject(sharedFL);

auto b = sharedFLObj.allocate(100);
assert(b.length == 100);
assert(sharedFLObj.deallocate(b));

std.file.readText now checks for a BOM. If a BOM is present and it is for UTF-8, UTF-16, or UTF-32, std.file.readText verifies that it matches the requested string type and the endianness of the machine, and if there is a mismatch, a std.utf.UTFException is thrown without bothering to validate the string.

If there is no BOM, or if the BOM is not for UTF-8, UTF-16, or UTF-32, then the behavior is what it's always been, and UTF validation continues as per normal, so if the text isn't valid for the requested string type, a std.utf.UTFException will be thrown.

In addition, before the buffer is cast to the requested string type, the alignment is checked (e.g. 5 bytes don't fit cleanly in an array of wchar or dchar), and a std.utf.UTFException is now throw if the number of bytes does not align with the requested string type. Previously, the alignment was not checked before casting, so if there was an alignment mismatch, the cast would throw an Error, killing the program.

std.parallelism.TaskPool.fold is functionally equivalent to TaskPool.reduce except the range parameter comes first and there is no need to use tuple for multiple seeds.

static int adder(int a, int b)
{
    return a + b;
}
static int multiplier(int a, int b)
{
    return a * b;
}

// Just the range
auto x = taskPool.fold!adder([1, 2, 3, 4]);
assert(x == 10);

// The range and the seeds (0 and 1 below; also note multiple
// functions in this example)
auto y = taskPool.fold!(adder, multiplier)([1, 2, 3, 4], 0, 1);
assert(y[0] == 10);
assert(y[1] == 24);

// The range, the seed (0), and the work unit size (20)
auto z = taskPool.fold!adder([1, 2, 3, 4], 0, 20);
assert(z == 10);

std.range.nullSink is a convenience wrapper for std.range.NullSink and creates an output range that discards the data it receives. It's the range analog of /dev/null.

import std.csv : csvNextToken;

string line = "a,b,c";

// ignore the first column
line.csvNextToken(nullSink, ',', '"');
line.popFront;

// look at the second column
Appender!string app;
line.csvNextToken(app, ',', '"');
assert(app.data == "b");

std.range.slide allows to iterate a range in sliding windows:

import std.array : array;
import std.algorithm.comparison : equal;

assert([0, 1, 2, 3].slide(2).equal!equal(
    [[0, 1], [1, 2], [2, 3]]
));
assert(5.iota.slide(3).equal!equal(
    [[0, 1, 2], [1, 2, 3], [2, 3, 4]]
));

assert(iota(7).slide(2, 2).equal!equal([[0, 1], [2, 3], [4, 5]]));
assert(iota(12).slide(2, 4).equal!equal([[0, 1], [4, 5], [8, 9]]));

// set a custom stepsize (default 1)
assert(6.iota.slide(1, 2).equal!equal(
    [[0], [2], [4]]
));

assert(6.iota.slide(2, 4).equal!equal(
    [[0, 1], [4, 5]]
));

// allow slide with less elements than the window size
assert(3.iota.slide!(No.withPartial)(4).empty);
assert(3.iota.slide!(Yes.withPartial)(4).equal!equal(
    [[0, 1, 2]]
));

New overload functions of std.string.strip, std.string.stripLeft and std.string.stripRight accepts a string of characters to be stripped instead of stripping only whitespace.

import std.string: stripLeft, stripRight, strip;

assert(stripLeft("www.dlang.org", "w.") == "dlang.org");
assert(stripRight("dlang.org/", "/") == "dlang.org");
assert(strip("www.dlang.org/", "w./") == "dlang.org");

Previously, enums whose base type was a string type were true for std.traits.isSomeString and std.traits.isNarrowString. Occasionally, this was useful, but in general, it was a source of bugs, because code that works with strings does not necessarily work with enums whose base type is a string, making it easy to write code where an enum would pass the template constraint and then the template would fail to compile. For instance, enums of base type string are false for std.range.primitives.isInputRange. As such, it really doesn't make sense for std.traits.isSomeString and std.traits.isNarrowString to be true for enums.

For some code, this will be a breaking change, but most code will either be unaffected, or it will then fail with enums at the template constraint instead of inside the function. So, the risk of code breakage is minimal but does exist. Other code will now be able to remove stuff like !is(T == enum) from its template constraints, since that is now part of std.traits.isSomeString and std.traits.isNarrowString, but it will continue to compile with the now unnecessary check for enums.

Code that uses std.traits.isSomeString or std.traits.isNarrowString in a template constraint and has no other conditions which would prevent enums from passing the constraint but then would fail to compile inside the template if an enum were passed to it will now fail to compile at the template constraint instead of inside the template. So, such code is fixed rather than broken.

The rare code that does break because of these changes is code that uses std.traits.isSomeString or std.traits.isNarrowString in a template constraint or static if and does not use other conditions to prevent enums from passing and actually has code in the template which compiles with both strings and enums with a base type of string. Such code will need to be changed to use std.traits.OriginalType, std.conv.asOriginalType, or std.traits.StringTypeOf instead. e.g. for enums to pass the constraint, instead of

auto foo(S)(S str)
if (isSomeString!S)
{
    ...
}

the code would be

auto foo(S)(S str)
if (isSomeString!(OriginalType!S))
{
    ...
}

As a rule of thumb, generic code should either disallow implicit conversions and force the caller to do the conversion explicitly (which generally is the least error-prone approach), or it should force the conversion to the desired type before the parameter is used in the function so that the function is definitely operating on the desired type and not just on one that implicitly converts to it and thus will work regardless of whether the argument was the exact type or implicitly converted to it.

However, great care should be taken if the implicit conversion will result in slicing the parameter or otherwise referring to its address, since that makes it very easy for a reference to local memory to escape the function, whereas if the conversion is done at the call site, then the slicing is done at the call site, so the dynamic array is still valid when the function returns. Fortunately, enums with a base type of string do not have that problem, but other types which implicitly convert to dynamic arrays (such as static arrays) do have that problem, which is a big reason why it's generally recommended to not have generic functions accept types based on implicit conversions.

It is recommended that if a function is supposed to accept both strings and types which implicitly convert to a string type, then it should explicitly accept dynamic arrays but be templated on the element type. e.g.

auto foo(C)(C[] str)
if (isSomeChar!C)
{
    ...
}

That way, any implicit conversions are done at the call site and do not introduce @safety problems inside the function. It also tends to result in template constraints which are shorter and more easily understood.

The standard library has been modified to recognize and use toString overloads that accept output ranges when such overloads exist.

import std.range.primitives;
import std.stdio;

struct MyType
{
    void toString(W)(ref W writer) if (isOutputRange!(W, char))
    {
        put(writer, "Custom toString");
    }
}

auto t = MyType();
writeln(t); // writes "Custom toString"

This has several benefits for the user. First, this design is much friendlier to inlining than the toString(scope void delegate(const(char)[]) sink) method of toString. Second, this cuts down on memory usage, as characters are placed right into the output buffers of functions like std.format.format. Third, because toString is now a template, can be marked @safe via inference much more often.

All previous forms of toString will continue to work.