[Breaking] Deprecate alias get this in Nullable.

changelog/remove-nullable-alias-get-this.dd

@@ -0,0 +1,15 @@
+`Nullable` `alias get this` has been deprecated
+
+`Nullable`'s feature of implicitly converting to its contained value has been marked deprecated.
+
+All sites where a `Nullable!T` is implicitly converted to `T`, potentially throwing an error at runtime,
+will now produce a deprecation warning or error with `-de`.
+
+To fix this, explicitly use `.get` to access the contained value of the `Nullable`.
+
+`alias get this` is the only implicit conversion in D that may throw an error. Since it is an implicit conversion,
+it can easily be used accidentally and lead to unexpected runtime errors. By explicitly invoking `get`,
+the developer acknowledges that the operation has a chance of error. Conversely, by not implicitly converting
+`Nullable!T` to `T`, the compiler gives the developer a chance to spot forgotten `isNull` checks.
+
+This feature will be removed after release 2.096.

std/typecons.d

@@ -2960,7 +2960,6 @@ Params:
 Gets the value if not null. If `this` is in the null state, and the optional
 parameter `fallback` was provided, it will be returned. Without `fallback`,
 calling `get` with a null state is invalid.
-This function is also called for the implicit conversion to `T`.
 
 Params:
     fallback = the value to return in case the `Nullable` is null.
@@ -2981,7 +2980,9 @@ Returns:
         return isNull ? fallback : _value.payload;
     }
 
-///
+//@@@DEPRECATED_2.096@@@
+deprecated(
+    "Implicit conversion with `alias Nullable.get this` will be removed after 2.096. Please use `.get` explicitly.")
 @system unittest
 {
     import core.exception : AssertError;
@@ -2999,24 +3000,33 @@ Returns:
     assert(i == 5);
 }
 
+    //@@@DEPRECATED_2.096@@@
+    deprecated(
+        "Implicit conversion with `alias Nullable.get this` will be removed after 2.096. Please use `.get` explicitly.")
+    @property ref inout(T) get_() inout @safe pure nothrow
+    {
+        return get;
+    }
+
 ///
 @safe pure nothrow unittest
 {
     int i = 42;
-    Nullable!int ni2;
-    int x = ni2.get(i);
+    Nullable!int ni;
+    int x = ni.get(i);
     assert(x == i);
 
-    ni2 = 7;
-    x = ni2.get(i);
+    ni = 7;
+    x = ni.get(i);
     assert(x == 7);
 }
 
 /**
 Implicitly converts to `T`.
 `this` must not be in the null state.
+This feature is deprecated and will be removed after 2.096.
  */
-    alias get this;
+    alias get_ this;
 }
 
 /// ditto
@@ -3046,8 +3056,8 @@ auto nullable(T)(T t)
     if (!queryResult.isNull)
     {
         //Process Mr. Doe's customer record
-        auto address = queryResult.address;
-        auto customerNum = queryResult.customerNum;
+        auto address = queryResult.get.address;
+        auto customerNum = queryResult.get.customerNum;
 
         //Do some things with this customer's info
     }
@@ -3071,6 +3081,9 @@ auto nullable(T)(T t)
     assertThrown!Throwable(a.get);
 }
 
+//@@@DEPRECATED_2.096@@@
+deprecated(
+    "Implicit conversion with `alias Nullable.get this` will be removed after 2.096. Please use `.get` explicitly.")
 @system unittest
 {
     import std.exception : assertThrown;
@@ -3124,10 +3137,10 @@ auto nullable(T)(T t)
     assert(s == S(6));
     assert(s != S(0));
     assert(s.get != S(0));
-    s.x = 9190;
-    assert(s.x == 9190);
+    s.get.x = 9190;
+    assert(s.get.x == 9190);
     s.nullify();
-    assertThrown!Throwable(s.x = 9441);
+    assertThrown!Throwable(s.get.x = 9441);
 }
 @safe unittest
 {
@@ -3156,7 +3169,7 @@ auto nullable(T)(T t)
     assert(s.isNull);
     s = S(5);
     assert(!s.isNull);
-    assert(s.x == 5);
+    assert(s.get.x == 5);
     s.nullify();
     assert(s.isNull);
 }
@@ -3276,10 +3289,10 @@ auto nullable(T)(T t)
         auto x2 = immutable Nullable!S1(sm);
         auto x3 =           Nullable!S1(si);
         auto x4 = immutable Nullable!S1(si);
-        assert(x1.val == 1);
-        assert(x2.val == 1);
-        assert(x3.val == 1);
-        assert(x4.val == 1);
+        assert(x1.get.val == 1);
+        assert(x2.get.val == 1);
+        assert(x3.get.val == 1);
+        assert(x4.get.val == 1);
     }
 
     auto nm = 10;
@@ -3292,8 +3305,8 @@ auto nullable(T)(T t)
         static assert(!__traits(compiles, { auto x2 = immutable Nullable!S2(sm); }));
         static assert(!__traits(compiles, { auto x3 =           Nullable!S2(si); }));
         auto x4 = immutable Nullable!S2(si);
-        assert(*x1.val == 10);
-        assert(*x4.val == 10);
+        assert(*x1.get.val == 10);
+        assert(*x4.get.val == 10);
     }
 
     {
@@ -3303,10 +3316,10 @@ auto nullable(T)(T t)
         auto x2 = immutable Nullable!S3(sm);
         auto x3 =           Nullable!S3(si);
         auto x4 = immutable Nullable!S3(si);
-        assert(*x1.val == 10);
-        assert(*x2.val == 10);
-        assert(*x3.val == 10);
-        assert(*x4.val == 10);
+        assert(*x1.get.val == 10);
+        assert(*x2.get.val == 10);
+        assert(*x3.get.val == 10);
+        assert(*x4.get.val == 10);
     }
 }
 @safe unittest
@@ -3397,16 +3410,6 @@ auto nullable(T)(T t)
     assert(c.canary == 0xA71FE);
 }
 
-// Regression test for issue 18539
-@safe unittest
-{
-    import std.math : approxEqual;
-
-    auto foo = nullable(2.0);
-    auto bar = nullable(2.0);
-
-    assert(foo.approxEqual(bar));
-}
 // bugzilla issue 19037
 @safe unittest
 {