fix Issue 22134 - Deprecate returning a discarded void value from a function (#12898)

fix Issue 22134 - Deprecate returning a discarded void value from a function

Signed-off-by: Martin Kinkelin <kinke@users.noreply.github.com>
Signed-off-by: Razvan Nitu <RazvanN7@users.noreply.github.com>
Merged-on-behalf-of: unknown

changelog/fix22134.dd

@@ -0,0 +1,26 @@
+Returning a discarded void value from a function is now deprecated
+
+An expression statement of type void that has no side effects should
+be discarded since it has no effect. The compiler, generally, does not
+allow such statements, however, in the case of return statements this
+error is circumvented. For example:
+
+---
+struct StackBuffer
+{
+    auto opIndex(size_t i)
+    {
+        return arr[i];
+    }
+
+private:
+    void[] arr;
+}
+---
+
+Although this code compiles, any call to `opIndex` is going to result in an
+error because the return type must either be stored somewhere (and variables
+cannot be of type void) or the call will have no effect.
+
+Starting with this release, returning a discarded void value from a function
+is deprecated. Such code can be deleted as it most certainly is dead code.

src/dmd/statementsem.d

@@ -2913,6 +2913,17 @@ package (dmd) extern (C++) final class StatementSemanticVisitor : Visitor
                     rs.exp.type = texp;
                 }
 
+                // @@@DEPRECATED_2.111@@@
+                const olderrors = global.startGagging();
+                // uncomment to turn deprecation into an error when
+                // deprecation cycle is over
+                if (discardValue(rs.exp))
+                {
+                    //errors = true;
+                }
+                if (global.endGagging(olderrors))
+                    rs.exp.deprecation("`%s` has no effect", rs.exp.toChars());
+
                 /* Replace:
                  *      return exp;
                  * with:

test/fail_compilation/fail22134.d

@@ -0,0 +1,17 @@
+// https://issues.dlang.org/show_bug.cgi?id=22134
+/* REQUIRED_ARGS: -de
+TEST_OUTPUT:
+---
+fail_compilation/fail22134.d(12): Deprecation: `this.arr[i]` has no effect
+---
+*/
+struct StackBuffer
+{
+    auto opIndex(size_t i)
+    {
+        return arr[i];
+    }
+
+private:
+    void[] arr;
+}