Merge pull request #8457 from iK4tsu/fix-sumtype-canMatch

Issue 23101 - [std.sumtype] canMatch does not account ref 

Signed-off-by: Luís Ferreira <contact@lsferreira.net>
Signed-off-by: Paul Backus <snarwin@gmail.com>
Merged-on-behalf-of: Atila Neves <atilaneves@users.noreply.github.com>

Author: dlang-bot

std/sumtype.d

@@ -1830,7 +1830,7 @@ class MatchException : Exception
 template canMatch(alias handler, Ts...)
 if (Ts.length > 0)
 {
-    enum canMatch = is(typeof((Ts args) => handler(args)));
+    enum canMatch = is(typeof((ref Ts args) => handler(args)));
 }
 
 ///
@@ -2575,6 +2575,27 @@ version (D_Exceptions)
     }
 }
 
+// return ref
+// issue: https://issues.dlang.org/show_bug.cgi?id=23101
+@safe unittest
+{
+    static assert(!__traits(compiles, () {
+        SumType!(int, string) st;
+        return st.match!(
+            function int* (string x) => assert(0),
+            function int* (return ref int i) => &i,
+        );
+    }));
+
+    SumType!(int, string) st;
+    static assert(__traits(compiles, () {
+        return st.match!(
+            function int* (string x) => null,
+            function int* (return ref int i) => &i,
+        );
+    }));
+}
+
 private void destroyIfOwner(T)(ref T value)
 {
     static if (hasElaborateDestructor!T)