fix Issue 15624 - opApply with @safe and @System variants can't be us…

…ed with foreach syntax

src/dmd/opover.d

@@ -2014,12 +2014,20 @@ private FuncDeclaration findBestOpApplyMatch(Expression ethis, FuncDeclaration f
             fd_ambig = null;
             match = m;
         }
-        else if (m == match)
-            fd_ambig = f;
+        else if (m == match && m > MATCH.nomatch)
+        {
+            assert(fd_best);
+            /* Ignore covariant matches, as later on it can be redone
+             * after the opApply delegate has its attributes inferred.
+             */
+            if (tf.covariant(fd_best.type) != 1 &&
+                fd_best.type.covariant(tf) != 1)
+                fd_ambig = f;                           // not covariant, so ambiguous
+        }
         return 0;               // continue
     });
 
-    if (fd_ambig && fd_best)
+    if (fd_ambig)
     {
         .error(ethis.loc, "`%s.%s` matches more than one declaration:\n`%s`:     `%s`\nand:\n`%s`:     `%s`",
             ethis.toChars(), fstart.ident.toChars(),

test/runnable/test15624.d

@@ -0,0 +1,51 @@
+/* PERMUTE_ARGS:
+ */
+
+// https://issues.dlang.org/show_bug.cgi?id=15624
+
+struct Foo {
+        int x;
+        int opApply(int delegate(int, string, string) @safe dg) @safe {
+                x = 1;
+                return 0;
+        }
+        int opApply(int delegate(int, string, string) @system dg) @system {
+                x = 2;
+                return 0;
+        }
+}
+
+void testSafe() @safe {
+        Foo foo;
+        foreach (i, k, v; foo) {
+        }
+        assert(foo.x == 1);
+}
+
+void testSystem() @system {
+        Foo foo;
+        foreach (i, k, v; foo) {
+        }
+        assert(foo.x == 2);
+}
+
+void test() @system
+{
+    Foo f;
+
+    int dgsafe  (int x, string s, string t) @safe   { return 1; }
+    int dgsystem(int x, string s, string t) @system { return 1; }
+
+    f.opApply(&dgsafe);
+    assert(f.x == 1);
+    f.opApply(&dgsystem);
+    assert(f.x == 2);
+}
+
+int main()
+{
+    testSafe();
+    testSystem();
+    test();
+    return 0;
+}