fix Issue 23045 - importC: casted function type is missing extern(C)

src/dmd/semantic3.d

@@ -327,7 +327,7 @@ private extern(C++) final class Semantic3Visitor : Visitor
             sc2.scontinue = null;
             sc2.sw = null;
             sc2.fes = funcdecl.fes;
-            sc2.linkage = LINK.d;
+            sc2.linkage = funcdecl.isCsymbol() ? LINK.c : LINK.d;
             sc2.stc &= STC.flowThruFunction;
             sc2.visibility = Visibility(Visibility.Kind.public_);
             sc2.explicitVisibility = 0;

test/runnable/test23045.c

@@ -0,0 +1,33 @@
+/* https://issues.dlang.org/show_bug.cgi?id=23045
+ */
+
+int printf(const char *s, ...);
+void exit(int);
+
+void assert(int b, int line)
+{
+    if (!b)
+    {
+        printf("failed test %d\n", line);
+        exit(1);
+    }
+}
+
+void other(int a, int b)
+{
+    printf("a=%d b=%d\n", a, b);
+    assert(a == 1, "1");
+    assert(b == 2, "2");
+}
+
+int main()
+{
+    // called like extern(D)
+    ((void (*)(int, int))other)(1, 2);
+
+    // Error: incompatible types for `(cast(void function(int, int))& other) is (other)`: `void function(int, int)` and `extern (C) void(int a, int b)`
+    if (((void (*)(int, int))other) == other)
+	    {   }
+
+    return 0;
+}