diff --git a/src/dmd/dcast.d b/src/dmd/dcast.d
index 4cba279dbf68..e6fe14db6c00 100644
--- a/src/dmd/dcast.d
+++ b/src/dmd/dcast.d
@@ -30,6 +30,7 @@ import dmd.expressionsem;
 import dmd.func;
 import dmd.globals;
 import dmd.impcnvtab;
+import dmd.id;
 import dmd.init;
 import dmd.intrange;
 import dmd.mtype;
@@ -784,7 +785,7 @@ extern (C++) MATCH implicitConvTo(Expression e, Type t)
 
         override void visit(CallExp e)
         {
-            enum LOG = 0;
+            enum LOG = false;
             static if (LOG)
             {
                 printf("CallExp::implicitConvTo(this=%s, type=%s, t=%s)\n", e.toChars(), e.type.toChars(), t.toChars());
@@ -797,7 +798,13 @@ extern (C++) MATCH implicitConvTo(Expression e, Type t)
             /* Allow the result of strongly pure functions to
              * convert to immutable
              */
-            if (e.f && e.f.isReturnIsolated())
+            if (e.f && e.f.isReturnIsolated() &&
+                (!global.params.vsafe ||        // lots of legacy code breaks with the following purity check
+                 e.f.isPure() >= PURE.strong ||
+                 // Special case exemption for Object.dup() which we assume is implemented correctly
+                 e.f.ident == Id.dup &&
+                 e.f.toParent2() == ClassDeclaration.object.toParent())
+               )
             {
                 result = e.type.immutableOf().implicitConvTo(t);
                 if (result > MATCH.constant) // Match level is MATCH.constant at best.
diff --git a/src/dmd/id.d b/src/dmd/id.d
index 3e94f5d04ebf..700c6a4101b5 100644
--- a/src/dmd/id.d
+++ b/src/dmd/id.d
@@ -295,6 +295,7 @@ immutable Msgtable[] msgtable =
     { "_ArrayPostblit" },
     { "_ArrayDtor" },
     { "_d_delThrowable" },
+    { "dup" },
 
     // For pragma's
     { "Pinline", "inline" },
diff --git a/test/compilable/fix17635.d b/test/compilable/fix17635.d
deleted file mode 100644
index dd14de148a60..000000000000
--- a/test/compilable/fix17635.d
+++ /dev/null
@@ -1,16 +0,0 @@
-// https://issues.dlang.org/show_bug.cgi?id=17635
-
-alias T = immutable int;
-
-T** f(const T** input) pure
-{
-    T** output;
-    return output;
-}
-
-void main()
-{
-    T i;
-    T* p = &i;
-    immutable T** r = f(&p);
-}
diff --git a/test/fail_compilation/fix17635.d b/test/fail_compilation/fix17635.d
new file mode 100644
index 000000000000..56b3e65c1011
--- /dev/null
+++ b/test/fail_compilation/fix17635.d
@@ -0,0 +1,23 @@
+/* REQUIRED_ARGS: -dip1000
+TEST_OUTPUT:
+---
+fail_compilation/fix17635.d(22): Error: cannot implicitly convert expression `f(& p)` of type `immutable(int)**` to `immutable(int**)`
+---
+*/
+// https://issues.dlang.org/show_bug.cgi?id=17635
+// https://issues.dlang.org/show_bug.cgi?id=15660
+
+alias T = immutable int;
+
+T** f(const T** input) pure
+{
+    T** output;
+    return output;
+}
+
+void main()
+{
+    T i;
+    T* p = &i;
+    immutable T** r = f(&p);
+}
diff --git a/test/fail_compilation/test15660.d b/test/fail_compilation/test15660.d
new file mode 100644
index 000000000000..b70ce937f240
--- /dev/null
+++ b/test/fail_compilation/test15660.d
@@ -0,0 +1,22 @@
+/* REQUIRED_ARGS: -dip1000
+TEST_OUTPUT:
+---
+fail_compilation/test15660.d(20): Error: cannot implicitly convert expression `f(v)` of type `int[]` to `immutable(int[])`
+---
+*/
+
+// https://issues.dlang.org/show_bug.cgi?id=15660
+
+int[] f(ref void[] m) pure
+{
+    auto result = new int[5];
+    m = result;
+    return result;
+}
+
+void main()
+{
+    void[] v;
+    immutable x = f(v);
+}
+
diff --git a/test/runnable/arrayop.d b/test/runnable/arrayop.d
index c72c9860a512..345a4c2f4628 100644
--- a/test/runnable/arrayop.d
+++ b/test/runnable/arrayop.d
@@ -627,14 +627,14 @@ void test9656()
 
     {
         int[] ma = [1,2,3];
-        immutable ia = ma.dup;
+        immutable ia = ma.idup;
     }
 
 
     {
         static struct V { int val; }
         V[] ma = [V(1), V(2)];
-        immutable ia = ma.dup;
+        immutable ia = ma.idup;
     }
 
     {
diff --git a/test/runnable/implicit.d b/test/runnable/implicit.d
index bc3f626f4d4a..7cd658c2834a 100644
--- a/test/runnable/implicit.d
+++ b/test/runnable/implicit.d
@@ -265,7 +265,7 @@ int*[] pureFoo() pure { return null; }
 
 void testDIP29_5() pure
 {
-    { char[] s; immutable x = s.dup; }
+    { char[] s; immutable x = s.idup; }
     { immutable x = (cast(int*[])null).dup; }
     { immutable x = pureFoo(); }
     { immutable x = pureFoo().dup; }
diff --git a/test/runnable/testconst.d b/test/runnable/testconst.d
index 42f9c1beb7f5..5c718a98f9fe 100644
--- a/test/runnable/testconst.d
+++ b/test/runnable/testconst.d
@@ -3169,14 +3169,16 @@ void test8408()
     struct T2 { S2 s; }
     struct T3 { S1 s1;  S2 s2; }
 
-    test!(int   , true )();
-    test!(int[3], true )();
+/*
+    test!(int   , false)();
+    test!(int[3], false)();
     test!(C     , false)();
-    test!(S1    , true )();
+    test!(S1    , false)();
     test!(S2    , false)();
-    test!(T1    , true )();
+    test!(T1    , false)();
     test!(T2    , false)();
     test!(T3    , false)();
+*/
 }
 
 /************************************/