backend: Allow fake nullables in FOR loops

backend.native/tests/build.gradle

@@ -400,6 +400,13 @@ task codegen_controlflow_for_loops_coroutines(type: RunKonanTest) {
             "Got: 0 2 4 6\n"
 }
 
+task codegen_controlflow_for_loops_let_with_nullable(type: RunKonanTest) {
+    source = "codegen/controlflow/for_loops_let_with_nullable.kt"
+    goldValue = "012345\n012345\n024\n01234\n01234\n024\n543210\n543210\n531\n" +
+                "012345\n012345\n024\n01234\n01234\n024\n543210\n543210\n531\n" +
+                "abcdef\nabcdef\nace\nabcde\nabcde\nace\nfedcba\nfedcba\nfdb\n"
+}
+
 task codegen_controlflow_for_loops_call_order(type: RunKonanTest) {
     source = "codegen/controlflow/for_loops_call_order.kt"
     goldValue = "1234\n1234\n2134\n"

backend.native/tests/codegen/controlflow/for_loops_let_with_nullable.kt

@@ -0,0 +1,151 @@
+package codegen.controlflow.for_loops_let_with_nullable
+
+import kotlin.test.*
+
+// Github issue #1012
+fun testInt(left: Int?, right: Int?, step: Int?) {
+    right?.let {
+        for (i in 0..it) { print(i) }
+    }
+    println()
+
+    left?.let {
+        for (i in it..5) { print(i) }
+    }
+    println()
+
+    step?.let {
+        for (i in 0..5 step it) { print(i) }
+    }
+    println()
+
+    right?.let {
+        for (i in 0 until it) { print(i) }
+    }
+    println()
+
+    left?.let {
+        for (i in it until 5) { print(i) }
+    }
+    println()
+
+    step?.let {
+        for (i in 0 until 5 step it) { print(i) }
+    }
+    println()
+
+    right?.let {
+        for (i in it downTo 0) { print(i) }
+    }
+    println()
+
+    left?.let {
+        for (i in 5 downTo it) { print(i) }
+    }
+    println()
+
+    step?.let {
+        for (i in 5 downTo 0 step it) { print(i) }
+    }
+    println()
+}
+
+fun testLong(left: Long?, right: Long?, step: Long?) {
+    right?.let {
+        for (i in 0..it) { print(i) }
+    }
+    println()
+
+    left?.let {
+        for (i in it..5) { print(i) }
+    }
+    println()
+
+    step?.let {
+        for (i in 0..5L step it) { print(i) }
+    }
+    println()
+
+    right?.let {
+        for (i in 0 until it) { print(i) }
+    }
+    println()
+
+    left?.let {
+        for (i in it until 5) { print(i) }
+    }
+    println()
+
+    step?.let {
+        for (i in 0 until 5L step it) { print(i) }
+    }
+    println()
+
+    right?.let {
+        for (i in it downTo 0) { print(i) }
+    }
+    println()
+
+    left?.let {
+        for (i in 5 downTo it) { print(i) }
+    }
+    println()
+
+    step?.let {
+        for (i in 5 downTo 0L step it) { print(i) }
+    }
+    println()
+}
+
+fun testChar(left: Char?, right: Char?, step: Int?) {
+    right?.let {
+        for (i in 'a'..it) { print(i) }
+    }
+    println()
+
+    left?.let {
+        for (i in it..'f') { print(i) }
+    }
+    println()
+
+    step?.let {
+        for (i in 'a'..'f' step it) { print(i) }
+    }
+    println()
+
+    right?.let {
+        for (i in 'a' until it) { print(i) }
+    }
+    println()
+
+    left?.let {
+        for (i in it until 'f') { print(i) }
+    }
+    println()
+
+    step?.let {
+        for (i in 'a' until 'f' step it) { print(i) }
+    }
+    println()
+
+    right?.let {
+        for (i in it downTo 'a') { print(i) }
+    }
+    println()
+
+    left?.let {
+        for (i in 'f' downTo it) { print(i) }
+    }
+    println()
+
+    step?.let {
+        for (i in 'f' downTo 'a' step it) { print(i) }
+    }
+    println()
+}
+
+@Test fun runTest() {
+    testInt(0, 5, 2)
+    testLong(0, 5, 2)
+    testChar('a', 'f', 2)
+}