FE1.0: keep nullability when approximating local types (in 1.9)

Report an error that inference will change and type has to be provided
manually in other language versions, since the current behavior is an
unsoundness that can cause runtime NPEs while the new behavior may
silently change overload resolution.

^KT-30054 Fixed

compiler/testData/diagnostics/tests/inference/substitutions/hideNullableLocalTypeInPublicPosition.kt

@@ -1,4 +1,6 @@
 // ISSUE: KT-30054
+// FIR_IDENTICAL
+// !LANGUAGE: +KeepNullabilityWhenApproximatingLocalType
 interface I {
     fun foo(): String
 }
@@ -13,6 +15,6 @@ fun bar(condition: Boolean) /*: I? */ =
 
 fun main() {
     bar(false).<!UNRESOLVED_REFERENCE!>baz<!>()
-    bar(false).foo()
-    bar(false)<!UNNECESSARY_SAFE_CALL!>?.<!>foo()
+    bar(false)<!UNSAFE_CALL!>.<!>foo()
+    bar(false)?.foo()
 }

compiler/testData/diagnostics/tests/inference/substitutions/hideNullableLocalTypeInPublicPosition.txt

@@ -1,6 +1,6 @@
 package
 
-public fun bar(/*0*/ condition: kotlin.Boolean): I
+public fun bar(/*0*/ condition: kotlin.Boolean): I?
 public fun main(): kotlin.Unit
 
 public interface I {

compiler/testData/diagnostics/tests/inference/substitutions/hideNullableLocalTypeInPublicPosition_before.kt

@@ -0,0 +1,18 @@
+// ISSUE: KT-30054
+interface I {
+    fun foo(): String
+}
+
+<!APPROXIMATED_LOCAL_TYPE_WILL_BECOME_NULLABLE!>fun bar(condition: Boolean)<!> /*: I? */ =
+    if (condition)
+        object : I {
+            override fun foo() = "should check for null first"
+            fun baz() = "invisible"
+        }
+    else null
+
+fun main() {
+    bar(false).<!UNRESOLVED_REFERENCE!>baz<!>()
+    bar(false).foo()
+    bar(false)<!UNNECESSARY_SAFE_CALL!>?.<!>foo()
+}

compiler/testData/diagnostics/tests/inference/substitutions/hideNullableLocalTypeInPublicPosition_before.txt

@@ -0,0 +1,11 @@
+package
+
+public fun bar(/*0*/ condition: kotlin.Boolean): I
+public fun main(): kotlin.Unit
+
+public interface I {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.String
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}