FIR: keep nullability when approximating local types

^KT-30054 Open

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

@@ -0,0 +1,18 @@
+// ISSUE: KT-30054
+interface I {
+    fun foo(): String
+}
+
+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)<!UNSAFE_CALL!>.<!>foo()
+    bar(false)?.foo()
+}

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

@@ -0,0 +1,18 @@
+// ISSUE: KT-30054
+interface I {
+    fun foo(): String
+}
+
+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.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
+}