[FIR] Comparison to String implies smart cast to String

When the left-hand side of an equality comparison is known to be a
String, and the equality condition resolves to true, then the right-hand
side can be smart-cast to a String as well. This was working for String
expressions on the left-hand side but not for String constants.

^KT-57513 Fixed

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/kt1778.fir.kt

@@ -8,8 +8,8 @@ import checkSubtype
 fun main(args : Array<String>) {
     val x = checkSubtype<Any>(args[0])
     if(x is <!PLATFORM_CLASS_MAPPED_TO_KOTLIN!>java.lang.CharSequence<!>) {
-        if (<!EQUALITY_NOT_APPLICABLE_WARNING!>"a" == x<!>) x.<!FUNCTION_CALL_EXPECTED!>length<!> else x.length() // OK
-        if (<!EQUALITY_NOT_APPLICABLE_WARNING!>"a" == x<!> || <!EQUALITY_NOT_APPLICABLE_WARNING!>"b" == x<!>) x.<!FUNCTION_CALL_EXPECTED!>length<!> else x.length() // <– THEN ERROR
-        if (<!EQUALITY_NOT_APPLICABLE_WARNING!>"a" == x<!> && <!EQUALITY_NOT_APPLICABLE_WARNING!>"a" == x<!>) x.<!FUNCTION_CALL_EXPECTED!>length<!> else x.length() // <– ELSE ERROR
+        if (<!EQUALITY_NOT_APPLICABLE_WARNING!>"a" == x<!>) x.length else x.length() // OK
+        if (<!EQUALITY_NOT_APPLICABLE_WARNING!>"a" == x<!> || <!EQUALITY_NOT_APPLICABLE_WARNING!>"b" == x<!>) x.length else x.length() // <– THEN ERROR
+        if (<!EQUALITY_NOT_APPLICABLE_WARNING!>"a" == x<!> && "a" == x) x.length else x.length() // <– ELSE ERROR
     }
 }

compiler/testData/diagnostics/tests/smartCasts/equals.fir.kt

@@ -23,3 +23,31 @@ fun gav(i: TestWithEquals?, j: TestWithEquals?) {
         if (i == j) foo(i)
     }
 }
+
+fun string(foo: Any) {
+    val string = ""
+    if ("" == foo) foo.length
+    if (string == foo) foo.length
+    if (foo == "") foo.<!UNRESOLVED_REFERENCE!>length<!>
+}
+
+fun int(foo: Any) {
+    val int = 1
+    if (1 == foo) foo.<!UNRESOLVED_REFERENCE!>plus<!>(1)
+    if (int == foo) foo.<!UNRESOLVED_REFERENCE!>plus<!>(1)
+    if (foo == 1) foo.<!UNRESOLVED_REFERENCE!>plus<!>(1)
+}
+
+fun long(foo: Any) {
+    val long = 1L
+    if (1L == foo) foo.<!UNRESOLVED_REFERENCE!>plus<!>(1L)
+    if (long == foo) foo.<!UNRESOLVED_REFERENCE!>plus<!>(1L)
+    if (foo == 1L) foo.<!UNRESOLVED_REFERENCE!>plus<!>(1L)
+}
+
+fun char(foo: Any) {
+    val char = 'a'
+    if ('a' == foo) foo.<!UNRESOLVED_REFERENCE!>compareTo<!>('a')
+    if (char == foo) foo.<!UNRESOLVED_REFERENCE!>compareTo<!>('a')
+    if (foo == 'a') foo.<!UNRESOLVED_REFERENCE!>compareTo<!>('a')
+}

compiler/testData/diagnostics/tests/smartCasts/equals.kt

@@ -22,4 +22,32 @@ fun gav(i: TestWithEquals?, j: TestWithEquals?) {
     if (j == null) {
         if (i == <!DEBUG_INFO_CONSTANT!>j<!>) foo(<!DEBUG_INFO_CONSTANT!>i<!>)
     }
-}
+}
+
+fun string(foo: Any) {
+    val string = ""
+    if ("" == foo) <!DEBUG_INFO_SMARTCAST!>foo<!>.length
+    if (string == foo) <!DEBUG_INFO_SMARTCAST!>foo<!>.length
+    if (foo == "") foo.<!UNRESOLVED_REFERENCE!>length<!>
+}
+
+fun int(foo: Any) {
+    val int = 1
+    if (1 == foo) foo.<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>plus<!>(1)
+    if (int == foo) foo.<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>plus<!>(1)
+    if (foo == 1) foo.<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>plus<!>(1)
+}
+
+fun long(foo: Any) {
+    val long = 1L
+    if (1L == foo) foo.<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>plus<!>(1L)
+    if (long == foo) foo.<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>plus<!>(1L)
+    if (foo == 1L) foo.<!UNRESOLVED_REFERENCE_WRONG_RECEIVER!>plus<!>(1L)
+}
+
+fun char(foo: Any) {
+    val char = 'a'
+    if ('a' == foo) foo.<!UNRESOLVED_REFERENCE!>compareTo<!>('a')
+    if (char == foo) foo.<!UNRESOLVED_REFERENCE!>compareTo<!>('a')
+    if (foo == 'a') foo.<!UNRESOLVED_REFERENCE!>compareTo<!>('a')
+}

compiler/testData/diagnostics/tests/smartCasts/equals.txt

@@ -2,8 +2,12 @@ package
 
 public fun bar(/*0*/ i: Test?): kotlin.Unit
 public fun bar(/*0*/ i: TestWithEquals?): kotlin.Unit
+public fun char(/*0*/ foo: kotlin.Any): kotlin.Unit
 public fun foo(/*0*/ x: kotlin.String?): kotlin.String?
 public fun gav(/*0*/ i: TestWithEquals?, /*1*/ j: TestWithEquals?): kotlin.Unit
+public fun int(/*0*/ foo: kotlin.Any): kotlin.Unit
+public fun long(/*0*/ foo: kotlin.Any): kotlin.Unit
+public fun string(/*0*/ foo: kotlin.Any): kotlin.Unit
 
 public final class Test {
     public constructor Test()