[FIR] Report no return diagnostic on functions returning Nothing

Previously, the NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY diagnostic was not
reported at all on functions which return Nothing. However, in K1, if
the function returns Nothing but the body is empty or doesn't contain a
Nothing expression, this diagnostic would be reported. The checker in K2
has been updated to report this diagnostic on functions which return
Nothing as well.

^KT-55965 Fixed

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirFunctionReturnChecker.kt

@@ -35,7 +35,7 @@ object FirFunctionReturnChecker : FirFunctionChecker() {
         if (declaration is FirConstructor) return
         if (declaration is FirAnonymousFunction && declaration.isLambda) return
         val returnType = declaration.returnTypeRef.coneType.fullyExpandedType(context.session)
-        if (returnType.isUnit || returnType.isNothing) return
+        if (returnType.isUnit) return
         val graph = declaration.controlFlowGraphReference?.controlFlowGraph ?: return
 
         val blockExitNode = graph.exitNode.previousNodes.lastOrNull { it is BlockExitNode } ?: return

compiler/testData/diagnostics/tests/FunctionReturnTypes.fir.kt

@@ -158,6 +158,8 @@ fun illegalReturnIf(): Char {
 fun returnNothing(): Nothing {
     throw <!TYPE_MISMATCH!>1<!>
 }
+fun returnNothingEmpty(): Nothing {
+<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 fun f(): Int {
     if (1 < 2) { return 1 } else returnNothing()
 }

compiler/testData/diagnostics/tests/FunctionReturnTypes.kt

@@ -158,6 +158,8 @@ fun illegalReturnIf(): Char {
 fun returnNothing(): Nothing {
     throw <!CONSTANT_EXPECTED_TYPE_MISMATCH!>1<!>
 }
+fun returnNothingEmpty(): Nothing {
+<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>
 fun f(): Int {
     if (1 < 2) { return 1 } else returnNothing()
 }

compiler/testData/diagnostics/tests/FunctionReturnTypes.txt

@@ -47,6 +47,7 @@ public fun nonBlockNoReturnIfUnitInOneBranch(): kotlin.Int
 public fun nonBlockReturnIfEmptyIf(): kotlin.Int
 public fun none(): kotlin.Unit
 public fun returnNothing(): kotlin.Nothing
+public fun returnNothingEmpty(): kotlin.Nothing
 public fun test1(): kotlin.Any
 public fun test2(): kotlin.Any
 public fun test3(): kotlin.Any