Introduce FirFunctionReturnTypeMismatchChecker

compiler/testData/diagnostics/tests/generics/nullability/nullToGeneric.fir.kt

@@ -9,12 +9,12 @@ fun <T> foo(): T {
     bar<T>(<!ARGUMENT_TYPE_MISMATCH!>null<!>)
     bar<T?>(null)
 
-    return null
+    return <!RETURN_TYPE_MISMATCH!>null<!>
 }
 
 fun <T> baz(): T? = null
 
-fun <T> foobar(): T = null
+fun <T> foobar(): T = <!RETURN_TYPE_MISMATCH!>null<!>
 
 class A<F> {
     fun xyz(x: F) {}
@@ -26,10 +26,10 @@ class A<F> {
         xyz(<!ARGUMENT_TYPE_MISMATCH!>null<!>)
         bar<F?>(null)
 
-        return null
+        return <!RETURN_TYPE_MISMATCH!>null<!>
     }
 
     fun baz(): F? = null
 
-    fun foobar(): F = null
+    fun foobar(): F = <!RETURN_TYPE_MISMATCH!>null<!>
 }

compiler/testData/diagnostics/tests/generics/projectionsScope/unsafeVarianceInAliasedFunctionalType.fir.kt

@@ -0,0 +1,11 @@
+class Foo<out T>(val baz: Baz<T>)
+
+class Bar {
+    val foo: Foo<*> = TODO()
+
+    fun <T> bar(): Baz<T> {
+        return <!RETURN_TYPE_MISMATCH!>foo.baz<!>
+    }
+}
+
+typealias Baz<T> = (@UnsafeVariance T) -> Unit

compiler/testData/diagnostics/tests/generics/projectionsScope/unsafeVarianceInAliasedFunctionalType.kt

@@ -1,5 +1,3 @@
-// FIR_IDENTICAL
-
 class Foo<out T>(val baz: Baz<T>)
 
 class Bar {
@@ -10,4 +8,4 @@ class Bar {
     }
 }
 
-typealias Baz<T> = (@UnsafeVariance T) -> Unit
+typealias Baz<T> = (@UnsafeVariance T) -> Unit