FIR: intersect return types of declarations in intersection scopes

^KT-54378 Fixed

compiler/testData/diagnostics/tests/smartCasts/intersectionScope/capturedSpecificity.kt

@@ -0,0 +1,14 @@
+// FIR_IDENTICAL
+// SKIP_TXT
+class C<T>(val value: T?)
+
+fun <T> assignable(x: () -> T) {}
+
+fun <V> foo(t: C<out Any>, v: C<out V>) {
+    assignable<V?> { v.value }
+    if (t == v) {
+        // `value: CapturedType(out V)?` <: `value: CapturedType(out Any)?` - same instantiation
+        assignable<V?> { v.value }
+        assignable<V?> { t.value }
+    }
+}

compiler/testData/diagnostics/tests/smartCasts/intersectionScope/intersectReturnType.fir.kt

@@ -0,0 +1,17 @@
+// SKIP_TXT
+class C<T>(val value: T)
+
+fun <T> assignable(x: () -> T) {}
+
+fun <T, V> foo(t: C<out T>, v: C<out V>) {
+    assignable<T> { t.value } // sure
+    assignable<V> { v.value } // obviously
+    if (t == v) {
+        // => {t,v} is C<out T> & C<out V>
+        // => {t,v}.value is T & V
+        assignable<T> { t.value }
+        assignable<V> { v.value }
+        assignable<T> { v.value }
+        assignable<V> { t.value }
+    }
+}

compiler/testData/diagnostics/tests/smartCasts/intersectionScope/intersectReturnType.kt

@@ -0,0 +1,17 @@
+// SKIP_TXT
+class C<T>(val value: T)
+
+fun <T> assignable(x: () -> T) {}
+
+fun <T, V> foo(t: C<out T>, v: C<out V>) {
+    assignable<T> { t.value } // sure
+    assignable<V> { v.value } // obviously
+    if (t == v) {
+        // => {t,v} is C<out T> & C<out V>
+        // => {t,v}.value is T & V
+        assignable<T> { <!TYPE_MISMATCH!>t.value<!> }
+        assignable<V> { <!TYPE_MISMATCH!>v.value<!> }
+        assignable<T> { v.value }
+        assignable<V> { t.value }
+    }
+}