KT-10139: any if without else used in expression is an error

regardless of expected type: it can't be an expression, even of type Unit.

 #KT-10139 Fixed

compiler/testData/diagnostics/tests/controlStructures/ifWhenWithoutElse.kt

@@ -0,0 +1,72 @@
+fun idAny(x: Any) = x
+fun <T> id(x: T) = x
+fun idUnit(x: Unit) = x
+
+class MList {
+    // MutableCollection<T>.add returns Boolean, but nobody cares
+    fun add(): Boolean = true
+}
+val mlist = MList()
+
+fun work() {}
+
+val xx1 = <!INVALID_IF_AS_EXPRESSION!>if (true) 42<!>
+val xx2: Unit = <!INVALID_IF_AS_EXPRESSION!>if (true) 42<!>
+val xx3 = idAny(<!INVALID_IF_AS_EXPRESSION!>if (true) 42<!>)
+val xx4 = id(<!INVALID_IF_AS_EXPRESSION!>if (true) 42<!>)
+val xx5 = idUnit(<!INVALID_IF_AS_EXPRESSION!>if (true) 42<!>)
+val xx6 = null ?: <!INVALID_IF_AS_EXPRESSION!>if (true) 42<!>
+val xx7 = "" + <!INVALID_IF_AS_EXPRESSION!>if (true) 42<!>
+
+val wxx1 = <!NO_ELSE_IN_WHEN!>when<!> { true -> 42 }
+val wxx2: Unit = when { true -> <!CONSTANT_EXPECTED_TYPE_MISMATCH!>42<!> }
+val wxx3 = idAny(<!NO_ELSE_IN_WHEN!>when<!> { true -> 42 })
+val wxx4 = id(<!NO_ELSE_IN_WHEN!>when<!> { true -> 42 })
+val wxx5 = idUnit(when { true -> 42 }) // TODO illegal expression
+val wxx6 = null ?: <!NO_ELSE_IN_WHEN!>when<!> { true -> 42 }
+val wxx7 = "" + <!NO_ELSE_IN_WHEN!>when<!> { true -> 42 }
+
+val fn1 = { if (true) <!UNUSED_EXPRESSION!>42<!> }
+val fn2 = { if (true) mlist.add() }
+val fn3 = { if (true) work() }
+val fn4 = { <!NO_ELSE_IN_WHEN!>when<!> { true -> 42 } }
+val fn5 = { <!NO_ELSE_IN_WHEN!>when<!> { true -> mlist.add() } }
+val fn6 = { when { true -> work() } }
+
+val ufn1: () -> Unit = { if (true) <!UNUSED_EXPRESSION!>42<!> }
+val ufn2: () -> Unit = { if (true) mlist.add() }
+val ufn3: () -> Unit = { if (true) work() }
+val ufn4: () -> Unit = { when { true -> <!UNUSED_EXPRESSION!>42<!> } }
+val ufn5: () -> Unit = { when { true -> mlist.add() } }
+val ufn6: () -> Unit = { when { true -> work() } }
+
+fun foo1(x: String?) {
+    "" + <!INVALID_IF_AS_EXPRESSION!>if (true) 42<!>
+    w@while (true) {
+        x ?: if (true) break
+        x ?: <!NO_ELSE_IN_WHEN!>when<!> { true -> break@w }
+    }
+}
+
+fun foo2() {
+    if (true) {
+        mlist.add()
+    }
+    else if (true) {
+        mlist.add()
+    }
+    else if (true) {
+        mlist.add()
+    }
+
+    when {
+        true -> mlist.add()
+        else -> when {
+            true -> mlist.add()
+            else -> when {
+                true -> mlist.add()
+            }
+        }
+    }
+}
+

compiler/testData/diagnostics/tests/controlStructures/ifWhenWithoutElse.txt

@@ -0,0 +1,43 @@
+package
+
+public val fn1: () -> kotlin.Unit
+public val fn2: () -> kotlin.Unit
+public val fn3: () -> kotlin.Unit
+public val fn4: () -> kotlin.Int
+public val fn5: () -> kotlin.Boolean
+public val fn6: () -> kotlin.Unit
+public val mlist: MList
+public val ufn1: () -> kotlin.Unit
+public val ufn2: () -> kotlin.Unit
+public val ufn3: () -> kotlin.Unit
+public val ufn4: () -> kotlin.Unit
+public val ufn5: () -> kotlin.Unit
+public val ufn6: () -> kotlin.Unit
+public val wxx1: kotlin.Int
+public val wxx2: kotlin.Unit
+public val wxx3: kotlin.Any
+public val wxx4: kotlin.Int
+public val wxx5: kotlin.Unit
+public val wxx6: kotlin.Int
+public val wxx7: kotlin.String
+public val xx1: kotlin.Unit
+public val xx2: kotlin.Unit
+public val xx3: kotlin.Any
+public val xx4: kotlin.Unit
+public val xx5: kotlin.Unit
+public val xx6: kotlin.Unit
+public val xx7: kotlin.String
+public fun foo1(/*0*/ x: kotlin.String?): kotlin.Unit
+public fun foo2(): kotlin.Unit
+public fun </*0*/ T> id(/*0*/ x: T): T
+public fun idAny(/*0*/ x: kotlin.Any): kotlin.Any
+public fun idUnit(/*0*/ x: kotlin.Unit): kotlin.Unit
+public fun work(): kotlin.Unit
+
+public final class MList {
+    public constructor MList()
+    public final fun add(): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/controlStructures/kt4310.kt

@@ -4,7 +4,7 @@ fun test(a: Boolean, b: Boolean): Int {
     return if(a) {
         1
     } else {
-        <!TYPE_MISMATCH!>if (b) {
+        <!TYPE_MISMATCH, INVALID_IF_AS_EXPRESSION!>if (b) {
             3
         }<!>
     }