[FIR] Forbid no then branch in if expression

#KT-59883

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirExhaustiveWhenChecker.kt

@@ -22,6 +22,7 @@ import org.jetbrains.kotlin.fir.declarations.utils.modality
 import org.jetbrains.kotlin.fir.expressions.ExhaustivenessStatus
 import org.jetbrains.kotlin.fir.expressions.FirWhenExpression
 import org.jetbrains.kotlin.fir.expressions.impl.FirElseIfTrueCondition
+import org.jetbrains.kotlin.fir.expressions.impl.FirEmptyExpressionBlock
 import org.jetbrains.kotlin.fir.expressions.isExhaustive
 import org.jetbrains.kotlin.fir.languageVersionSettings
 import org.jetbrains.kotlin.fir.resolve.fullyExpandedType
@@ -37,8 +38,23 @@ object FirExhaustiveWhenChecker : FirWhenExpressionChecker(MppCheckerKind.Common
         reportElseMisplaced(expression, reporter, context)
     }
 
+    private fun reportEmptyThenInExpression(whenExpression: FirWhenExpression, context: CheckerContext, reporter: DiagnosticReporter) {
+        val source = whenExpression.source ?: return
+
+        if (source.isIfExpression && whenExpression.usedAsExpression) {
+            val thenBranch = whenExpression.branches.firstOrNull()
+            if (thenBranch == null || thenBranch.result is FirEmptyExpressionBlock) {
+                reporter.reportOn(source, FirErrors.INVALID_IF_AS_EXPRESSION, context)
+            }
+        }
+    }
+
     private fun reportNotExhaustive(whenExpression: FirWhenExpression, context: CheckerContext, reporter: DiagnosticReporter) {
-        if (whenExpression.isExhaustive) return
+        if (whenExpression.isExhaustive) {
+            // whenExpression.isExhaustive is checked as otherwise the constraint is checked below
+            reportEmptyThenInExpression(whenExpression, context, reporter)
+            return
+        }
 
         val source = whenExpression.source ?: return
 

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

@@ -72,7 +72,7 @@ fun blockReturnValueTypeMatch2() : Int {
     return <!INVALID_IF_AS_EXPRESSION!>if<!> (1 > 2) 1
 }
 fun blockReturnValueTypeMatch3() : Int {
-    return <!RETURN_TYPE_MISMATCH!>if (1 > 2) else 1<!>
+    return <!RETURN_TYPE_MISMATCH!><!INVALID_IF_AS_EXPRESSION!>if<!> (1 > 2) else 1<!>
 }
 fun blockReturnValueTypeMatch4() : Int {
     if (1 > 2)

compiler/testData/diagnostics/tests/controlStructures/improperElseInExpression.fir.kt

@@ -2,7 +2,7 @@
 
 fun example() {
     val a = if (true) true else false
-    val b = if (true) else false
+    val b = <!INVALID_IF_AS_EXPRESSION!>if<!> (true) else false
     val c = <!INVALID_IF_AS_EXPRESSION!>if<!> (true) true
     val d = <!INVALID_IF_AS_EXPRESSION!>if<!> (true) true else;
     val e = if (true) {} else false