psi2ir: Handle missing "then" branches as empty blocks

#KT-27933

compiler/ir/ir.psi2ir/src/org/jetbrains/kotlin/psi2ir/generators/BranchingExpressionGenerator.kt

@@ -28,6 +28,7 @@ import org.jetbrains.kotlin.ir.types.IrType
 import org.jetbrains.kotlin.ir.types.classifierOrNull
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.psiUtil.endOffset
+import org.jetbrains.kotlin.psi.psiUtil.startOffset
 import org.jetbrains.kotlin.psi.psiUtil.startOffsetSkippingComments
 import org.jetbrains.kotlin.psi2ir.deparenthesize
 import org.jetbrains.kotlin.psi2ir.intermediate.defaultLoad
@@ -45,7 +46,8 @@ class BranchingExpressionGenerator(statementGenerator: StatementGenerator) : Sta
 
         whenBranches@ while (true) {
             val irCondition = ktLastIf.condition!!.genExpr()
-            val irThenBranch = ktLastIf.then!!.genExpr()
+
+            val irThenBranch = ktLastIf.then?.genExpr() ?: generateEmptyBlockForMissingBranch(ktLastIf)
             irBranches.add(IrBranchImpl(irCondition, irThenBranch))
 
             val ktElse = ktLastIf.`else`?.deparenthesize()
@@ -63,6 +65,9 @@ class BranchingExpressionGenerator(statementGenerator: StatementGenerator) : Sta
         return createIrWhen(expression, irBranches, irElseBranch, resultType)
     }
 
+    private fun generateEmptyBlockForMissingBranch(ktLastIf: KtIfExpression) =
+        IrBlockImpl(ktLastIf.startOffset, ktLastIf.endOffset, context.irBuiltIns.nothingType, IrStatementOrigin.IF, listOf())
+
     private fun createIrWhen(
         ktIf: KtIfExpression,
         irBranches: List<IrBranch>,

compiler/testData/ir/irText/expressions/kt27933.kt

@@ -0,0 +1,7 @@
+fun box(): String {
+    var r = ""
+    if (r != "") else r += "O"
+    if (r == "O") r += "K" else;
+    return r
+}
+

compiler/testData/ir/irText/expressions/kt27933.txt

@@ -0,0 +1,29 @@
+FILE fqName:<root> fileName:/kt27933.kt
+  FUN name:box visibility:public modality:FINAL <> () returnType:kotlin.String flags:
+    BLOCK_BODY
+      VAR name:r type:kotlin.String flags:var
+        CONST String type=kotlin.String value=""
+      WHEN type=kotlin.Unit origin=null
+        BRANCH
+          if: CALL 'NOT(Boolean): Boolean' type=kotlin.Boolean origin=EXCLEQ
+            arg0: CALL 'EQEQ(Any?, Any?): Boolean' type=kotlin.Boolean origin=EXCLEQ
+              arg0: GET_VAR 'r: String' type=kotlin.String origin=null
+              arg1: CONST String type=kotlin.String value=""
+          then: BLOCK type=kotlin.Nothing origin=IF
+        BRANCH
+          if: CONST Boolean type=kotlin.Boolean value=true
+          then: SET_VAR 'r: String' type=kotlin.Unit origin=PLUSEQ
+            CALL 'plus(Any?): String' type=kotlin.String origin=PLUSEQ
+              $this: GET_VAR 'r: String' type=kotlin.String origin=PLUSEQ
+              other: CONST String type=kotlin.String value="O"
+      WHEN type=kotlin.Unit origin=null
+        BRANCH
+          if: CALL 'EQEQ(Any?, Any?): Boolean' type=kotlin.Boolean origin=EQEQ
+            arg0: GET_VAR 'r: String' type=kotlin.String origin=null
+            arg1: CONST String type=kotlin.String value="O"
+          then: SET_VAR 'r: String' type=kotlin.Unit origin=PLUSEQ
+            CALL 'plus(Any?): String' type=kotlin.String origin=PLUSEQ
+              $this: GET_VAR 'r: String' type=kotlin.String origin=PLUSEQ
+              other: CONST String type=kotlin.String value="K"
+      RETURN type=kotlin.Nothing from='box(): String'
+        GET_VAR 'r: String' type=kotlin.String origin=null

compiler/tests/org/jetbrains/kotlin/ir/IrTextTestCaseGenerated.java

@@ -947,6 +947,11 @@ public class IrTextTestCaseGenerated extends AbstractIrTextTestCase {
             runTest("compiler/testData/ir/irText/expressions/kt23030.kt");
         }
 
+        @TestMetadata("kt27933.kt")
+        public void testKt27933() throws Exception {
+            runTest("compiler/testData/ir/irText/expressions/kt27933.kt");
+        }
+
         @TestMetadata("kt28006.kt")
         public void testKt28006() throws Exception {
             runTest("compiler/testData/ir/irText/expressions/kt28006.kt");