KT-16437 Incorrect type inference for some when coerced to Unit

If the result of 'when' is not used in an expression,
this 'when' expression has type 'Unit' despite of whatever FE has inferred.

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

@@ -85,7 +85,12 @@ class BranchingExpressionGenerator(statementGenerator: StatementGenerator) : Sta
             scope.createTemporaryVariable(statementGenerator.generateExpression(it), "subject")
         }
 
-        val resultType = getInferredTypeWithImplicitCastsOrFail(expression)
+        // TODO relies on ControlFlowInformationProvider, get rid of it
+        val resultType =
+                if (context.bindingContext[BindingContext.USED_AS_EXPRESSION, expression] ?: false)
+                    getInferredTypeWithImplicitCastsOrFail(expression)
+                else
+                    context.builtIns.unitType
 
         val irWhen = IrWhenImpl(expression.startOffset, expression.endOffset, resultType, IrStatementOrigin.WHEN)
 

compiler/testData/ir/irCfg/when/whenReturn.txt

@@ -5,7 +5,7 @@ CONTENT
       1 FUN public fun toString(grade: kotlin.String): kotlin.String
       2 GET_VAR 'value-parameter grade: String' type=kotlin.String origin=null
       3 VAR IR_TEMPORARY_VARIABLE val tmp0_subject: kotlin.String
-      4 WHEN type=kotlin.Nothing origin=WHEN
+      4 WHEN type=kotlin.Unit origin=WHEN
       5 GET_VAR 'tmp0_subject: String' type=kotlin.String origin=null
       6 CONST String type=kotlin.String value='A'
 OUTGOING -> BB 1, 5

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

@@ -0,0 +1,7 @@
+// WITH_RUNTIME
+
+fun foo(x: Int) {
+    when (x) {
+        0 -> 0
+    }
+}

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

@@ -0,0 +1,13 @@
+FILE /whenCoercedToUnit.kt
+  FUN public fun foo(x: kotlin.Int): kotlin.Unit
+    BLOCK_BODY
+      BLOCK type=kotlin.Unit origin=WHEN
+        VAR IR_TEMPORARY_VARIABLE val tmp0_subject: kotlin.Int
+          GET_VAR 'value-parameter x: Int' type=kotlin.Int origin=null
+        WHEN type=kotlin.Unit origin=WHEN
+          BRANCH
+            if: CALL 'EQEQ(Any?, Any?): Boolean' type=kotlin.Boolean origin=EQEQ
+              arg0: GET_VAR 'tmp0_subject: Int' type=kotlin.Int origin=null
+              arg1: CONST Int type=kotlin.Int value='0'
+            then: TYPE_OP origin=IMPLICIT_COERCION_TO_UNIT typeOperand=kotlin.Unit
+              CONST Int type=kotlin.Int value='0'

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

@@ -1,10 +1,10 @@
 FILE /whenReturn.kt
   FUN public fun toString(grade: kotlin.String): kotlin.String
     BLOCK_BODY
-      BLOCK type=kotlin.Nothing origin=WHEN
+      BLOCK type=kotlin.Unit origin=WHEN
         VAR IR_TEMPORARY_VARIABLE val tmp0_subject: kotlin.String
           GET_VAR 'value-parameter grade: String' type=kotlin.String origin=null
-        WHEN type=kotlin.Nothing origin=WHEN
+        WHEN type=kotlin.Unit origin=WHEN
           BRANCH
             if: CALL 'EQEQ(Any?, Any?): Boolean' type=kotlin.Boolean origin=EQEQ
               arg0: GET_VAR 'tmp0_subject: String' type=kotlin.String origin=null

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

@@ -689,6 +689,12 @@ public class IrTextTestCaseGenerated extends AbstractIrTextTestCase {
             doTest(fileName);
         }
 
+        @TestMetadata("whenCoercedToUnit.kt")
+        public void testWhenCoercedToUnit() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/ir/irText/expressions/whenCoercedToUnit.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("whenElse.kt")
         public void testWhenElse() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/ir/irText/expressions/whenElse.kt");