FIR2IR: coerce to Unit when "when" expr is not effectively exhaustive

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt

@@ -581,11 +581,15 @@ class Fir2IrVisitor(
             KtNodeTypes.POSTFIX_EXPRESSION -> IrStatementOrigin.EXCLEXCL
             else -> null
         }
+        // If the constant true branch has empty body, it won't be converted. Thus, the entire `when` expression is effectively _not_
+        // exhaustive anymore. In that case, coerce the return type of `when` expression to Unit as per the backend expectation.
+        val effectivelyNotExhaustive = !whenExpression.isExhaustive ||
+                whenExpression.branches.any { it.condition is FirElseIfTrueCondition && it.result.statements.isEmpty() }
         return conversionScope.withWhenSubject(subjectVariable) {
             whenExpression.convertWithOffsets { startOffset, endOffset ->
                 val irWhen = IrWhenImpl(
                     startOffset, endOffset,
-                    whenExpression.typeRef.toIrType(),
+                    if (effectivelyNotExhaustive) irBuiltIns.unitType else whenExpression.typeRef.toIrType(),
                     origin
                 ).apply {
                     var unconditionalBranchFound = false

compiler/testData/codegen/box/regressions/kt528.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM
 
 // WITH_RUNTIME

compiler/testData/codegen/box/regressions/kt529.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM
 
 // WITH_RUNTIME

compiler/testData/codegen/box/regressions/kt533.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM
 
 // WITH_RUNTIME

compiler/testData/ir/irText/expressions/ifElseIf.fir.txt

@@ -31,7 +31,7 @@ FILE fqName:<root> fileName:/ifElseIf.kt
         BRANCH
           if: CONST Boolean type=kotlin.Boolean value=true
           then: CONST Boolean type=kotlin.Boolean value=true
-      WHEN type=kotlin.Any origin=IF
+      WHEN type=kotlin.Unit origin=IF
         BRANCH
           if: GET_VAR 'flag: kotlin.Boolean declared in <root>.testEmptyBranches1' type=kotlin.Boolean origin=null
           then: CONST Boolean type=kotlin.Boolean value=true
@@ -45,7 +45,7 @@ FILE fqName:<root> fileName:/ifElseIf.kt
         BRANCH
           if: CONST Boolean type=kotlin.Boolean value=true
           then: CONST Boolean type=kotlin.Boolean value=true
-      WHEN type=kotlin.Any origin=IF
+      WHEN type=kotlin.Unit origin=IF
         BRANCH
           if: GET_VAR 'flag: kotlin.Boolean declared in <root>.testEmptyBranches2' type=kotlin.Boolean origin=null
           then: CONST Boolean type=kotlin.Boolean value=true