WASM: Cover frontend issue where unit-type is casted to the non-unit type

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/lower/TryCatchCanonicalization.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.backend.wasm.lower
 
+import org.jetbrains.kotlin.backend.common.CommonBackendContext
 import org.jetbrains.kotlin.backend.common.FileLoweringPass
 import org.jetbrains.kotlin.backend.common.IrElementTransformerVoidWithContext
 import org.jetbrains.kotlin.backend.common.lower.*
@@ -66,7 +67,7 @@ import org.jetbrains.kotlin.name.Name
 //   tmp // result
 
 
-internal class TryCatchCanonicalization(private val ctx: WasmBackendContext) : FileLoweringPass {
+internal class TryCatchCanonicalization(private val ctx: CommonBackendContext) : FileLoweringPass {
     override fun lower(irFile: IrFile) {
         irFile.transformChildrenVoid(CatchMerger(ctx))
 
@@ -84,7 +85,7 @@ internal class TryCatchCanonicalization(private val ctx: WasmBackendContext) : F
     }
 }
 
-internal class CatchMerger(private val ctx: WasmBackendContext) : IrElementTransformerVoidWithContext() {
+internal class CatchMerger(private val ctx: CommonBackendContext) : IrElementTransformerVoidWithContext() {
     override fun visitTry(aTry: IrTry): IrExpression {
         // First, handle all nested constructs
         aTry.transformChildrenVoid(this)

compiler/ir/backend.wasm/src/org/jetbrains/kotlin/backend/wasm/lower/WasmTypeOperatorLowering.kt

@@ -168,6 +168,22 @@ class WasmBaseTypeOperatorTransformer(val context: WasmBackendContext) : IrEleme
             }
         }
 
+        // Workaround for the bug in frontend.
+        // In code like this:
+        //    val p: Int = try {
+        //        1
+        //    } catch (e: Exception) {
+        //        throw RuntimeException()
+        //    }
+        // Frontend will emit IMPLICIT_CAST from Unit into Int. Such code can't be correctly compiled to wasm as it is.
+        // This supposed to only happen in the unreachable code so we emit unreachable intrinsic.
+        if (fromType == builtIns.unitType && toType !in listOf(builtIns.unitType, builtIns.anyType, builtIns.anyNType)) {
+            return builder.irComposite(resultType = toType) {
+                +value
+                +builder.irCall(symbols.wasmUnreachable)
+            }
+        }
+
         // Handled by autoboxing transformer
         if (toType.isInlined() && !fromType.isInlined()) {
             return builder.irCall(