[IR] Move isAssignable to IrValueDeclaration.

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrValueDeclaration.kt

@@ -16,4 +16,6 @@ abstract class IrValueDeclaration : IrDeclarationBase(), IrDeclarationWithName,
 
     abstract override val symbol: IrValueSymbol
     abstract var type: IrType
+
+    abstract val isAssignable: Boolean
 }

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrValueParameter.kt

@@ -39,11 +39,6 @@ abstract class IrValueParameter : IrValueDeclaration(), IrSymbolDeclaration<IrVa
     // Once we are able to load any top-level declaration from klib this hack should be deprecated and removed.
     abstract val isHidden: Boolean
 
-    // Parameters are not assignable by default. However, in the IR, assignments to parameters are used
-    // to implement default argument stubs. If the default value is to be used, the value is assigned
-    // to the parameter.
-    abstract val isAssignable: Boolean
-
     abstract var defaultValue: IrExpressionBody?
 
     override fun <R, D> accept(visitor: IrElementVisitor<R, D>, data: D): R =

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/impl/IrVariableImpl.kt

@@ -61,6 +61,12 @@ class IrVariableImpl(
 
     override var initializer: IrExpression? = null
 
+    // Variables are assignable by default. This means that they can be used in IrSetValue.
+    // Variables are assigned in the IR even though they are not 'var' in the input. Hence
+    // the separate assignability flag.
+    override val isAssignable: Boolean
+        get() = true
+
     override val factory: IrFactory
         get() = error("Create IrVariableImpl directly")
 

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/expressions/impl/IrSetValueImpl.kt

@@ -34,21 +34,8 @@ class IrSetValueImpl(
     override val origin: IrStatementOrigin?
 ) : IrSetValue() {
 
-    companion object {
-        private fun isVariableOrAssignable(symbol: IrValueSymbol): Boolean {
-            val declaration = symbol.owner
-            return if (declaration is IrValueParameter) {
-                declaration.isAssignable
-            } else {
-                true
-            }
-        }
-    }
-
     init {
-        assert(isVariableOrAssignable(symbol)) {
+        assert(symbol.owner.isAssignable) { "Only assignable IrValues can be set" }
-            "Only IrVariables and assignable IrValueParameters can be set"
-        }
     }
 
     override fun <R, D> accept(visitor: IrElementVisitor<R, D>, data: D): R {