Improve changetype check (#898)

Author: dcodeIO

src/builtins.ts

@@ -2037,10 +2037,10 @@ export function compileCall(
         checkArgsRequired(operands, 1, reportNode, compiler)
       ) return module.unreachable();
       let toType = typeArguments![0];
-      let arg0 = compiler.compileExpression(operands[0], toType);
+      let arg0 = compiler.compileExpression(operands[0], Type.auto);
       let fromType = compiler.currentType;
       compiler.currentType = toType;
-      if (fromType.size != toType.size) {
+      if (!fromType.isChangeableTo(toType)) {
         compiler.error(
           DiagnosticCode.Type_0_cannot_be_changed_to_type_1,
           reportNode.range, fromType.toString(), toType.toString()

src/types.ts

@@ -280,6 +280,15 @@ export class Type {
     return this.kind == target.kind;
   }
 
+  /** Tests if a value of this type can be changed to the target type using `changetype`. */
+  isChangeableTo(target: Type): bool {
+    if (this.is(TypeFlags.INTEGER) && target.is(TypeFlags.INTEGER)) {
+      let size = this.size;
+      return size == target.size && (size >= 32 || this.is(TypeFlags.SIGNED) == target.is(TypeFlags.SIGNED));
+    }
+    return this.kind == target.kind;
+  }
+
   /** Determines the common denominator type of two types, if there is any. */
   static commonDenominator(left: Type, right: Type, signednessIsImportant: bool): Type | null {
     if (right.isAssignableTo(left, signednessIsImportant)) return left;