feat: update error message for reserved self function argument

dev-docs/CHANGELOG.md

@@ -47,6 +47,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Better error message for `extend function without parameters` error: PR [#1624](https://github.com/tact-lang/tact/pull/1624)
 - Don't generate `lazy_deployment_completed` by default: PR [#1717](https://github.com/tact-lang/tact/pull/1717)
 - Optimized `emptyCell()` and `emptySlice()` functions: PR [#1696](https://github.com/tact-lang/tact/pull/1696)
+- Error message for reserved `self` function parameter now suggests using `extends` function modifier: PR [#1737](https://github.com/tact-lang/tact/pull/1737)
 
 ### Fixed
 

src/types/__snapshots__/resolveDescriptors.spec.ts.snap

@@ -495,6 +495,15 @@ exports[`resolveDescriptors should fail descriptors for native-fun-getter 1`] =
 "
 `;
 
+exports[`resolveDescriptors should fail descriptors for not-extend-function-self-param 1`] = `
+"<unknown>:3:30: Parameter name "self" is reserved for functions with "extends" modifier
+  2 | 
+> 3 | asm(other self) fun testFunc(self: Int, other: Int) { 2 BLKDROP }
+                                   ^~~~~~~~~
+  4 | 
+"
+`;
+
 exports[`resolveDescriptors should fail descriptors for override-for-non-existing-constant-in-contract-from-base-trait 1`] = `
 "<unknown>:8:5: Constant "BAR" overrides nothing, remove "override" modifier or inherit any traits with this constant
   7 | contract Test {

src/types/resolveDescriptors.ts

@@ -841,12 +841,19 @@ export function resolveDescriptors(ctx: CompilerContext, Ast: FactoryAst) {
 
         // Check parameter names
         const exNames: Set<string> = new Set();
-        for (const param of params) {
+        for (const [i, param] of params.entries()) {
             if (isSelfId(param.name)) {
-                throwCompilationError(
-                    'Parameter name "self" is reserved',
-                    param.loc,
-                );
+                if (i === 0) {
+                    throwCompilationError(
+                        'Parameter name "self" is reserved for functions with "extends" modifier',
+                        param.loc,
+                    );
+                } else {
+                    throwCompilationError(
+                        'Parameter name "self" is reserved',
+                        param.loc,
+                    );
+                }
             }
             if (exNames.has(idText(param.name))) {
                 throwCompilationError(

src/types/test-failed/not-extend-function-self-param.tact

@@ -0,0 +1,3 @@
+primitive Int;
+
+asm(other self) fun testFunc(self: Int, other: Int) { 2 BLKDROP }