patch 8.2.2983: Vim9: an inline function requires specifying the return type

Problem:    Vim9: an inline function requires specifying the return type.
Solution:   Make the return type optional.

src/eval.c

@@ -3530,9 +3530,13 @@ eval7(
 		    {
 			ufunc_T *ufunc = rettv->vval.v_partial->pt_func;
 
-			// compile it here to get the return type
+			// Compile it here to get the return type.  The return
+			// type is optional, when it's missing use t_unknown.
+			// This is recognized in compile_return().
+			if (ufunc->uf_ret_type->tt_type == VAR_VOID)
+			    ufunc->uf_ret_type = &t_unknown;
 			if (compile_def_function(ufunc,
-					 TRUE, PROFILING(ufunc), NULL) == FAIL)
+					 FALSE, PROFILING(ufunc), NULL) == FAIL)
 			{
 			    clear_tv(rettv);
 			    ret = FAIL;

src/testdir/test_vim9_func.vim

@@ -948,6 +948,26 @@ def Test_lambda_return_type()
       echo FilterWithCond('foo', (v) => v .. '^b')
   END
   CheckDefAndScriptFailure(lines, 'E1013: Argument 2: type mismatch, expected func(string): bool but got func(any): string', 1)
+
+  lines =<< trim END
+      var Lambda1 = (x) => {
+              return x
+              }
+      assert_equal('asdf', Lambda1('asdf'))
+      var Lambda2 = (x): string => {
+              return x
+              }
+      assert_equal('foo', Lambda2('foo'))
+  END
+  CheckDefAndScriptSuccess(lines)
+
+  lines =<< trim END
+      var Lambda = (x): string => {
+              return x
+              }
+      echo Lambda(['foo'])
+  END
+  CheckDefExecAndScriptFailure(lines, 'E1012:')
 enddef
 
 def Test_lambda_uses_assigned_var()

src/userfunc.c

@@ -1377,7 +1377,7 @@ get_lambda_tv(
 		    goto errret;
 	    }
 	    else
-		fp->uf_ret_type = &t_any;
+		fp->uf_ret_type = &t_unknown;
 	}
 
 	fp->uf_lines = newlines;

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2983,
 /**/
     2982,
 /**/

src/vim9compile.c

@@ -3565,8 +3565,12 @@ compile_lambda(char_u **arg, cctx_T *cctx)
     ++ufunc->uf_refcount;
     clear_tv(&rettv);
 
-    // Compile the function into instructions.
+    // Compile it here to get the return type.  The return type is optional,
-    compile_def_function(ufunc, TRUE, PROFILING(ufunc), cctx);
+    // when it's missing use t_unknown.  This is recognized in
+    // compile_return().
+    if (ufunc->uf_ret_type->tt_type == VAR_VOID)
+	ufunc->uf_ret_type = &t_unknown;
+    compile_def_function(ufunc, FALSE, PROFILING(ufunc), cctx);
 
     // evalarg.eval_tofree_cmdline may have a copy of the last line and "*arg"
     // points into it.  Point to the original line to avoid a dangling pointer.
@@ -5398,10 +5402,15 @@ compile_return(char_u *arg, int check_return_type, int legacy, cctx_T *cctx)
 
 	if (cctx->ctx_skip != SKIP_YES)
 	{
+	    // "check_return_type" with uf_ret_type set to &t_unknown is used
+	    // for an inline function without a specified return type.  Set the
+	    // return type here.
 	    stack_type = ((type_T **)stack->ga_data)[stack->ga_len - 1];
-	    if (check_return_type && (cctx->ctx_ufunc->uf_ret_type == NULL
+	    if ((check_return_type && (cctx->ctx_ufunc->uf_ret_type == NULL
 				|| cctx->ctx_ufunc->uf_ret_type == &t_unknown
 				|| cctx->ctx_ufunc->uf_ret_type == &t_any))
+		    || (!check_return_type
+				&& cctx->ctx_ufunc->uf_ret_type == &t_unknown))
 	    {
 		cctx->ctx_ufunc->uf_ret_type = stack_type;
 	    }