all: allow functions to return `shared` object (#8606)

vlib/v/gen/c/cgen.v

@@ -2548,7 +2548,7 @@ fn (mut g Gen) expr(node ast.Expr) {
 				node.return_type.clear_flag(.optional)
 			}
 			mut shared_styp := ''
-			if g.is_shared {
+			if g.is_shared && !ret_type.has_flag(.shared_f) {
 				ret_sym := g.table.get_type_symbol(ret_type)
 				shared_typ := ret_type.set_flag(.shared_f)
 				shared_styp = g.typ(shared_typ)
@@ -2576,7 +2576,7 @@ fn (mut g Gen) expr(node ast.Expr) {
 				g.strs_to_free0 = []
 				// println('pos=$node.pos.pos')
 			}
-			if g.is_shared {
+			if g.is_shared && !ret_type.has_flag(.shared_f) {
 				g.writeln('}, sizeof($shared_styp))')
 			}
 			// if g.autofree && node.autofree_pregen != '' { // g.strs_to_free0.len != 0 {

vlib/v/parser/fn.v

@@ -207,6 +207,9 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 				p.warn_with_pos('use `(mut f Foo)` instead of `(f mut Foo)`', lpar_pos.extend(p.peek_tok2.position()))
 			}
 		}
+		if p.tok.kind == .key_shared {
+			p.error_with_pos('use `(shared f Foo)` instead of `(f shared Foo)`', lpar_pos.extend(p.peek_tok2.position()))
+		}
 		receiver_pos = rec_start_pos.extend(p.tok.position())
 		is_amp := p.tok.kind == .amp
 		if p.tok.kind == .name && p.tok.lit == 'JS' {
@@ -685,6 +688,9 @@ fn (mut p Parser) fn_args() ([]table.Param, bool, bool) {
 				}
 				is_mut = true
 			}
+			if p.tok.kind == .key_shared {
+				p.error_with_pos('use `shared f Foo` instead of `f shared Foo`', p.tok.position())
+			}
 			if p.tok.kind == .ellipsis {
 				p.next()
 				is_variadic = true

vlib/v/tests/fn_shared_return_test.v

@@ -0,0 +1,54 @@
+struct St {
+mut:
+	x f64
+}
+
+fn f() shared St {
+	shared x := St{ x: 3.25 }
+	return x
+}
+
+fn g(good bool) ?shared St {
+	if !good {
+		return error('no shared St created')
+	}
+	shared x := St{ x: 12.75 }
+	return x
+}
+
+fn test_shared_fn_return() {
+	shared x := f()
+	val := rlock x { x.x }
+	assert val == 3.25
+}
+
+fn shared_opt_propagate(good bool) ?f64 {
+	shared x := g(good) ?
+	ret := rlock x { x.x }
+	return ret
+}
+
+fn test_shared_opt_propagate() {
+	x := shared_opt_propagate(true) or { 1.25 }
+	y := shared_opt_propagate(false) or { 2.125 }
+	assert x == 12.75
+	assert y == 2.125
+}
+
+fn test_shared_opt_good() {
+	shared yy := g(true) or {
+		shared my_default := St{ x: 37.5 }
+	    my_default
+	}
+	val := rlock yy { yy.x }
+	assert val == 12.75
+}
+
+fn test_shared_opt_bad() {
+	shared yy := g(false) or {
+		shared my_default := St{ x: 37.5 }
+	    my_default
+	}
+	val := rlock yy { yy.x }
+	assert val == 37.5
+}

vlib/v/token/token.v

@@ -427,7 +427,7 @@ pub fn (tok Kind) is_relational() bool {
 }
 
 pub fn (k Kind) is_start_of_type() bool {
-	return k in [.name, .lpar, .amp, .lsbr, .question]
+	return k in [.name, .lpar, .amp, .lsbr, .question, .key_shared]
 }
 
 pub fn (kind Kind) is_prefix() bool {