cgen: fix sumtype with fntype using fn directly (fix #15674) (#15679)

vlib/v/gen/c/cgen.v

@@ -2098,16 +2098,22 @@ fn (mut g Gen) write_sumtype_casting_fn(fun SumtypeCastingFn) {
 	got_sym, exp_sym := g.table.sym(got), g.table.sym(exp)
 	mut got_cname, exp_cname := got_sym.cname, exp_sym.cname
 	mut type_idx := g.type_sidx(got)
+	mut sb := strings.new_builder(128)
+	mut is_anon_fn := false
 	if got_sym.info is ast.FnType {
-		if got_sym.info.is_anon {
+		if got_sym.info.is_anon || g.table.known_fn(got_sym.name) {
 			got_name := 'fn ${g.table.fn_type_source_signature(got_sym.info.func)}'
 			got_cname = 'anon_fn_${g.table.fn_type_signature(got_sym.info.func)}'
 			type_idx = g.table.type_idxs[got_name].str()
+			sb.writeln('static inline $exp_cname ${fun.fn_name}($got_cname x) {')
+			sb.writeln('\t$got_cname ptr = x;')
+			is_anon_fn = true
 		}
 	}
-	mut sb := strings.new_builder(128)
+	if !is_anon_fn {
 		sb.writeln('static inline $exp_cname ${fun.fn_name}($got_cname* x) {')
 		sb.writeln('\t$got_cname* ptr = memdup(x, sizeof($got_cname));')
+	}
 	for embed_hierarchy in g.table.get_embeds(got_sym) {
 		// last embed in the hierarchy
 		mut embed_cname := ''
@@ -2149,14 +2155,14 @@ fn (mut g Gen) write_sumtype_casting_fn(fun SumtypeCastingFn) {
 	g.auto_fn_definitions << sb.str()
 }
 
-fn (mut g Gen) call_cfn_for_casting_expr(fname string, expr ast.Expr, exp_is_ptr bool, exp_styp string, got_is_ptr bool, got_styp string) {
+fn (mut g Gen) call_cfn_for_casting_expr(fname string, expr ast.Expr, exp_is_ptr bool, exp_styp string, got_is_ptr bool, got_is_fn bool, got_styp string) {
 	mut rparen_n := 1
 	if exp_is_ptr {
 		g.write('HEAP($exp_styp, ')
 		rparen_n++
 	}
 	g.write('${fname}(')
-	if !got_is_ptr {
+	if !got_is_ptr && !got_is_fn {
 		if !expr.is_lvalue()
 			|| (expr is ast.Ident && (expr as ast.Ident).obj.is_simple_define_const()) {
 			// Note: the `_to_sumtype_` family of functions do call memdup internally, making
@@ -2207,7 +2213,7 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 				fname = g.generic_fn_name(exp_sym.info.concrete_types, fname)
 			}
 			g.call_cfn_for_casting_expr(fname, expr, expected_is_ptr, exp_styp, true,
-				got_styp)
+				false, got_styp)
 			g.inside_cast_in_heap--
 		} else {
 			got_styp := g.cc_type(got_type, true)
@@ -2227,16 +2233,18 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 				fname = g.generic_fn_name(exp_sym.info.concrete_types, fname)
 			}
 			g.call_cfn_for_casting_expr(fname, expr, expected_is_ptr, exp_styp, got_is_ptr,
-				got_styp)
+				false, got_styp)
 		}
 		return
 	}
 	// cast to sum type
 	exp_styp := g.typ(expected_type)
 	mut got_styp := g.typ(got_type)
+	mut got_is_fn := false
 	if got_sym.info is ast.FnType {
 		if got_sym.info.is_anon {
 			got_styp = 'anon_fn_${g.table.fn_type_signature(got_sym.info.func)}'
+			got_is_fn = true
 		}
 	}
 	if expected_type != ast.void_type {
@@ -2276,7 +2284,7 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 				}
 				fname := g.get_sumtype_casting_fn(unwrapped_got_type, unwrapped_expected_type)
 				g.call_cfn_for_casting_expr(fname, expr, expected_is_ptr, unwrapped_exp_sym.cname,
-					got_is_ptr, got_styp)
+					got_is_ptr, got_is_fn, got_styp)
 			}
 			return
 		}
@@ -5074,7 +5082,13 @@ fn (mut g Gen) write_types(symbols []&ast.TypeSymbol) {
 				g.type_definitions.writeln('\tunion {')
 				for variant in sym.info.variants {
 					variant_sym := g.table.sym(variant)
-					g.type_definitions.writeln('\t\t${g.typ(variant.ref())} _$variant_sym.cname;')
+					mut var := variant.ref()
+					if variant_sym.info is ast.FnType {
+						if variant_sym.info.is_anon {
+							var = variant
+						}
+					}
+					g.type_definitions.writeln('\t\t${g.typ(var)} _$variant_sym.cname;')
 				}
 				g.type_definitions.writeln('\t};')
 				g.type_definitions.writeln('\tint _typ;')
@@ -5608,7 +5622,11 @@ fn (mut g Gen) as_cast(node ast.AsCast) {
 	mut expr_type_sym := g.table.sym(g.unwrap_generic(node.expr_type))
 	if mut expr_type_sym.info is ast.SumType {
 		dot := if node.expr_type.is_ptr() { '->' } else { '.' }
+		if sym.info is ast.FnType {
+			g.write('/* as */ ($styp)__as_cast(')
+		} else {
 			g.write('/* as */ *($styp*)__as_cast(')
+		}
 		g.write('(')
 		g.expr(node.expr)
 		g.write(')')

vlib/v/tests/sumtype_with_fntype_test.v

@@ -0,0 +1,32 @@
+type Expr = fn () int | fn (int) int
+
+fn id(n int) int {
+	return n
+}
+
+fn func() Expr {
+	return id
+}
+
+fn test_sumtype_with_fntype() {
+	f := func()
+
+	f1 := f as fn (int) int
+	println(123)
+	assert f1(123) == 123
+
+	if f is fn (int) int {
+		ret := f(123)
+		println(ret)
+		assert ret == 123
+	}
+
+	match f {
+		fn (int) int {
+			ret := f(321)
+			println(ret)
+			assert ret == 321
+		}
+		else {}
+	}
+}