cgen: fix blank identifier in IfGard (`if _ := <-ch { ... }`, etc) (#9650)

vlib/v/gen/c/cgen.v

@@ -4576,10 +4576,15 @@ fn (mut g Gen) if_expr(node ast.IfExpr) {
 						g.expr(branch.cond.expr)
 						g.writeln(', ${var_name}.state == 0) {')
 					}
-					if branch.cond.var_name != '_' {
+					if short_opt || branch.cond.var_name != '_' {
 						base_type := g.base_type(branch.cond.expr_type)
 						if short_opt {
-							g.write('\t$base_type $branch.cond.var_name = ')
+							cond_var_name := if branch.cond.var_name == '_' {
+								'_dummy_${g.tmp_count + 1}'
+							} else {
+								branch.cond.var_name
+							}
+							g.write('\t$base_type $cond_var_name = ')
 							g.expr(branch.cond.expr)
 							g.writeln(';')
 						} else {

vlib/v/tests/if_guard_test.v

@@ -17,6 +17,14 @@ fn test_fn_return() {
 	assert res == [31.0, 7.5]
 }
 
+fn test_fn_return_empty() {
+	if _ := f(-3) {
+		assert false
+	} else {
+		assert true
+	}
+}
+
 fn test_map_get() {
 	mut m := map{
 		'xy': 5
@@ -33,6 +41,18 @@ fn test_map_get() {
 	assert res == [-17, 7]
 }
 
+fn test_map_get_empty() {
+	mut m := map{
+		'xy': 5
+		'zu': 7
+	}
+	if _ := m['jk'] {
+		assert false
+	} else {
+		assert true
+	}
+}
+
 fn test_array_get() {
 	mut a := [12.5, 6.5, -17.25]
 	mut res := []f64{cap: 2}
@@ -46,6 +66,15 @@ fn test_array_get() {
 	assert res == [6.5, -23.0]
 }
 
+fn test_array_get_empty() {
+	mut a := [12.5, 6.5, -17.25]
+	if _ := a[7] {
+		assert false
+	} else {
+		assert true
+	}
+}
+
 fn test_chan_pop() {
 	mut res := []f64{cap: 3}
 	ch := chan f64{cap: 10}
@@ -62,6 +91,20 @@ fn test_chan_pop() {
 	assert res == [6.75, -3.25, -37.5]
 }
 
+fn test_chan_pop_empty() {
+	ch := chan f64{cap: 10}
+	ch <- 6.75
+	ch <- -3.25
+	ch.close()
+	for i in 0 .. 3 {
+		if _ := <-ch {
+			assert i < 2
+		} else {
+			assert i == 2
+		}
+	}
+}
+
 struct Thing {
 	name string
 }