cgen: fix comptime if in struct field default (fix #15058) (#15152)

vlib/v/gen/c/comptime.v

@@ -234,6 +234,7 @@ fn (mut g Gen) comptime_if(node ast.IfExpr) {
 			}
 		}
 	}
+	tmp_var := g.new_tmp_var()
 	line := if node.is_expr {
 		stmt_str := g.go_before_stmt(0)
 		g.write(util.tabs(g.indent))
@@ -265,21 +266,23 @@ fn (mut g Gen) comptime_if(node ast.IfExpr) {
 			len := branch.stmts.len
 			if len > 0 {
 				last := branch.stmts.last() as ast.ExprStmt
+				styp := g.typ(node.typ)
 				if len > 1 {
-					tmp := g.new_tmp_var()
-					styp := g.typ(last.typ)
 					g.indent++
-					g.writeln('$styp $tmp;')
+					g.writeln('$styp $tmp_var;')
 					g.writeln('{')
 					g.stmts(branch.stmts[..len - 1])
-					g.write('\t$tmp = ')
+					g.write('\t$tmp_var = ')
 					g.stmt(last)
+					g.writeln(';')
 					g.writeln('}')
 					g.indent--
-					g.writeln('$line $tmp;')
 				} else {
-					g.write('$line ')
+					g.indent++
+					g.write('$styp $tmp_var = ')
 					g.stmt(last)
+					g.writeln(';')
+					g.indent--
 				}
 			}
 		} else {
@@ -298,6 +301,9 @@ fn (mut g Gen) comptime_if(node ast.IfExpr) {
 		g.defer_ifdef = ''
 	}
 	g.writeln('#endif')
+	if node.is_expr {
+		g.write('$line $tmp_var')
+	}
 }
 
 // returns the value of the bool comptime expression

vlib/v/tests/comptime_if_expr_in_struct_field_default_test.v

@@ -0,0 +1,14 @@
+struct Foo {
+	text string = $if linux {
+		'linux'
+	} $else {
+		println('else')
+		'else'
+	}
+}
+
+fn test_comptime_if_expr_in_struct_field_default() {
+	f := Foo{}
+	println(f)
+	assert f.text.len > 0
+}