autofree: fix a bug with $if and add a test

vlib/v/gen/cgen.v

@@ -782,9 +782,25 @@ fn (mut g Gen) stmts_with_tmp_var(stmts []ast.Stmt, tmp_var string) {
 			// go back 1 position is important so we dont get the
 			// internal scope of for loops and possibly other nodes
 			// g.autofree_scope_vars(stmt.position().pos - 1)
-			stmt_pos := stmt.position()
-			g.writeln('// af scope_vars')
-			g.autofree_scope_vars(stmt_pos.pos - 1, stmt_pos.line_nr, false)
+			mut stmt_pos := stmt.position()
+			if stmt_pos.pos == 0 {
+				// Do not autofree if the position is 0, since the correct scope won't be found.
+				// Report a bug, since position shouldn't be 0 for most nodes.
+				if stmt is ast.Module {
+					return
+				}
+				if stmt is ast.ExprStmt {
+					// For some reason ExprStmt.pos is 0 when ExprStmt.expr is comp if expr
+					// Extract the pos. TODO figure out why and fix.
+					stmt_pos = stmt.expr.position()
+				}
+				if stmt_pos.pos == 0 {
+					print('autofree: first stmt pos = 0. ')
+					println(typeof(stmt))
+					return
+				}
+			}
+			g.autofree_scope_vars(stmt_pos.pos, stmt_pos.line_nr, false)
 		}
 	}
 }

vlib/v/parser/if_match.v

@@ -18,7 +18,8 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 	pos := if is_comptime {
 		p.inside_ct_if_expr = true
 		p.next() // `$`
-		p.prev_tok.position().extend(p.tok.position())
+		// p.prev_tok.position().extend(p.tok.position())
+		p.tok.position()
 	} else {
 		p.tok.position()
 	}

vlib/v/tests/valgrind/1.strings_and_arrays.v

@@ -278,6 +278,15 @@ fn string_array_get() {
 	println(s)
 }
 
+fn comp_if() {
+	// compif pos used to be 0, if it was the first statement in a block, vars wouldn't be freed
+	$if macos {
+		println('macos')
+	}
+	s := 'a' + 'b'
+	println(s)
+}
+
 fn main() {
 	println('start')
 	simple()
@@ -297,6 +306,7 @@ fn main() {
 	q := if_expr()
 	s := return_if_expr()
 	free_inside_opt_block()
+	comp_if()
 	// free_before_return()
 	// free_before_return_bool()
 	// free_before_break()