From 299d6ef308ac5642b1dfc07247ad69b1a547fd6e Mon Sep 17 00:00:00 2001
From: Stephen Chung <schungx@live.com>
Date: Tue, 19 Apr 2022 21:45:11 +0800
Subject: [PATCH] Type checking in switch case condition.

---
 CHANGELOG.md    | 1 +
 src/ast/stmt.rs | 4 +++-
 src/parser.rs   | 7 ++++++-
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 81b7cd2b..e5edd37d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,7 @@ Enhancements
 ------------
 
 * `Module::eval_ast_as_new_raw` is made public as a low-level API.
+* Improper `switch` case condition syntax is now caught at parse time.
 
 
 Version 1.6.1
diff --git a/src/ast/stmt.rs b/src/ast/stmt.rs
index 3b73a91a..788ff368 100644
--- a/src/ast/stmt.rs
+++ b/src/ast/stmt.rs
@@ -122,7 +122,9 @@ impl fmt::Debug for OpAssignment<'_> {
     }
 }
 
-/// A statements block with an optional condition.
+/// A statements block with a condition.
+///
+/// The condition may simply be [`Expr::BoolConstant`] with `true` if there is actually no condition.
 #[derive(Debug, Clone, Hash)]
 pub struct ConditionalStmtBlock {
     /// Condition.
diff --git a/src/parser.rs b/src/parser.rs
index 33c315f8..6abb4586 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -1054,7 +1054,12 @@ impl Engine {
                         Some(self.parse_expr(input, state, lib, settings.level_up())?);
 
                     let condition = if match_token(input, Token::If).0 {
-                        self.parse_expr(input, state, lib, settings.level_up())?
+                        ensure_not_statement_expr(input, "a boolean")?;
+                        let guard = self
+                            .parse_expr(input, state, lib, settings.level_up())?
+                            .ensure_bool_expr()?;
+                        ensure_not_assignment(input)?;
+                        guard
                     } else {
                         Expr::BoolConstant(true, Position::NONE)
                     };