diff --git a/mers/src/main.rs b/mers/src/main.rs
index ede4906..69853b8 100755
--- a/mers/src/main.rs
+++ b/mers/src/main.rs
@@ -125,7 +125,8 @@ fn main() {
                     exit(20);
                 }
                 Ok(parsed) => {
-                    let (i1, _, i3) = config.infos();
+                    let (i1, _, mut i3) = config.infos();
+                    i3.global.show_warnings_to_stderr();
                     match compile(&*parsed, i1) {
                         Err(e) => {
                             eprintln!("{e:?}");
diff --git a/mers_lib/src/errors/mod.rs b/mers_lib/src/errors/mod.rs
index 61eb64c..0ff07e0 100644
--- a/mers_lib/src/errors/mod.rs
+++ b/mers_lib/src/errors/mod.rs
@@ -97,6 +97,8 @@ pub enum EColor {
     MaximumRuntimeExceeded,
 
     InCodePositionLine,
+
+    Warning,
 }
 
 pub trait ETheme: ThemeGen<C = EColor, T = String> {}
diff --git a/mers_lib/src/errors/themes.rs b/mers_lib/src/errors/themes.rs
index 3b4f20d..8e7ded5 100644
--- a/mers_lib/src/errors/themes.rs
+++ b/mers_lib/src/errors/themes.rs
@@ -42,6 +42,10 @@ pub fn default_theme<C>(
             _ => return None,
         });
     }
+    if let Warning = color {
+        // same color as runtime errors, because warnings are only displayed in `check` mode, where the program won't run.
+        return Some(yellow);
+    }
     let hard_err = red;
     let type_right = blue;
     let type_wrong = magenta;
@@ -56,7 +60,7 @@ pub fn default_theme<C>(
     drop(cyan_bright);
     use EColor::*;
     Some(match color {
-        Indent(_) => unreachable!(),
+        Indent(_) | Warning => unreachable!(),
 
         // macros (#...)
         WhitespaceAfterHashtag
diff --git a/mers_lib/src/program/run/if.rs b/mers_lib/src/program/run/if.rs
index 33ebf67..d2ead0a 100755
--- a/mers_lib/src/program/run/if.rs
+++ b/mers_lib/src/program/run/if.rs
@@ -1,7 +1,7 @@
 use std::sync::Arc;
 
 use crate::{
-    data::{self, tuple::TupleT, Data, MersType, Type},
+    data::{self, tuple::TupleT, Data, Type},
     errors::{CheckError, EColor, SourceRange},
 };
 
@@ -47,18 +47,44 @@ impl MersStatement for If {
                     ),
                 ]));
         }
-        let mut t = if Type::new(data::bool::TrueT).is_included_in(&cond_return_type) {
+        let may_be_true = Type::new(data::bool::TrueT).is_included_in(&cond_return_type);
+        let may_be_false = Type::new(data::bool::FalseT).is_included_in(&cond_return_type);
+        let mut t = if may_be_true {
             self.on_true.check(info, None)?
         } else {
             Type::empty()
         };
-        if Type::new(data::bool::FalseT).is_included_in(&cond_return_type) {
+        if may_be_false {
             if let Some(f) = &self.on_false {
                 t.add_all(&f.check(info, None)?);
             } else {
                 t.add(Arc::new(TupleT(vec![])));
             }
         }
+        if let Some(show_warning) = &info.global.show_warnings {
+            if !may_be_false || !may_be_true {
+                let mut e = CheckError::new().src(vec![
+                    (self.pos_in_src.clone(), None),
+                    (
+                        self.condition.source_range(),
+                        Some(EColor::IfConditionNotBool),
+                    ),
+                ]);
+                if !may_be_true {
+                    e.msg_mut(vec![(
+                        "Condition in this if-statement is never true".to_owned(),
+                        Some(EColor::Warning),
+                    )]);
+                }
+                if !may_be_false {
+                    e.msg_mut(vec![(
+                        "Condition in this if-statement is never false".to_owned(),
+                        Some(EColor::Warning),
+                    )]);
+                }
+                show_warning(e);
+            }
+        }
         Ok(t)
     }
     fn run_custom(&self, info: &mut super::Info) -> Result<Data, CheckError> {
diff --git a/mers_lib/src/program/run/mod.rs b/mers_lib/src/program/run/mod.rs
index 3718091..91e6ceb 100755
--- a/mers_lib/src/program/run/mod.rs
+++ b/mers_lib/src/program/run/mod.rs
@@ -142,10 +142,11 @@ pub struct CheckLocal {
         >,
     >,
 }
-#[derive(Clone, Debug, Default)]
+#[derive(Clone, Default)]
 pub struct CheckLocalGlobalInfo {
     pub depth: usize,
     pub enable_hooks: bool,
+    pub show_warnings: Option<Arc<dyn Fn(CheckError) + Send + Sync>>,
     /// ((results, byte_pos_in_src, deepest_statement))
     /// you only have to set `byte_pos_in_src`. `deepest` is used internally.
     /// These values should be initialized to `(vec![], _, 0)`, but `0` can be replaced by a minimum statement depth, i.e. `2` to exclude the outer scope (which has depth `1`).
@@ -160,6 +161,22 @@ pub struct CheckLocalGlobalInfo {
     >,
     pub unused_try_statements: Arc<Mutex<Vec<(SourceRange, Vec<Option<SourceRange>>)>>>,
 }
+impl CheckLocalGlobalInfo {
+    pub fn show_warnings_to_stderr(&mut self) {
+        self.show_warnings = Some(Arc::new(|e| {
+            #[cfg(feature = "ecolor-term")]
+            let theme = crate::errors::themes::TermDefaultTheme;
+            #[cfg(not(feature = "ecolor-term"))]
+            let theme = crate::errors::themes::NoTheme;
+            eprintln!("{}", e.display(theme));
+        }));
+    }
+}
+impl Debug for CheckLocalGlobalInfo {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "CheckLocalGlobalInfo {{ depth: {}, enable_hooks: {}, show_warnings: {}, unused_try_statements: {} }}", self.depth, self.enable_hooks, self.show_warnings.is_some(), self.unused_try_statements.lock().unwrap().len())
+    }
+}
 impl Debug for CheckLocal {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "CheckLocal {:?}, {:?}", self.vars, self.types.keys())