diff --git a/crates/compiler/can/src/copy.rs b/crates/compiler/can/src/copy.rs
index dd17d65c66..956ea867a0 100644
--- a/crates/compiler/can/src/copy.rs
+++ b/crates/compiler/can/src/copy.rs
@@ -1,6 +1,6 @@
 use crate::{
     def::Def,
-    expr::{AccessorData, ClosureData, Expr, Field, OpaqueWrapFunctionData},
+    expr::{AccessorData, ClosureData, Expr, Field, OpaqueWrapFunctionData, WhenBranchPattern},
     pattern::{DestructType, Pattern, RecordDestruct},
 };
 use roc_module::{
@@ -295,7 +295,16 @@ fn deep_copy_expr_help<C: CopyEnv>(env: &mut C, copied: &mut Vec<Variable>, expr
                      }| crate::expr::WhenBranch {
                         patterns: patterns
                             .iter()
-                            .map(|lp| lp.map(|p| deep_copy_pattern_help(env, copied, p)))
+                            .map(
+                                |WhenBranchPattern {
+                                     pattern,
+                                     degenerate,
+                                 }| WhenBranchPattern {
+                                    pattern: pattern
+                                        .map(|p| deep_copy_pattern_help(env, copied, p)),
+                                    degenerate: *degenerate,
+                                },
+                            )
                             .collect(),
                         value: value.map(|e| go_help!(e)),
                         guard: guard.as_ref().map(|le| le.map(|e| go_help!(e))),
diff --git a/crates/compiler/can/src/effect_module.rs b/crates/compiler/can/src/effect_module.rs
index e70989c6a3..f19eb35ef3 100644
--- a/crates/compiler/can/src/effect_module.rs
+++ b/crates/compiler/can/src/effect_module.rs
@@ -1,6 +1,6 @@
 use crate::annotation::IntroducedVariables;
 use crate::def::Def;
-use crate::expr::{AnnotatedMark, ClosureData, Declarations, Expr, Recursive};
+use crate::expr::{AnnotatedMark, ClosureData, Declarations, Expr, Recursive, WhenBranchPattern};
 use crate::pattern::Pattern;
 use crate::scope::Scope;
 use roc_collections::{SendMap, VecSet};
@@ -475,11 +475,15 @@ fn build_effect_after(
             type_arguments,
             lambda_set_variables,
         };
+        let pattern = WhenBranchPattern {
+            pattern: Loc::at_zero(pattern),
+            degenerate: false,
+        };
 
         let branches = vec![crate::expr::WhenBranch {
             guard: None,
             value: Loc::at_zero(force_inner_thunk_call),
-            patterns: vec![Loc::at_zero(pattern)],
+            patterns: vec![pattern],
             redundant: RedundantMark::new(var_store),
         }];
 
@@ -1256,9 +1260,13 @@ fn build_effect_loop_inner_body(
         let step_tag_name = TagName("Step".into());
 
         let step_pattern = applied_tag_pattern(step_tag_name, &[new_state_symbol], var_store);
+        let step_pattern = WhenBranchPattern {
+            pattern: Loc::at_zero(step_pattern),
+            degenerate: false,
+        };
 
         crate::expr::WhenBranch {
-            patterns: vec![Loc::at_zero(step_pattern)],
+            patterns: vec![step_pattern],
             value: Loc::at_zero(force_thunk2),
             guard: None,
             redundant: RedundantMark::new(var_store),
@@ -1268,9 +1276,13 @@ fn build_effect_loop_inner_body(
     let done_branch = {
         let done_tag_name = TagName("Done".into());
         let done_pattern = applied_tag_pattern(done_tag_name, &[done_symbol], var_store);
+        let done_pattern = WhenBranchPattern {
+            pattern: Loc::at_zero(done_pattern),
+            degenerate: false,
+        };
 
         crate::expr::WhenBranch {
-            patterns: vec![Loc::at_zero(done_pattern)],
+            patterns: vec![done_pattern],
             value: Loc::at_zero(Expr::Var(done_symbol)),
             guard: None,
             redundant: RedundantMark::new(var_store),
diff --git a/crates/compiler/can/src/exhaustive.rs b/crates/compiler/can/src/exhaustive.rs
index 31a7665596..41c06c85f8 100644
--- a/crates/compiler/can/src/exhaustive.rs
+++ b/crates/compiler/can/src/exhaustive.rs
@@ -260,16 +260,19 @@ pub fn sketch_when_branches(
                     // NB: ordering the guard pattern first seems to be better at catching
                     // non-exhaustive constructors in the second argument; see the paper to see if
                     // there is a way to improve this in general.
-                    vec![guard_pattern, sketch_pattern(target_var, &loc_pat.value)],
+                    vec![
+                        guard_pattern,
+                        sketch_pattern(target_var, &loc_pat.pattern.value),
+                    ],
                 )]
             } else {
                 // Simple case
-                vec![sketch_pattern(target_var, &loc_pat.value)]
+                vec![sketch_pattern(target_var, &loc_pat.pattern.value)]
             };
 
             let row = SketchedRow {
                 patterns,
-                region: loc_pat.region,
+                region: loc_pat.pattern.region,
                 guard,
                 redundant_mark: *redundant,
             };
diff --git a/crates/compiler/can/src/expr.rs b/crates/compiler/can/src/expr.rs
index 464ad0cf27..5d3ac7f037 100644
--- a/crates/compiler/can/src/expr.rs
+++ b/crates/compiler/can/src/expr.rs
@@ -481,9 +481,18 @@ impl Recursive {
     }
 }
 
+#[derive(Clone, Debug)]
+pub struct WhenBranchPattern {
+    pub pattern: Loc<Pattern>,
+    /// Degenerate branch patterns are those that don't fully bind symbols that the branch body
+    /// needs. For example, in `A x | B y -> x`, the `B y` pattern is degenerate.
+    /// Degenerate patterns emit a runtime error if reached in a program.
+    pub degenerate: bool,
+}
+
 #[derive(Clone, Debug)]
 pub struct WhenBranch {
-    pub patterns: Vec<Loc<Pattern>>,
+    pub patterns: Vec<WhenBranchPattern>,
     pub value: Loc<Expr>,
     pub guard: Option<Loc<Expr>>,
     /// Whether this branch is redundant in the `when` it appears in
@@ -497,11 +506,13 @@ impl WhenBranch {
                 .patterns
                 .first()
                 .expect("when branch has no pattern?")
+                .pattern
                 .region,
             &self
                 .patterns
                 .last()
                 .expect("when branch has no pattern?")
+                .pattern
                 .region,
         )
     }
@@ -512,7 +523,7 @@ impl WhenBranch {
         Region::across_all(
             self.patterns
                 .iter()
-                .map(|p| &p.region)
+                .map(|p| &p.pattern.region)
                 .chain([self.value.region].iter()),
         )
     }
@@ -1350,14 +1361,20 @@ fn canonicalize_when_branch<'a>(
         );
 
         multi_pattern_variables.add_pattern(&can_pattern);
-        patterns.push(can_pattern);
+        patterns.push(WhenBranchPattern {
+            pattern: can_pattern,
+            degenerate: false,
+        });
     }
 
+    let mut some_symbols_not_bound_in_all_patterns = false;
     for (unbound_symbol, region) in multi_pattern_variables.get_unbound() {
         env.problem(Problem::NotBoundInAllPatterns {
             unbound_symbol,
             region,
-        })
+        });
+
+        some_symbols_not_bound_in_all_patterns = true;
     }
 
     let (value, mut branch_output) = canonicalize_expr(
@@ -1384,12 +1401,33 @@ fn canonicalize_when_branch<'a>(
 
     // Now that we've collected all the references for this branch, check to see if
     // any of the new idents it defined were unused. If any were, report it.
-    for (symbol, region) in BindingsFromPattern::new_many(patterns.iter()) {
-        if !output.references.has_value_lookup(symbol) {
+    let mut pattern_bound_symbols_body_needs = VecSet::default();
+    for (symbol, region) in BindingsFromPattern::new_many(patterns.iter().map(|pat| &pat.pattern)) {
+        if output.references.has_value_lookup(symbol) {
+            pattern_bound_symbols_body_needs.insert(symbol);
+        } else {
             env.problem(Problem::UnusedDef(symbol, region));
         }
     }
 
+    if some_symbols_not_bound_in_all_patterns && !pattern_bound_symbols_body_needs.is_empty() {
+        // There might be branches that don't bind all the symbols needed by the body; mark those
+        // branches degenerate.
+        for pattern in patterns.iter_mut() {
+            let bound_by_pattern: VecSet<_> = BindingsFromPattern::new(&pattern.pattern)
+                .map(|(sym, _)| sym)
+                .collect();
+
+            let binds_all_needed = pattern_bound_symbols_body_needs
+                .iter()
+                .all(|sym| bound_by_pattern.contains(sym));
+
+            if !binds_all_needed {
+                pattern.degenerate = true;
+            }
+        }
+    }
+
     (
         WhenBranch {
             patterns,
diff --git a/crates/compiler/can/src/module.rs b/crates/compiler/can/src/module.rs
index d107e9485a..f35f8b9f36 100644
--- a/crates/compiler/can/src/module.rs
+++ b/crates/compiler/can/src/module.rs
@@ -910,7 +910,10 @@ fn fix_values_captured_in_closure_expr(
 
                 // patterns can contain default expressions, so much go over them too!
                 for loc_pat in branch.patterns.iter_mut() {
-                    fix_values_captured_in_closure_pattern(&mut loc_pat.value, no_capture_symbols);
+                    fix_values_captured_in_closure_pattern(
+                        &mut loc_pat.pattern.value,
+                        no_capture_symbols,
+                    );
                 }
 
                 if let Some(guard) = &mut branch.guard {
diff --git a/crates/compiler/can/src/traverse.rs b/crates/compiler/can/src/traverse.rs
index 31ad25f1f2..83ffe7c14f 100644
--- a/crates/compiler/can/src/traverse.rs
+++ b/crates/compiler/can/src/traverse.rs
@@ -323,9 +323,13 @@ pub fn walk_when_branch<V: Visitor>(
         redundant: _,
     } = branch;
 
-    patterns
-        .iter()
-        .for_each(|pat| visitor.visit_pattern(&pat.value, pat.region, pat.value.opt_var()));
+    patterns.iter().for_each(|pat| {
+        visitor.visit_pattern(
+            &pat.pattern.value,
+            pat.pattern.region,
+            pat.pattern.value.opt_var(),
+        )
+    });
     visitor.visit_expr(&value.value, value.region, expr_var);
     if let Some(guard) = guard {
         visitor.visit_expr(&guard.value, guard.region, Variable::BOOL);
diff --git a/crates/compiler/constrain/src/expr.rs b/crates/compiler/constrain/src/expr.rs
index 4bcade1dd2..ad7028ce01 100644
--- a/crates/compiler/constrain/src/expr.rs
+++ b/crates/compiler/constrain/src/expr.rs
@@ -1837,14 +1837,15 @@ fn constrain_when_branch_help(
     };
 
     for (i, loc_pattern) in when_branch.patterns.iter().enumerate() {
-        let pattern_expected = pattern_expected(HumanIndex::zero_based(i), loc_pattern.region);
+        let pattern_expected =
+            pattern_expected(HumanIndex::zero_based(i), loc_pattern.pattern.region);
 
         let mut partial_state = PatternState::default();
         constrain_pattern(
             constraints,
             env,
-            &loc_pattern.value,
-            loc_pattern.region,
+            &loc_pattern.pattern.value,
+            loc_pattern.pattern.region,
             pattern_expected,
             &mut partial_state,
         );
diff --git a/crates/compiler/derive/src/encoding.rs b/crates/compiler/derive/src/encoding.rs
index 461f099ce9..3efcd835e9 100644
--- a/crates/compiler/derive/src/encoding.rs
+++ b/crates/compiler/derive/src/encoding.rs
@@ -3,7 +3,9 @@
 use std::iter::once;
 
 use roc_can::abilities::SpecializationLambdaSets;
-use roc_can::expr::{AnnotatedMark, ClosureData, Expr, Field, Recursive, WhenBranch};
+use roc_can::expr::{
+    AnnotatedMark, ClosureData, Expr, Field, Recursive, WhenBranch, WhenBranchPattern,
+};
 use roc_can::module::ExposedByModule;
 use roc_can::pattern::Pattern;
 use roc_collections::SendMap;
@@ -672,6 +674,10 @@ fn to_encoder_tag_union(
                     .map(|(var, sym)| (*var, Loc::at_zero(Pattern::Identifier(*sym))))
                     .collect(),
             };
+            let branch_pattern = WhenBranchPattern {
+                pattern: Loc::at_zero(pattern),
+                degenerate: false,
+            };
 
             // whole type of the elements in [ Encode.toEncoder v1, Encode.toEncoder v2 ]
             let whole_payload_encoders_var = env.subs.fresh_unnamed_flex_var();
@@ -792,7 +798,7 @@ fn to_encoder_tag_union(
             env.unify(this_encoder_var, whole_tag_encoders_var);
 
             WhenBranch {
-                patterns: vec![Loc::at_zero(pattern)],
+                patterns: vec![branch_pattern],
                 value: Loc::at_zero(encode_tag_call),
                 guard: None,
                 redundant: RedundantMark::known_non_redundant(),
diff --git a/crates/compiler/mono/src/ir.rs b/crates/compiler/mono/src/ir.rs
index c302eda017..084f1fb36f 100644
--- a/crates/compiler/mono/src/ir.rs
+++ b/crates/compiler/mono/src/ir.rs
@@ -20,7 +20,7 @@ use roc_debug_flags::{
 };
 use roc_derive::SharedDerivedModule;
 use roc_error_macros::{internal_error, todo_abilities};
-use roc_exhaustive::{Ctor, CtorName, Guard, RenderAs, TagId};
+use roc_exhaustive::{Ctor, CtorName, RenderAs, TagId};
 use roc_late_solve::{resolve_ability_specialization, AbilitiesView, Resolved, UnificationFailed};
 use roc_module::ident::{ForeignSymbol, Lowercase, TagName};
 use roc_module::low_level::LowLevel;
@@ -2533,7 +2533,7 @@ fn pattern_to_when<'a>(
     body: Loc<roc_can::expr::Expr>,
 ) -> (Symbol, Loc<roc_can::expr::Expr>) {
     use roc_can::expr::Expr::*;
-    use roc_can::expr::WhenBranch;
+    use roc_can::expr::{WhenBranch, WhenBranchPattern};
     use roc_can::pattern::Pattern::*;
 
     match &pattern.value {
@@ -2580,7 +2580,10 @@ fn pattern_to_when<'a>(
                 region: Region::zero(),
                 loc_cond: Box::new(Loc::at_zero(Var(symbol))),
                 branches: vec![WhenBranch {
-                    patterns: vec![pattern],
+                    patterns: vec![WhenBranchPattern {
+                        pattern,
+                        degenerate: false,
+                    }],
                     value: body,
                     guard: None,
                     // If this type-checked, it's non-redundant
@@ -5184,7 +5187,7 @@ pub fn with_hole<'a>(
             }
         }
         TypedHole(_) => Stmt::RuntimeError("Hit a blank"),
-        RuntimeError(e) => Stmt::RuntimeError(env.arena.alloc(format!("{:?}", e))),
+        RuntimeError(e) => Stmt::RuntimeError(env.arena.alloc(e.runtime_message())),
     }
 }
 
@@ -6033,56 +6036,50 @@ fn to_opt_branches<'a>(
 )> {
     debug_assert!(!branches.is_empty());
 
-    let mut loc_branches = std::vec::Vec::new();
     let mut opt_branches = std::vec::Vec::new();
 
     for when_branch in branches {
-        let exhaustive_guard = if when_branch.guard.is_some() {
-            Guard::HasGuard
-        } else {
-            Guard::NoGuard
-        };
-
         if when_branch.redundant.is_redundant(env.subs) {
             // Don't codegen this branch since it's redundant.
             continue;
         }
 
         for loc_pattern in when_branch.patterns {
-            match from_can_pattern(env, procs, layout_cache, &loc_pattern.value) {
+            match from_can_pattern(env, procs, layout_cache, &loc_pattern.pattern.value) {
                 Ok((mono_pattern, assignments)) => {
-                    loc_branches.push((
-                        Loc::at(loc_pattern.region, mono_pattern.clone()),
-                        exhaustive_guard,
-                    ));
+                    let loc_expr = if !loc_pattern.degenerate {
+                        let mut loc_expr = when_branch.value.clone();
 
-                    let mut loc_expr = when_branch.value.clone();
-                    let region = loc_pattern.region;
-                    for (symbol, variable, expr) in assignments.into_iter().rev() {
-                        let def = roc_can::def::Def {
-                            annotation: None,
-                            expr_var: variable,
-                            loc_expr: Loc::at(region, expr),
-                            loc_pattern: Loc::at(
-                                region,
-                                roc_can::pattern::Pattern::Identifier(symbol),
-                            ),
-                            pattern_vars: std::iter::once((symbol, variable)).collect(),
-                        };
-                        let new_expr =
-                            roc_can::expr::Expr::LetNonRec(Box::new(def), Box::new(loc_expr));
-                        loc_expr = Loc::at(region, new_expr);
-                    }
+                        let region = loc_pattern.pattern.region;
+                        for (symbol, variable, expr) in assignments.into_iter().rev() {
+                            let def = roc_can::def::Def {
+                                annotation: None,
+                                expr_var: variable,
+                                loc_expr: Loc::at(region, expr),
+                                loc_pattern: Loc::at(
+                                    region,
+                                    roc_can::pattern::Pattern::Identifier(symbol),
+                                ),
+                                pattern_vars: std::iter::once((symbol, variable)).collect(),
+                            };
+                            let new_expr =
+                                roc_can::expr::Expr::LetNonRec(Box::new(def), Box::new(loc_expr));
+                            loc_expr = Loc::at(region, new_expr);
+                        }
+
+                        loc_expr
+                    } else {
+                        // This pattern is degenerate; when it's reached we must emit a runtime
+                        // error.
+                        Loc::at_zero(roc_can::expr::Expr::RuntimeError(
+                            RuntimeError::DegenerateBranch(loc_pattern.pattern.region),
+                        ))
+                    };
 
                     // TODO remove clone?
                     opt_branches.push((mono_pattern, when_branch.guard.clone(), loc_expr.value));
                 }
                 Err(runtime_error) => {
-                    loc_branches.push((
-                        Loc::at(loc_pattern.region, Pattern::Underscore),
-                        exhaustive_guard,
-                    ));
-
                     // TODO remove clone?
                     opt_branches.push((
                         Pattern::Underscore,
diff --git a/crates/compiler/problem/src/can.rs b/crates/compiler/problem/src/can.rs
index 8f90017cd4..38ffc7788c 100644
--- a/crates/compiler/problem/src/can.rs
+++ b/crates/compiler/problem/src/can.rs
@@ -329,6 +329,24 @@ pub enum RuntimeError {
     EmptySingleQuote(Region),
     /// where 'aa'
     MultipleCharsInSingleQuote(Region),
+
+    DegenerateBranch(Region),
+}
+
+impl RuntimeError {
+    pub fn runtime_message(self) -> String {
+        use RuntimeError::*;
+
+        match self {
+            DegenerateBranch(region) => {
+                format!(
+                    "Hit a branch pattern that does not bind all symbols its body needs, at {:?}",
+                    region
+                )
+            }
+            err => format!("{:?}", err),
+        }
+    }
 }
 
 #[derive(Clone, Copy, Debug, PartialEq)]
diff --git a/crates/compiler/test_derive/src/pretty_print.rs b/crates/compiler/test_derive/src/pretty_print.rs
index e5c99604f6..f83b4f355c 100644
--- a/crates/compiler/test_derive/src/pretty_print.rs
+++ b/crates/compiler/test_derive/src/pretty_print.rs
@@ -221,7 +221,7 @@ fn branch<'a>(c: &Ctx, f: &'a Arena<'a>, b: &'a WhenBranch) -> DocBuilder<'a, Ar
     f.intersperse(
         patterns
             .iter()
-            .map(|lp| pattern(c, PPrec::Free, f, &lp.value)),
+            .map(|lp| pattern(c, PPrec::Free, f, &lp.pattern.value)),
         f.text(" | "),
     )
     .append(match guard {
diff --git a/crates/compiler/test_gen/src/gen_primitives.rs b/crates/compiler/test_gen/src/gen_primitives.rs
index d2c281c576..e531e1ace7 100644
--- a/crates/compiler/test_gen/src/gen_primitives.rs
+++ b/crates/compiler/test_gen/src/gen_primitives.rs
@@ -3651,7 +3651,25 @@ fn shared_pattern_variable_in_when_branches() {
 }
 
 #[test]
-#[ignore = "TODO currently fails in alias analysis because `B y` does not introduce `x`"]
+#[cfg(any(feature = "gen-llvm"))]
+fn symbol_not_bound_in_all_patterns_runs_when_no_bound_symbol_used() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            f = \t -> when t is
+                        A x | B y -> 31u8
+
+            {a: f (A 15u8), b: f (B 15u8)}
+            "#
+        ),
+        31u8,
+        u8,
+        |x| x,
+        true // allow errors
+    );
+}
+
+#[test]
 #[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
 fn symbol_not_bound_in_all_patterns_runs_when_bound_pattern_reached() {
     assert_evals_to!(
@@ -3662,6 +3680,28 @@ fn symbol_not_bound_in_all_patterns_runs_when_bound_pattern_reached() {
             "#
         ),
         15u8,
-        u8
+        u8,
+        |x| x,
+        true // allow errors
+    );
+}
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+#[should_panic(
+    expected = r#"Roc failed with message: "Hit a branch pattern that does not bind all symbols its body needs"#
+)]
+fn runtime_error_when_degenerate_pattern_reached() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            when B 15u8 is
+                A x | B y -> x + 5u8
+            "#
+        ),
+        15u8,
+        u8,
+        |x| x,
+        true // allow errors
     );
 }
diff --git a/crates/compiler/test_gen/src/helpers/llvm.rs b/crates/compiler/test_gen/src/helpers/llvm.rs
index 5ca82a9c09..f1e1e9afb6 100644
--- a/crates/compiler/test_gen/src/helpers/llvm.rs
+++ b/crates/compiler/test_gen/src/helpers/llvm.rs
@@ -272,7 +272,12 @@ fn create_llvm_module<'a>(
     // Uncomment this to see the module's optimized LLVM instruction output:
     // env.module.print_to_stderr();
 
-    (main_fn_name, delayed_errors.join("\n"), env.module)
+    let delayed_errors = if config.ignore_problems {
+        String::new()
+    } else {
+        delayed_errors.join("\n")
+    };
+    (main_fn_name, delayed_errors, env.module)
 }
 
 #[derive(Debug, Clone, Copy)]
diff --git a/crates/reporting/src/error/canonicalize.rs b/crates/reporting/src/error/canonicalize.rs
index 804a0296af..28919156c8 100644
--- a/crates/reporting/src/error/canonicalize.rs
+++ b/crates/reporting/src/error/canonicalize.rs
@@ -1899,6 +1899,14 @@ fn pretty_runtime_error<'b>(
 
             title = OPAQUE_OVER_APPLIED;
         }
+        RuntimeError::DegenerateBranch(region) => {
+            doc = alloc.stack([
+                alloc.reflow("This branch pattern does not bind all symbols its body needs:"),
+                alloc.region(lines.convert_region(region)),
+            ]);
+
+            title = "DEGENERATE BRANCH";
+        }
     }
 
     (doc, title)