diff --git a/compiler/can/src/def.rs b/compiler/can/src/def.rs
index 374d882f18..1634ade5a0 100644
--- a/compiler/can/src/def.rs
+++ b/compiler/can/src/def.rs
@@ -1326,7 +1326,14 @@ fn to_pending_def<'a>(
                     }
                 }
 
-                Err(err) => panic!("TODO gracefully handle shadowing of type alias {:?}", err),
+                Err((original_region, shadow)) => {
+                    env.problem(Problem::ShadowingInAnnotation {
+                        original_region,
+                        shadow,
+                    });
+
+                    panic!("TODO gracefully handle shadowing of type alias {:?}", err)
+                }
             }
         }
 
diff --git a/compiler/reporting/src/report.rs b/compiler/reporting/src/report.rs
index 7f38c7edf3..69156c3a1e 100644
--- a/compiler/reporting/src/report.rs
+++ b/compiler/reporting/src/report.rs
@@ -1,12 +1,14 @@
 use crate::report::ReportText::{BinOp, Concat, Module, Region, Value};
 use roc_module::symbol::{Interns, ModuleId, Symbol};
 use roc_problem::can::PrecedenceProblem::BothNonAssociative;
-use roc_problem::can::Problem;
+use roc_problem::can::{Problem, RuntimeError};
 use roc_types::pretty_print::content_to_string;
 use roc_types::subs::{Content, Subs};
 use roc_types::types::{write_error_type, ErrorType};
 use std::path::PathBuf;
 
+use roc_module::ident::Ident;
+use roc_region::all::Located;
 use std::fmt;
 use ven_pretty::{BoxAllocator, DocAllocator, DocBuilder, Render, RenderAnnotated};
 
@@ -110,14 +112,21 @@ pub fn can_problem(filename: PathBuf, problem: Problem) -> Report {
             original_region,
             shadow,
         } => {
-            // v-- just to satisfy clippy
-            let _a = original_region;
-            let _b = shadow;
-            panic!("TODO implement shadow report");
-        }
-        Problem::RuntimeError(_runtime_error) => {
-            panic!("TODO implement run time error report");
+            shadowing_report(&mut texts, original_region, shadow);
         }
+        Problem::RuntimeError(runtime_error) => match runtime_error {
+            RuntimeError::Shadowing {
+                original_region,
+                shadow,
+            } => {
+                shadowing_report(&mut texts, original_region, shadow);
+            }
+
+            _ => {
+                dbg!(runtime_error);
+                panic!("TODO implement run time error reporting");
+            }
+        },
     };
 
     Report {
@@ -126,11 +135,27 @@ pub fn can_problem(filename: PathBuf, problem: Problem) -> Report {
     }
 }
 
+fn shadowing_report(
+    texts: &mut Vec<ReportText>,
+    original_region: roc_region::all::Region,
+    shadow: Located<Ident>,
+) {
+    texts.push(name(shadow.value));
+    texts.push(plain_text(" is first defined here:"));
+    texts.push(Region(original_region));
+    texts.push(plain_text("But then it's defined a second time here:"));
+    texts.push(Region(shadow.region));
+    texts.push(plain_text("Since these variables have the same name, it's easy to use the wrong one on accident. Give one of them a new name."));
+}
+
 #[derive(Debug, Clone)]
 pub enum ReportText {
     /// A value. Render it qualified unless it was defined in the current module.
     Value(Symbol),
 
+    /// An identifier, should probably be rendered the same way as a symbol.
+    Name(Ident),
+
     /// A module,
     Module(ModuleId),
 
@@ -181,6 +206,10 @@ pub fn plain_text(str: &str) -> ReportText {
     ReportText::Plain(Box::from(str))
 }
 
+pub fn name(ident: Ident) -> ReportText {
+    ReportText::Name(ident)
+}
+
 pub fn em_text(str: &str) -> ReportText {
     ReportText::EmText(Box::from(str))
 }
@@ -658,6 +687,9 @@ impl ReportText {
                     .append(alloc.line())
                     .append(alloc.line())
             }
+            Name(ident) => alloc
+                .text(format!("{}", ident.as_inline_str()))
+                .annotate(Annotation::Symbol),
         }
     }
 }
diff --git a/compiler/reporting/tests/test_reporting.rs b/compiler/reporting/tests/test_reporting.rs
index ba785aa362..d8147653c2 100644
--- a/compiler/reporting/tests/test_reporting.rs
+++ b/compiler/reporting/tests/test_reporting.rs
@@ -305,22 +305,67 @@ mod test_reporting {
         )
     }
 
-    // #[test]
-    // fn report_shadow() {
-    //     report_problem_as(
-    //         indoc!(
-    //             r#"
-    //            i = 1
-    //
-    //            s = \i ->
-    //                i + 1
-    //
-    //            s i
-    //        "#
-    //         ),
-    //         indoc!(r#"     "#),
-    //     )
-    // }
+    #[test]
+    fn report_shadowing() {
+        report_problem_as(
+            indoc!(
+                r#"
+               i = 1
+
+               s = \i ->
+                   i + 1
+
+               s i
+           "#
+            ),
+            indoc!(
+                r#"
+                i is first defined here:
+
+                1 ┆  i = 1
+                  ┆  ^
+
+                But then it's defined a second time here:
+
+                3 ┆  s = \i ->
+                  ┆       ^
+
+                Since these variables have the same name, it's easy to use the wrong one on accident. Give one of them a new name."#
+            ),
+        )
+    }
+
+    #[test]
+    fn report_shadowing_in_annotation() {
+        report_problem_as(
+            indoc!(
+                r#"
+                Booly : [ Yes, No ]
+
+                Booly : [ Yes, No, Maybe ]
+
+                x =
+                    No
+
+                x
+           "#
+            ),
+            indoc!(
+                r#"
+                Booly is first defined here:
+
+                1 ┆  Booly : [ Yes, No ]
+                  ┆  ^^^^^
+
+                But then it's defined a second time here:
+
+                3 ┆ Booly : [ Yes, No, Maybe ]
+                  ┆ ^^^^^
+
+                Since these variables have the same name, it's easy to use the wrong one on accident. Give one of them a new name."#
+            ),
+        )
+    }
 
     // #[test]
     // fn report_unsupported_top_level_def() {