implement canonicalization error when signature pattern does not match definition pattern.

compiler/can/src/def.rs

@@ -168,7 +168,15 @@ pub fn canonicalize_defs<'a>(
                                 pattern_type,
                             )
                         } else {
-                            panic!("TODO gracefully handle the case where a type annotation appears immediately before a body def, but the patterns are different. This should be an error; put a newline or comment between them!");
+                            // the pattern of the annotation does not match the pattern of the body directly below it
+                            env.problems.push(Problem::SignatureDefMismatch {
+                                annotation_pattern: pattern.region,
+                                def_pattern: body_pattern.region,
+                            });
+
+                            // both the annotation and definition are skipped!
+                            iter.next();
+                            continue;
                         }
                     }
                     _ => to_pending_def(env, var_store, &loc_def.value, &mut scope, pattern_type),

compiler/problem/src/can.rs

@@ -46,6 +46,10 @@ pub enum Problem {
         replaced_region: Region,
     },
     RuntimeError(RuntimeError),
+    SignatureDefMismatch {
+        annotation_pattern: Region,
+        def_pattern: Region,
+    },
 }
 
 #[derive(Clone, Debug, PartialEq)]

compiler/reporting/src/error/canonicalize.rs

@@ -1,6 +1,7 @@
 use roc_collections::all::MutSet;
 use roc_problem::can::PrecedenceProblem::BothNonAssociative;
 use roc_problem::can::{Problem, RuntimeError};
+use roc_region::all::Region;
 use std::path::PathBuf;
 
 use crate::report::{Annotation, Report, RocDocAllocator, RocDocBuilder};
@@ -238,6 +239,19 @@ pub fn can_problem<'b>(
                 alloc.reflow(" definitions from this tag union type."),
             ]),
         ]),
+        Problem::SignatureDefMismatch {
+            ref annotation_pattern,
+            ref def_pattern,
+        } => alloc.stack(vec![
+            alloc.reflow("This annotation does not match the definition immediately following it:"),
+            alloc.region(Region::span_across(annotation_pattern, def_pattern)),
+            alloc.reflow("Is it a typo? If not, put either a newline or comment between them."),
+            // TODO add link to this guide section
+            //            alloc.hint().append(alloc.reflow(
+            //                "If you want an unused type parameter (a so-called \"phantom type\"), \
+            //                read the guide section on phantom data.",
+            //            )),
+        ]),
         Problem::RuntimeError(runtime_error) => pretty_runtime_error(alloc, runtime_error),
     };
 

compiler/reporting/tests/test_reporting.rs

@@ -2684,6 +2684,34 @@ mod test_reporting {
         )
     }
 
+    #[test]
+    fn annotation_definition_mismatch() {
+        report_problem_as(
+            indoc!(
+                r#"
+                bar : Int
+                foo = \x -> x
+
+                # NOTE: neither bar or foo are defined at this point
+                4
+                "#
+            ),
+            indoc!(
+                r#"
+                -- SYNTAX PROBLEM --------------------------------------------------------------
+
+                This annotation does not match the definition immediately following
+                it:
+
+                1 ┆>  bar : Int
+                2 ┆>  foo = \x -> x
+
+                Is it a typo? If not, put either a newline or comment between them.
+                "#
+            ),
+        )
+    }
+
     #[test]
     fn invalid_num() {
         report_problem_as(