de-alias when diffing with a non-alias

When one of the types in a comparison is an alias, elm does not de-alias it.
That is really annoying if e.g. the alias is of a record and you used an invalid field: the field suggestion error does not kick in

compiler/constrain/src/expr.rs

@@ -524,6 +524,9 @@ pub fn constrain_expr(
                     constraints.push(ast_con);
 
                     for (index, when_branch) in branches.iter().enumerate() {
+                        let pattern_region =
+                            Region::across_all(when_branch.patterns.iter().map(|v| &v.region));
+
                         let branch_con = constrain_when_branch(
                             env,
                             when_branch.value.region,
@@ -533,12 +536,14 @@ pub fn constrain_expr(
                                     index: Index::zero_based(index),
                                 },
                                 cond_type.clone(),
-                                Region::across_all(when_branch.patterns.iter().map(|v| &v.region)),
+                                pattern_region,
                             ),
                             FromAnnotation(
                                 name.clone(),
                                 *arity,
-                                TypedWhenBranch(Index::zero_based(index)),
+                                TypedWhenBranch {
+                                    index: Index::zero_based(index),
+                                },
                                 typ.clone(),
                             ),
                         );
@@ -552,6 +557,8 @@ pub fn constrain_expr(
                     let mut branch_cons = Vec::with_capacity(branches.len());
 
                     for (index, when_branch) in branches.iter().enumerate() {
+                        let pattern_region =
+                            Region::across_all(when_branch.patterns.iter().map(|v| &v.region));
                         let branch_con = constrain_when_branch(
                             env,
                             region,
@@ -561,7 +568,7 @@ pub fn constrain_expr(
                                     index: Index::zero_based(index),
                                 },
                                 cond_type.clone(),
-                                Region::across_all(when_branch.patterns.iter().map(|v| &v.region)),
+                                pattern_region,
                             ),
                             ForReason(
                                 Reason::WhenBranch {

compiler/constrain/src/uniq.rs

@@ -1085,6 +1085,9 @@ pub fn constrain_expr(
                     ));
 
                     for (index, when_branch) in branches.iter().enumerate() {
+                        let pattern_region =
+                            Region::across_all(when_branch.patterns.iter().map(|v| &v.region));
+
                         let branch_con = constrain_when_branch(
                             var_store,
                             var_usage,
@@ -1097,12 +1100,14 @@ pub fn constrain_expr(
                                     index: Index::zero_based(index),
                                 },
                                 cond_type.clone(),
-                                region,
+                                pattern_region,
                             ),
                             Expected::FromAnnotation(
                                 name.clone(),
                                 *arity,
-                                TypedWhenBranch(Index::zero_based(index)),
+                                TypedWhenBranch {
+                                    index: Index::zero_based(index),
+                                },
                                 typ.clone(),
                             ),
                         );
@@ -1120,6 +1125,8 @@ pub fn constrain_expr(
                     let mut branch_cons = Vec::with_capacity(branches.len());
 
                     for (index, when_branch) in branches.iter().enumerate() {
+                        let pattern_region =
+                            Region::across_all(when_branch.patterns.iter().map(|v| &v.region));
                         let branch_con = constrain_when_branch(
                             var_store,
                             var_usage,
@@ -1132,7 +1139,7 @@ pub fn constrain_expr(
                                     index: Index::zero_based(index),
                                 },
                                 cond_type.clone(),
-                                region,
+                                pattern_region,
                             ),
                             Expected::ForReason(
                                 Reason::WhenBranch {

compiler/reporting/src/report.rs

@@ -462,7 +462,7 @@ impl<'a> RocDocAllocator<'a> {
         self.intersperse(docs, self.line())
     }
 
-    /// live vcat, but adds a double line break between elements. Visually this means an empty line
+    /// like vcat, but adds a double line break between elements. Visually this means an empty line
     /// between elements.
     pub fn stack<A, I>(&'a self, docs: I) -> DocBuilder<'a, Self, A>
     where

compiler/reporting/src/type_error.rs

@@ -170,7 +170,7 @@ fn to_expr_report<'b>(
                     alloc.keyword("if"),
                     alloc.text(" expression:"),
                 ]),
-                TypedWhenBranch(index) => alloc.concat(vec![
+                TypedWhenBranch { index } => alloc.concat(vec![
                     alloc.string(index.ordinal()),
                     alloc.reflow(" branch of this "),
                     alloc.keyword("when"),
@@ -185,7 +185,7 @@ fn to_expr_report<'b>(
 
             let it_is = match annotation_source {
                 TypedIfBranch { index, .. } => format!("The {} branch is", index.ordinal()),
-                TypedWhenBranch(index) => format!("The {} branch is", index.ordinal()),
+                TypedWhenBranch { index, .. } => format!("The {} branch is", index.ordinal()),
                 TypedBody => "The body is".into(),
             };
 
@@ -1319,6 +1319,15 @@ fn to_diff<'b>(
             }
         }
 
+        (Alias(symbol, _, actual), other) if !symbol.module_id().is_builtin() => {
+            // when diffing an alias with a non-alias, de-alias
+            to_diff(alloc, parens, *actual, other)
+        }
+        (other, Alias(symbol, _, actual)) if !symbol.module_id().is_builtin() => {
+            // when diffing an alias with a non-alias, de-alias
+            to_diff(alloc, parens, other, *actual)
+        }
+
         (Record(fields1, ext1), Record(fields2, ext2)) => {
             diff_record(alloc, fields1, ext1, fields2, ext2)
         }

compiler/types/src/types.rs

@@ -605,7 +605,7 @@ pub enum PReason {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum AnnotationSource {
     TypedIfBranch { index: Index, num_branches: usize },
-    TypedWhenBranch(Index),
+    TypedWhenBranch { index: Index },
     TypedBody,
 }
 
@@ -619,8 +619,6 @@ pub enum Reason {
         name: Option<Symbol>,
         arity: u8,
     },
-    //    BinOpArg(BinOp, ArgSide),
-    //    BinOpRet(BinOp),
     FloatLiteral,
     IntLiteral,
     NumLiteral,