Add polymorphic expression reproduction cases from #1745

These work now that we have ways to compile polymorphic expressions.

Closes #1745

crates/compiler/test_gen/src/gen_list.rs

@@ -3369,3 +3369,23 @@ fn issue_3530_uninitialized_capacity_in_list_literal() {
         |(_, _, cap)| cap
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn list_let_generalization() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            empty : List a
+            empty = []
+
+            xs : List Str
+            xs = List.append empty "foo"
+
+            List.len xs
+            "#
+        ),
+        1,
+        usize
+    );
+}

crates/compiler/test_gen/src/gen_primitives.rs

@@ -4019,3 +4019,24 @@ fn mutually_recursive_captures() {
         RocStr
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn int_let_generalization() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            manyAux : {} -> I32 
+            manyAux = \_ ->
+                output = \_ -> 42
+
+                output {}
+
+            when manyAux {} is
+                _ -> "done"
+            "#
+        ),
+        RocStr::from("done"),
+        RocStr
+    );
+}

crates/compiler/test_gen/src/gen_tags.rs

@@ -1933,3 +1933,25 @@ fn issue_2165_recursive_tag_destructure() {
         RocStr
     )
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm", feature = "gen-wasm"))]
+fn tag_union_let_generalization() {
+    assert_evals_to!(
+        indoc!(
+            r#"
+            manyAux : {} -> [ Loop, Done ]
+            manyAux = \_ ->
+                output = Done
+
+                output
+
+            when manyAux {} is
+                Loop -> "loop"
+                Done -> "done"
+            "#
+        ),
+        RocStr::from("done"),
+        RocStr
+    );
+}