Mark patterns in lambda argument position as having a presence constraint

Closes #2299

compiler/constrain/src/expr.rs

@@ -77,7 +77,7 @@ fn constrain_untyped_args(
             loc_pattern.region,
             pattern_expected,
             &mut pattern_state,
-            false,
+            true,
         );
 
         vars.push(*pattern_var);

compiler/solve/tests/solve_expr.rs

@@ -1327,7 +1327,7 @@ mod solve_expr {
                     \Foo -> 42
                 "#
             ),
-            "[ Foo ]* -> Num *",
+            "[ Foo ] -> Num *",
         );
     }
 
@@ -1339,7 +1339,7 @@ mod solve_expr {
                     \@Foo -> 42
                 "#
             ),
-            "[ @Foo ]* -> Num *",
+            "[ @Foo ] -> Num *",
         );
     }
 
@@ -1419,7 +1419,7 @@ mod solve_expr {
                     \Foo x -> Foo x
                 "#
             ),
-            "[ Foo a ]* -> [ Foo a ]*",
+            "[ Foo a ] -> [ Foo a ]*",
         );
     }
 
@@ -1431,7 +1431,7 @@ mod solve_expr {
                     \Foo x _ -> Foo x "y"
                 "#
             ),
-            "[ Foo a * ]* -> [ Foo a Str ]*",
+            "[ Foo a * ] -> [ Foo a Str ]*",
         );
     }
 
@@ -4909,4 +4909,17 @@ mod solve_expr {
             "{ x : [ Blue, Red ], y ? Num a }* -> Num a",
         )
     }
+
+    #[test]
+    // Issue #2299
+    fn infer_union_argument_position() {
+        infer_eq_without_problem(
+            indoc!(
+                r#"
+                 \UserId id -> id + 1
+                 "#
+            ),
+            "[ UserId (Num a) ] -> Num a",
+        )
+    }
 }