Warns for structural equality on abstract types (#1972)

This is achieved by making promotion local to finding the lub of the arguments. I've changed the error message slightly, because "incompatible" seems like the wrong description now.

Author: kritzcreek

src/mo_frontend/typing.ml

@@ -770,9 +770,9 @@ and infer_exp'' env exp : T.typ =
     end;
     t
   | RelE (ot, exp1, op, exp2) ->
-    let t1 = infer_exp_promote env exp1 in
-    let t2 = infer_exp_promote env exp2 in
-    let t = Operator.type_relop op (T.lub t1 t2) in
+    let t1 = T.normalize (infer_exp env exp1) in
+    let t2 = T.normalize (infer_exp env exp2) in
+    let t = Operator.type_relop op (T.lub (T.promote t1) (T.promote t2)) in
     if not env.pre then begin
       assert (!ot = Type.Pre);
       if not (Operator.has_relop op t) then
@@ -781,11 +781,17 @@ and infer_exp'' env exp : T.typ =
           (T.string_of_typ_expand t1)
           (T.string_of_typ_expand t2);
       if not (T.eq t t1 || T.eq t t2) then
-        warn env exp.at
-          "comparing incompatible types\n  %s\nand\n  %s\nat common supertype\n  %s"
-          (T.string_of_typ_expand t1)
-          (T.string_of_typ_expand t2)
-          (T.string_of_typ_expand t);
+        if T.eq t1 t2 then
+          warn env exp.at
+            "comparing abstract type\n  %s\nto itself at supertype\n  %s"
+            (T.string_of_typ_expand t1)
+            (T.string_of_typ_expand t)
+        else
+          warn env exp.at
+            "comparing incompatible types\n  %s\nand\n  %s\nat common supertype\n  %s"
+            (T.string_of_typ_expand t1)
+            (T.string_of_typ_expand t2)
+            (T.string_of_typ_expand t);
       ot := t;
     end;
     T.bool
@@ -2277,4 +2283,3 @@ let check_lib scope lib : Scope.t Diag.result =
           Scope.lib lib.note imp_typ
         ) lib
     )
-

test/fail/ok/structural_equality.tc.ok

@@ -20,3 +20,12 @@ structural_equality.mo:10.8-10.18: warning, comparing incompatible types
 and
   Text
   Any
+structural_equality.mo:12.37-12.43: warning, comparing abstract type
+  A/10
+to itself at supertype
+  Any
+structural_equality.mo:13.41-13.47: warning, comparing incompatible types
+  A/11
+and
+  B
+  Any

test/fail/structural_equality.mo

@@ -8,3 +8,6 @@ A() == A();
 ignore ({ x = 10 } == { var x = 10 });
 
 assert(10 == "hi");
+
+func myEq<A>(x : A, y : A) : Bool = x == y;
+func myEq2<A, B>(x : A, y : B) : Bool = x == y;