Fix casts from json to custom scalars (#5624)

We've had a codepath to fix this bug for enum already, but now I extended it to anything that:

 * is a scalar,
 * it not the top-most concrete base of itself,

Closes #5616

edb/edgeql/compiler/casts.py

@@ -104,8 +104,6 @@ def compile_cast(
     ):
         return _find_object_by_id(ir_expr, new_stype, ctx=ctx)
 
-    json_t = ctx.env.get_schema_type_and_track(sn.QualName('std', 'json'))
-
     if isinstance(ir_set.expr, irast.Array):
         return _cast_array_literal(
             ir_set, orig_stype, new_stype, srcctx=srcctx, ctx=ctx)
@@ -145,38 +143,33 @@ def compile_cast(
             ir_set, orig_stype, new_stype,
             cardinality_mod=cardinality_mod, ctx=ctx)
 
+    json_t = ctx.env.get_schema_type_and_track(sn.QualName('std', 'json'))
     if (
         new_stype.issubclass(ctx.env.schema, json_t)
         and ir_set.path_id.is_objtype_path()
     ):
         # JSON casts of objects are special: we want the full shape
         # and not just an identity.
         viewgen.late_compile_view_shapes(ir_set, ctx=ctx)
-    else:
-        if orig_stype.issubclass(ctx.env.schema, json_t) and new_stype.is_enum(
-            ctx.env.schema
-        ):
-            # Casts from json to enums need some special handling
-            # here, where we have access to the enum type. Just turn
-            # it into json->str and str->enum.
-            str_typ = ctx.env.get_schema_type_and_track(
-                sn.QualName('std', 'str')
-            )
-            str_ir = compile_cast(ir_expr, str_typ, srcctx=srcctx, ctx=ctx)
+    elif orig_stype.issubclass(ctx.env.schema, json_t):
+
+        if base_stype := _get_concrete_scalar_base(new_stype, ctx):
+            # Casts from json to custom scalars may have special handling.
+            # So we turn the type cast json->x into json->base and base->x.
+            base_ir = compile_cast(ir_expr, base_stype, srcctx=srcctx, ctx=ctx)
+
             return compile_cast(
-                str_ir,
+                base_ir,
                 new_stype,
                 cardinality_mod=cardinality_mod,
                 srcctx=srcctx,
                 ctx=ctx,
             )
 
-        if (
-            orig_stype.issubclass(ctx.env.schema, json_t)
-            and isinstance(new_stype, s_types.Array)
-            and not new_stype.get_subtypes(ctx.env.schema)[0].issubclass(
-                ctx.env.schema, json_t
-            )
+        elif isinstance(
+            new_stype, s_types.Array
+        ) and not new_stype.get_subtypes(ctx.env.schema)[0].issubclass(
+            ctx.env.schema, json_t
         ):
             # Turn casts from json->array<T> into json->array<json>
             # and array<json>->array<T>.
@@ -194,9 +187,7 @@ def compile_cast(
                 json_array_ir, new_stype, srcctx=srcctx, ctx=ctx
             )
 
-        if orig_stype.issubclass(ctx.env.schema, json_t) and isinstance(
-            new_stype, s_types.Tuple
-        ):
+        elif isinstance(new_stype, s_types.Tuple):
             return _cast_json_to_tuple(
                 ir_set,
                 orig_stype,
@@ -206,9 +197,7 @@ def compile_cast(
                 ctx=ctx,
             )
 
-        if orig_stype.issubclass(ctx.env.schema, json_t) and isinstance(
-            new_stype, s_types.Range
-        ):
+        elif isinstance(new_stype, s_types.Range):
             return _cast_json_to_range(
                 ir_set,
                 orig_stype,
@@ -260,6 +249,23 @@ def _has_common_concrete_scalar(
     )
 
 
+def _get_concrete_scalar_base(
+    stype: s_types.Type,
+    ctx: context.ContextLevel
+) -> Optional[s_types.Type]:
+    """Returns None if stype is not scalar or if it is already topmost"""
+
+    if stype.is_enum(ctx.env.schema):
+        return ctx.env.get_schema_type_and_track(sn.QualName('std', 'str'))
+
+    if not isinstance(stype, s_scalars.ScalarType):
+        return None
+    if topmost := stype.maybe_get_topmost_concrete_base(ctx.env.schema):
+        if topmost != stype:
+            return topmost
+    return None
+
+
 def _compile_cast(
         ir_expr: Union[irast.Set, irast.Expr],
         orig_stype: s_types.Type,

tests/test_edgeql_casts.py

@@ -2624,6 +2624,25 @@ async def test_edgeql_casts_custom_scalar_05(self):
             [[42]],
         )
 
+    async def test_edgeql_casts_custom_scalar_06(self):
+        await self.con.execute(
+            '''
+            create scalar type x extending str {
+                create constraint expression on (false)
+            };
+        '''
+        )
+
+        async with self.assertRaisesRegexTx(
+            edgedb.ConstraintViolationError, 'invalid x'
+        ):
+            await self.con.query("""SELECT <x>42""")
+
+        async with self.assertRaisesRegexTx(
+            edgedb.ConstraintViolationError, 'invalid x'
+        ):
+            await self.con.query("""SELECT <x>to_json('"a"')""")
+
     async def test_edgeql_casts_tuple_params_01(self):
         # insert tuples into a nested array
         def nest(data):