[ty] Minor improvements to `assert_type` diagnostics (#21811)

crates/ty_python_semantic/resources/mdtest/directives/assert_type.md

@@ -7,10 +7,11 @@
 ```py
 from typing_extensions import assert_type
 
-def _(x: int):
+def _(x: int, y: bool):
     assert_type(x, int)  # fine
     assert_type(x, str)  # error: [type-assertion-failure]
     assert_type(assert_type(x, int), int)
+    assert_type(y, int)  # error: [type-assertion-failure]
 ```
 
 ## Narrowing

crates/ty_python_semantic/resources/mdtest/snapshots/assert_type.md_-_`assert_type`_-_Basic_(c507788da2659ec9).snap

@@ -14,10 +14,11 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/directives/assert_type.m
 ```
 1 | from typing_extensions import assert_type
 2 | 
-3 | def _(x: int):
+3 | def _(x: int, y: bool):
 4 |     assert_type(x, int)  # fine
 5 |     assert_type(x, str)  # error: [type-assertion-failure]
 6 |     assert_type(assert_type(x, int), int)
+7 |     assert_type(y, int)  # error: [type-assertion-failure]
 ```
 
 # Diagnostics
@@ -26,15 +27,32 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/directives/assert_type.m
 error[type-assertion-failure]: Argument does not have asserted type `str`
  --> src/mdtest_snippet.py:5:5
   |
-3 | def _(x: int):
+3 | def _(x: int, y: bool):
 4 |     assert_type(x, int)  # fine
 5 |     assert_type(x, str)  # error: [type-assertion-failure]
   |     ^^^^^^^^^^^^-^^^^^^
   |                 |
-  |                 Inferred type of argument is `int`
+  |                 Inferred type is `int`
 6 |     assert_type(assert_type(x, int), int)
+7 |     assert_type(y, int)  # error: [type-assertion-failure]
   |
 info: `str` and `int` are not equivalent types
 info: rule `type-assertion-failure` is enabled by default
 
 ```
+
+```
+error[type-assertion-failure]: Argument does not have asserted type `int`
+ --> src/mdtest_snippet.py:7:5
+  |
+5 |     assert_type(x, str)  # error: [type-assertion-failure]
+6 |     assert_type(assert_type(x, int), int)
+7 |     assert_type(y, int)  # error: [type-assertion-failure]
+  |     ^^^^^^^^^^^^-^^^^^^
+  |                 |
+  |                 Inferred type is `bool`
+  |
+info: `bool` is a subtype of `int`, but they are not equivalent
+info: rule `type-assertion-failure` is enabled by default
+
+```

crates/ty_python_semantic/src/types/function.rs

@@ -1483,17 +1483,22 @@ impl KnownFunction {
 
                     diagnostic.annotate(
                         Annotation::secondary(context.span(&call_expression.arguments.args[0]))
-                            .message(format_args!(
+                            .message(format_args!("Inferred type is `{}`", actual_ty.display(db),)),
-                                "Inferred type of argument is `{}`",
-                                actual_ty.display(db),
-                            )),
                     );
 
+                    if actual_ty.is_subtype_of(db, *asserted_ty) {
+                        diagnostic.info(format_args!(
+                            "`{inferred_type}` is a subtype of `{asserted_type}`, but they are not equivalent",
+                            asserted_type = asserted_ty.display(db),
+                            inferred_type = actual_ty.display(db),
+                        ));
+                    } else {
                         diagnostic.info(format_args!(
                             "`{asserted_type}` and `{inferred_type}` are not equivalent types",
                             asserted_type = asserted_ty.display(db),
                             inferred_type = actual_ty.display(db),
                         ));
+                    }
 
                     diagnostic.set_concise_message(format_args!(
                         "Type `{}` does not match asserted type `{}`",