Handle intersection with type alias correctly (#70)

In TS it's just fine to use the LHS of a type alias in an
intersection. However, in Python we can't inherit from a type
alias. So if the reference target is a type alias, we have to
unwind it until we hit something we can inherit from.

Author: hoodmane

type-generation/src/astToIR.ts

@@ -406,16 +406,24 @@ class SyntheticTypeConverter {
           .getTypeNodes()
           .map((ty, idx) => {
             this.nameContext.push(`Intersection${idx}`);
+            // In TS it's just fine to use the LHS of a type alias in an
+            // intersection. However, in Python we can't inherit from a type
+            // alias. So if the reference target is a type alias, we have to
+            // unwind it until we hit something we can inherit.
+            while (Node.isTypeReference(ty)) {
+              const classified = classifyIdentifier(
+                ty.getTypeName() as Identifier,
+              );
+              if (classified.kind !== "typeAlias") {
+                break;
+              }
+              ty = classified.decl.getTypeNode()!;
+            }
             const res = this.typeToIR(ty, modifiers);
             this.nameContext.pop();
             return res;
           })
           .filter((x): x is ReferenceTypeIR => !!x && x.kind === "reference");
-        for (const x of types) {
-          if (!x.name.endsWith("_iface")) {
-            x.name += "_iface";
-          }
-        }
         const name = this.nameContext.join("__") + "_iface";
         this.converter.extraTopLevels.push(
           this.converter.interfaceToIR(name, types, [], [], [], []),

type-generation/tests/a.test.ts

@@ -1456,19 +1456,46 @@ describe("emit", () => {
         dedent(`
           type D = D_iface
 
-          type F = F_iface
-
           def f() -> D: ...
 
+          class D__Intersection0_iface(Protocol):
+              a: str = ...
+
           class D__Intersection1_iface(Protocol):
               @property
               def id(self, /) -> int | float: ...
 
-          class D_iface(F_iface, D__Intersection1_iface, Protocol):
+          class D_iface(D__Intersection1_iface, D__Intersection0_iface, Protocol):
               pass
+        `).trim(),
+      );
+    });
+    it("intersection3", () => {
+      const res = emitFile(`
+        interface I<T> {
+          x: T;
+        }
+        type F = I<string>;
+        type D = F & {
+            id: number;
+        };
+        declare function f(): D;
+      `);
+      assert.strictEqual(
+        removeTypeIgnores(res.slice(1).join("\n\n")),
+        dedent(`
+          type D = D_iface
 
-          class F_iface(Protocol):
-              a: str = ...
+          def f() -> D: ...
+
+          class I_iface[T](Protocol):
+              x: T = ...
+
+          class D__Intersection1_iface(Protocol):
+              id: int | float = ...
+
+          class D_iface(D__Intersection1_iface, I_iface[str], Protocol):
+              pass
         `).trim(),
       );
     });