[TS] Create TSUnionType or TSIntersectionType when typealias has a leading operator (#12758)

* Create `TSUnionType` or `TSIntersectionType` when typealias has a leading operator

* Apply code review suggestions

* Test `TSIntersectionType`

packages/babel-parser/src/plugins/typescript/index.js

@@ -936,17 +936,16 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       operator: TokenType,
     ): N.TsType {
       const node: N.TsUnionType | N.TsIntersectionType = this.startNode();
-      this.eat(operator);
+      const hasLeadingOperator = this.eat(operator);
-      let type = parseConstituentType();
+      const types = [];
-      if (this.match(operator)) {
+      do {
-        const types = [type];
-        while (this.eat(operator)) {
         types.push(parseConstituentType());
+      } while (this.eat(operator));
+      if (types.length === 1 && !hasLeadingOperator) {
+        return types[0];
       }
       node.types = types;
-        type = this.finishNode(node, kind);
+      return this.finishNode(node, kind);
-      }
-      return type;
     }
 
     tsParseIntersectionTypeOrHigher(): N.TsType {

packages/babel-parser/test/fixtures/typescript/types/union-intersection/input.ts

@@ -7,3 +7,5 @@ type J = number | string
 type F = number & string
 type K = | number | string
 type M = & number & string
+type N = | number
+type O = & string

packages/babel-parser/test/fixtures/typescript/types/union-intersection/output.json

@@ -1,9 +1,9 @@
 {
   "type": "File",
-  "start":0,"end":265,"loc":{"start":{"line":1,"column":0},"end":{"line":9,"column":26}},
+  "start":0,"end":301,"loc":{"start":{"line":1,"column":0},"end":{"line":11,"column":17}},
   "program": {
     "type": "Program",
-    "start":0,"end":265,"loc":{"start":{"line":1,"column":0},"end":{"line":9,"column":26}},
+    "start":0,"end":301,"loc":{"start":{"line":1,"column":0},"end":{"line":11,"column":17}},
     "sourceType": "module",
     "interpreter": null,
     "body": [
@@ -262,6 +262,44 @@
             }
           ]
         }
+      },
+      {
+        "type": "TSTypeAliasDeclaration",
+        "start":266,"end":283,"loc":{"start":{"line":10,"column":0},"end":{"line":10,"column":17}},
+        "id": {
+          "type": "Identifier",
+          "start":271,"end":272,"loc":{"start":{"line":10,"column":5},"end":{"line":10,"column":6},"identifierName":"N"},
+          "name": "N"
+        },
+        "typeAnnotation": {
+          "type": "TSUnionType",
+          "start":275,"end":283,"loc":{"start":{"line":10,"column":9},"end":{"line":10,"column":17}},
+          "types": [
+            {
+              "type": "TSNumberKeyword",
+              "start":277,"end":283,"loc":{"start":{"line":10,"column":11},"end":{"line":10,"column":17}}
+            }
+          ]
+        }
+      },
+      {
+        "type": "TSTypeAliasDeclaration",
+        "start":284,"end":301,"loc":{"start":{"line":11,"column":0},"end":{"line":11,"column":17}},
+        "id": {
+          "type": "Identifier",
+          "start":289,"end":290,"loc":{"start":{"line":11,"column":5},"end":{"line":11,"column":6},"identifierName":"O"},
+          "name": "O"
+        },
+        "typeAnnotation": {
+          "type": "TSIntersectionType",
+          "start":293,"end":301,"loc":{"start":{"line":11,"column":9},"end":{"line":11,"column":17}},
+          "types": [
+            {
+              "type": "TSStringKeyword",
+              "start":295,"end":301,"loc":{"start":{"line":11,"column":11},"end":{"line":11,"column":17}}
+            }
+          ]
+        }
       }
     ],
     "directives": []