From 34c75afbab227b41586e97ec293a396f4176991e Mon Sep 17 00:00:00 2001
From: Mark Probst <mark.probst@gmail.com>
Date: Mon, 20 Jul 2026 17:57:54 -0400
Subject: [PATCH 1/2] fix(schema): don't collapse allOf-intersected string with
 date-time/etc to empty (#1770)

Co-Authored-By: gpt-5.6-sol via pi <noreply@openai.com>
---
 .../src/rewrites/ResolveIntersections.ts      | 26 +++++++++++++++
 .../transformed-string-intersection.1.json    |  5 +++
 .../transformed-string-intersection.schema    | 33 +++++++++++++++++++
 3 files changed, 64 insertions(+)
 create mode 100644 test/inputs/schema/transformed-string-intersection.1.json
 create mode 100644 test/inputs/schema/transformed-string-intersection.schema

diff --git a/packages/quicktype-core/src/rewrites/ResolveIntersections.ts b/packages/quicktype-core/src/rewrites/ResolveIntersections.ts
index 7f3ab80d84..19d701b7d9 100644
--- a/packages/quicktype-core/src/rewrites/ResolveIntersections.ts
+++ b/packages/quicktype-core/src/rewrites/ResolveIntersections.ts
@@ -32,6 +32,7 @@ import {
     UnionType,
     isNumberTypeKind,
     isPrimitiveTypeKind,
+    transformedStringTypeKinds,
 } from "../Type/index.js";
 import type { TypeBuilder } from "../Type/TypeBuilder.js";
 import type { StringTypeMapping } from "../Type/TypeBuilderUtils.js";
@@ -125,6 +126,11 @@ class IntersectionAccumulator
             iterableFind(this._primitiveTypes, isNumberTypeKind) !==
                 undefined &&
             iterableFind(kinds, isNumberTypeKind) !== undefined;
+        const narrowedStringTypes = this._primitiveTypes.has("string")
+            ? setIntersect(kinds, transformedStringTypeKinds)
+            : kinds.has("string")
+              ? setIntersect(this._primitiveTypes, transformedStringTypeKinds)
+              : new Set<PrimitiveTypeKind>();
         this._primitiveTypes = setIntersect(this._primitiveTypes, kinds);
         if (
             haveNumber &&
@@ -134,6 +140,11 @@ class IntersectionAccumulator
             // of that is integer.
             this._primitiveTypes = this._primitiveTypes.add("integer");
         }
+
+        // A transformed string is a specialization of a plain string.
+        if (!this._primitiveTypes.has("string")) {
+            setUnionInto(this._primitiveTypes, narrowedStringTypes);
+        }
     }
 
     private updateArrayItemTypes(members: Iterable<Type>): void {
@@ -339,6 +350,21 @@ class IntersectionAccumulator
             });
         }
 
+        const maybeStringAttributes = this._primitiveAttributes.get("string");
+        if (maybeStringAttributes !== undefined && !kinds.has("string")) {
+            for (const kind of transformedStringTypeKinds) {
+                if (kinds.has(kind)) {
+                    mapUpdateInto(kinds, kind, (a) =>
+                        combineTypeAttributes(
+                            "intersect",
+                            defined(a),
+                            maybeStringAttributes,
+                        ),
+                    );
+                }
+            }
+        }
+
         if (
             this._arrayItemTypes !== undefined &&
             this._arrayItemTypes !== false
diff --git a/test/inputs/schema/transformed-string-intersection.1.json b/test/inputs/schema/transformed-string-intersection.1.json
new file mode 100644
index 0000000000..e958405c28
--- /dev/null
+++ b/test/inputs/schema/transformed-string-intersection.1.json
@@ -0,0 +1,5 @@
+{
+  "committer": {
+    "date": "2024-01-02T03:04:05Z"
+  }
+}
diff --git a/test/inputs/schema/transformed-string-intersection.schema b/test/inputs/schema/transformed-string-intersection.schema
new file mode 100644
index 0000000000..89adcc72a2
--- /dev/null
+++ b/test/inputs/schema/transformed-string-intersection.schema
@@ -0,0 +1,33 @@
+{
+  "definitions": {
+    "committer": {
+      "type": "object",
+      "properties": {
+        "date": {
+          "type": "string",
+          "format": "date-time"
+        }
+      },
+      "required": ["date"]
+    }
+  },
+  "type": "object",
+  "properties": {
+    "committer": {
+      "allOf": [
+        {
+          "$ref": "#/definitions/committer"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "date": {
+              "type": "string"
+            }
+          }
+        }
+      ]
+    }
+  },
+  "required": ["committer"]
+}

From 257d3a10fa763aec332ae9ee68547e6465643b92 Mon Sep 17 00:00:00 2001
From: Mark Probst <mark.probst@gmail.com>
Date: Mon, 20 Jul 2026 19:36:33 -0400
Subject: [PATCH 2/2] test: add missing fixture cases for
 transformed-string-intersection.schema (#3026)

Co-Authored-By: Claude <noreply@anthropic.com>
---
 .../transformed-string-intersection.1.fail.date-time.json    | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 test/inputs/schema/transformed-string-intersection.1.fail.date-time.json

diff --git a/test/inputs/schema/transformed-string-intersection.1.fail.date-time.json b/test/inputs/schema/transformed-string-intersection.1.fail.date-time.json
new file mode 100644
index 0000000000..2e1669a4dd
--- /dev/null
+++ b/test/inputs/schema/transformed-string-intersection.1.fail.date-time.json
@@ -0,0 +1,5 @@
+{
+  "committer": {
+    "date": "not a real timestamp"
+  }
+}