fix(resolver): child tsconfig paths should override parent paths, not merge

Per TypeScript semantics, when a child tsconfig.json defines its own `paths`
mapping, it should completely replace the parent's `paths` rather than merging
them together.

Fixes #25622

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/resolver/resolver.zig

@@ -4235,9 +4235,11 @@ pub const Resolver = struct {
                             merged_config.preserve_imports_not_used_as_values = value;
                         }
 
-                        var iter = parent_config.paths.iterator();
-                        while (iter.next()) |c| {
-                            merged_config.paths.put(c.key_ptr.*, c.value_ptr.*) catch unreachable;
+                        // TypeScript's behavior: if a child config defines `paths`, it completely
+                        // overrides the parent's `paths` rather than merging.
+                        if (parent_config.paths.count() > 0) {
+                            merged_config.paths = parent_config.paths;
+                            merged_config.base_url_for_paths = parent_config.base_url_for_paths;
                         }
                         // todo deinit these parent configs somehow?
                     }

test/regression/issue/25622.test.ts

@@ -0,0 +1,77 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+// https://github.com/oven-sh/bun/issues/25622
+// TypeScript's behavior: child tsconfig `paths` should completely override parent's `paths`,
+// not merge with them.
+test("child tsconfig paths should override parent paths, not merge", async () => {
+  using dir = tempDir("issue-25622", {
+    "tsconfig.base.json": JSON.stringify({
+      compilerOptions: {
+        paths: {
+          "@helpers/*": ["./src/helpers/*"],
+        },
+      },
+    }),
+    "tsconfig.json": JSON.stringify({
+      extends: "./tsconfig.base.json",
+      compilerOptions: {
+        paths: {
+          "@/*": ["./src/*"],
+        },
+      },
+    }),
+    "src/helpers/x.ts": `export const x = "from helpers";`,
+    "src/index.ts": `import "@helpers/x";`,
+  });
+
+  // This should fail because child's paths should override parent's paths
+  // (the @helpers/* mapping from the parent should not be present)
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "src/index.ts"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  // Should fail with a resolution error since @helpers/* should not be mapped
+  expect(stderr).toContain("@helpers/x");
+  expect(exitCode).not.toBe(0);
+});
+
+test("child tsconfig inherits parent paths when child has no paths", async () => {
+  using dir = tempDir("issue-25622-inherit", {
+    "tsconfig.base.json": JSON.stringify({
+      compilerOptions: {
+        paths: {
+          "@helpers/*": ["./src/helpers/*"],
+        },
+      },
+    }),
+    "tsconfig.json": JSON.stringify({
+      extends: "./tsconfig.base.json",
+      compilerOptions: {
+        // No paths defined - should inherit from parent
+      },
+    }),
+    "src/helpers/x.ts": `console.log("inherited path works");`,
+    "src/index.ts": `import "@helpers/x";`,
+  });
+
+  // This should succeed because child inherits parent's paths
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "src/index.ts"],
+    env: bunEnv,
+    cwd: String(dir),
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stdout).toContain("inherited path works");
+  expect(exitCode).toBe(0);
+});