From 63c5b4353d50020e9bedd354c54f5cdf99437878 Mon Sep 17 00:00:00 2001
From: "Randall C. O'Reilly" <oreilly@ucdavis.edu>
Date: Sun, 25 Oct 2026 10:16:51 +0100
Subject: [PATCH] fix(builtin): wrap a uint64 converted to int in generic code

A conversion to a type parameter whose type argument is int did not wrap
a uint64 beyond the int range, so T(u) for u = 1<<64 - 7 gave
18446744073709552000 instead of -7, as the non-generic int(u) does.

Signed-off-by: Randall C. O'Reilly <oreilly@ucdavis.edu>
---
 gs/builtin/builtin.ts                         |  3 ++-
 .../generic_typeparam_conversion/expected.log |  2 ++
 .../generic_typeparam_conversion.go           |  9 ++++++++
 .../generic_typeparam_conversion.gs.ts        | 22 +++++++++++++++++++
 4 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/gs/builtin/builtin.ts b/gs/builtin/builtin.ts
index 228bfc72d..cfa2831b9 100644
--- a/gs/builtin/builtin.ts
+++ b/gs/builtin/builtin.ts
@@ -894,7 +894,8 @@ export function genericConvert<T>(
     case 'rune':
       return int(value, 32) as T
     case 'int':
-      return int(value) as T
+      // int is 64 bits, so a uint64 beyond its range wraps as in Go.
+      return int(value, 64) as T
     case 'uint8':
     case 'byte':
       return uint(value, 8) as T
diff --git a/tests/tests/generic_typeparam_conversion/expected.log b/tests/tests/generic_typeparam_conversion/expected.log
index 4df8258fb..f5b0ec566 100644
--- a/tests/tests/generic_typeparam_conversion/expected.log
+++ b/tests/tests/generic_typeparam_conversion/expected.log
@@ -6,3 +6,5 @@
 10 wide
 16
 7 44
+-7 -7 -7
+-1
diff --git a/tests/tests/generic_typeparam_conversion/generic_typeparam_conversion.go b/tests/tests/generic_typeparam_conversion/generic_typeparam_conversion.go
index f5256033a..cceb07a23 100644
--- a/tests/tests/generic_typeparam_conversion/generic_typeparam_conversion.go
+++ b/tests/tests/generic_typeparam_conversion/generic_typeparam_conversion.go
@@ -48,6 +48,12 @@ func toWide[T, E EnumConstraint](e E) T {
 	return T(e)
 }
 
+// fromUint64 converts a uint64 to a type parameter, which wraps a value
+// beyond the range of the type.
+func fromUint64[T Integer](u uint64) T {
+	return T(u)
+}
+
 // fromConst converts a constant and a basic value to a type parameter.
 func fromConst[T Integer](i int) (T, T) {
 	return T(7), T(i)
@@ -64,4 +70,7 @@ func main() {
 	println(int64(a) + int64(b))
 	c, d := fromConst[uint8](300)
 	println(c, d)
+	u := uint64(1<<64 - 7)
+	println(fromUint64[int](u), fromUint64[int64](u), fromUint64[int32](u))
+	println(fromUint64[int](u) % 3)
 }
diff --git a/tests/tests/generic_typeparam_conversion/generic_typeparam_conversion.gs.ts b/tests/tests/generic_typeparam_conversion/generic_typeparam_conversion.gs.ts
index 2377b951e..f6226be2b 100644
--- a/tests/tests/generic_typeparam_conversion/generic_typeparam_conversion.gs.ts
+++ b/tests/tests/generic_typeparam_conversion/generic_typeparam_conversion.gs.ts
@@ -115,6 +115,10 @@ export function toWide(__typeArgs: $.GenericTypeArgs | undefined, e: any): any {
 	return $.genericConvert<any>(__typeArgs, "T", e)
 }
 
+export function fromUint64(__typeArgs: $.GenericTypeArgs | undefined, u: bigint): any {
+	return $.genericConvert<any>(__typeArgs, "T", u)
+}
+
 export function fromConst(__typeArgs: $.GenericTypeArgs | undefined, i: number): [any, any] {
 	return [$.genericConvert<any>(__typeArgs, "T", 7), $.genericConvert<any>(__typeArgs, "T", i)]
 }
@@ -135,6 +139,9 @@ export async function main(): globalThis.Promise<void> {
 	let c: number = (__goscriptTuple1[0] as number)
 	let d: number = (__goscriptTuple1[1] as number)
 	await $.println(c, d)
+	let u = 18446744073709551609n
+	await $.println(fromUint64(__goscriptStatic4(), u), fromUint64(__goscriptStatic5(), u), fromUint64(__goscriptStatic6(), u))
+	await $.println(fromUint64(__goscriptStatic4(), u) % 3)
 }
 
 var __goscriptStatic0$value: any
@@ -157,6 +164,21 @@ function __goscriptStatic3(): any {
 	return __goscriptStatic3$value ??= ({[$.genericTypeArgsMarker]: $.genericTypeArgsBrand, T: { type: /* @__PURE__ */ $.basicType("uint8"), zero: () => 0 }})
 }
 
+var __goscriptStatic4$value: any
+function __goscriptStatic4(): any {
+	return __goscriptStatic4$value ??= ({[$.genericTypeArgsMarker]: $.genericTypeArgsBrand, T: { type: /* @__PURE__ */ $.basicType("int"), zero: () => 0 }})
+}
+
+var __goscriptStatic5$value: any
+function __goscriptStatic5(): any {
+	return __goscriptStatic5$value ??= ({[$.genericTypeArgsMarker]: $.genericTypeArgsBrand, T: { type: /* @__PURE__ */ $.basicType("int64"), zero: () => 0n }})
+}
+
+var __goscriptStatic6$value: any
+function __goscriptStatic6(): any {
+	return __goscriptStatic6$value ??= ({[$.genericTypeArgsMarker]: $.genericTypeArgsBrand, T: { type: /* @__PURE__ */ $.basicType("int32"), zero: () => 0 }})
+}
+
 if ($.isMainScript(import.meta)) {
 	await main()
 }