fix(compiler): returning negative bigints from asBigInit (#5295)

Co-authored-by: Will Temple <[email protected]>

Author: TomerAberbach

.chronus/changes/fix-negative-big-int-2024-12-06-23-51-23.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Fix incorrectly returning a positive `BigInt` for a negative `Numeric`.

packages/compiler/src/core/numeric.ts

@@ -195,7 +195,12 @@ const NumericPrototype = {
     return equals(this[InternalDataSym], Numeric(num.toString())[InternalDataSym]) ? num : null;
   },
   asBigInt: function (this: Numeric) {
-    return this.isInteger ? this[InternalDataSym].n : null;
+    if (!this.isInteger) {
+      return null;
+    }
+
+    const { s, n } = this[InternalDataSym];
+    return BigInt(s) * n;
   },
   equals: function (this: Numeric, other: Numeric) {
     return equals(this[InternalDataSym], other[InternalDataSym]);

packages/compiler/test/core/numeric.test.ts

@@ -215,6 +215,7 @@ describe("asNumber", () => {
     ["123.456", 123.456],
     ["123.00", 123],
     ["123456789123456789123456789123456789", null],
+    ["-123456789123456789123456789123456789", null],
     ["123456789123456789.123456789123456789", null],
   ])("%s => %d", (a, b) => {
     const numeric = Numeric(a);
@@ -229,6 +230,7 @@ describe("asBigInt", () => {
     ["123.456", null],
     ["123.00", 123n],
     ["123456789123456789123456789123456789", 123456789123456789123456789123456789n],
+    ["-123456789123456789123456789123456789", -123456789123456789123456789123456789n],
     ["123456789123456789.123456789123456789", null],
   ])("%s => %d", (a, b) => {
     const numeric = Numeric(a);