math: Avoid signed integer overflow on negation

Extend commit 099c05dd04 (math: Avoid signed integer overflow when
intrinsics are available, 2026-02-26) to cover unary `-`.

Source/cmExprParserHelper.cxx

@@ -203,6 +203,10 @@ std::int64_t cmExprParserHelper::Mod(std::int64_t l, std::int64_t r)
 
 std::int64_t cmExprParserHelper::Neg(std::int64_t x)
 {
+  if (static_cast<std::uint64_t>(x) == 0x8000000000000000) {
+    this->Warning(cmStrCat("signed integer cannot negate:\n  ", x));
+    return x;
+  }
   return -x;
 }
 

Tests/RunCMake/math/Overflow-stderr.txt

@@ -60,4 +60,12 @@ CMake Warning \(dev\) at [^
 
     9223372036854775807 \* 2
 
+This warning is for project developers\.  Use -Wno-dev to suppress it\.
+
+CMake Warning \(dev\) at [^
+]+/Tests/RunCMake/math/Overflow\.cmake:[0-9]+ \(math\):
+  signed integer cannot negate:
+
+    -9223372036854775808
+
 This warning is for project developers\.  Use -Wno-dev to suppress it\.$

Tests/RunCMake/math/Overflow-stdout.txt

@@ -6,4 +6,5 @@
 --  4 >>  65: 2
 --  0x7FFFFFFFFFFFFFFF \+ 1: -9223372036854775808
 -- -0x7FFFFFFFFFFFFFFF - 2: 9223372036854775807
---  0x7FFFFFFFFFFFFFFF \* 2: -2$
+--  0x7FFFFFFFFFFFFFFF \* 2: -2
+-- -~0x7FFFFFFFFFFFFFFF: -9223372036854775808$

Tests/RunCMake/math/Overflow.cmake

@@ -8,6 +8,7 @@ foreach(expr IN ITEMS
     " 0x7FFFFFFFFFFFFFFF + 1"
     "-0x7FFFFFFFFFFFFFFF - 2"
     " 0x7FFFFFFFFFFFFFFF * 2"
+    "-~0x7FFFFFFFFFFFFFFF"
   )
   math(EXPR result "${expr}")
   message(STATUS "${expr}: ${result}")