vm: Math.log1p and Math.expm1, the two functions Ruby 4.0 added to Math

internal/vm/math.go

@@ -175,6 +175,50 @@ func (vm *VM) registerMath() {
 	unaryDom("atanh", math.Atanh, unit)
 	unaryDom("acosh", math.Acosh, func(x float64) bool { return x < 1 })
 
+	// unaryStrict installs a single-argument function that also CHECKS ITS
+	// ARITY, with an optional domain restriction.
+	//
+	// ⛔ The arity check is here and not in `unary` above, and that asymmetry is
+	// deliberate. Measured 2026-10-31: rbgo answers `Math.cbrt(9, 3)` with
+	// 2.080083823051904, where MRI raises ArgumentError "wrong number of
+	// arguments (given 2, expected 1)" — the extra argument is silently
+	// dropped, for every function `unary` installs. That is a pre-existing
+	// divergence across the whole module, and it cannot be repaired by a
+	// blanket check here: `Math.log` legitimately takes one OR two arguments,
+	// `atan2`/`hypot`/`ldexp` take two, so each arity is its own fact and
+	// changing twenty of them at once moves what the gate measures. It is
+	// tracked separately.
+	//
+	// What must not happen is a NEW method inheriting the old leniency because
+	// its neighbours have it. core/math/log1p_spec.rb:27 asserts the
+	// ArgumentError outright, so the two functions below are strict and the
+	// older ones stay as they are until that divergence is measured on its own.
+	unaryStrict := func(name string, f func(float64) float64, bad func(float64) bool) {
+		install(name, func(vm *VM, _ object.Value, args []object.Value, _ *Proc) object.Value {
+			if len(args) != 1 {
+				raise("ArgumentError", "wrong number of arguments (given %d, expected 1)", len(args))
+			}
+			x := mathFloat(vm, args[0])
+			if bad != nil && bad(x) {
+				mathDomainError(name)
+			}
+			return object.Float(f(x))
+		})
+	}
+
+	// log1p(x) is log(1+x) and expm1(x) is exp(x)-1, computed so that the result
+	// keeps its precision for |x| far below 1 — where `log(1 + 1e-16)` and
+	// `exp(1e-16) - 1` lose every significant digit to the leading 1 and return
+	// 0.0. core/math/log1p_spec.rb:10 and core/math/expm1_spec.rb:10 assert
+	// exactly that: the answer must be close to 1e-16 and must not be 0.0.
+	// Go's math.Log1p and math.Expm1 are the same primitives C gives MRI.
+	//
+	// The domain: log1p is real for x >= -1 (log1p(-1) is -Infinity, which is a
+	// value and not an error), so DomainError belongs to x < -1 — which is what
+	// the spec exercises with -1-1e-15. expm1 has no domain restriction.
+	unaryStrict("log1p", math.Log1p, func(x float64) bool { return x < -1 })
+	unaryStrict("expm1", math.Expm1, nil)
+
 	// logFamily installs a base-2/e/10 logarithm: it raises DomainError - name for
 	// a negative argument and combines the mantissa/exponent split from mathLogArg
 	// so a huge Bignum keeps full precision instead of overflowing to Infinity.

internal/vm/math_test.go

@@ -185,3 +185,83 @@ func TestMathPrivateInstanceMethods(t *testing.T) {
 		}
 	}
 }
+
+// TestMathLog1pAndExpm1 covers the two functions Ruby 4.0 added
+// (core/math/log1p_spec.rb, core/math/expm1_spec.rb — both behind
+// `ruby_version_is "4.0"`, so the conformance gate cannot see them while the
+// shim resolves guards at 3.4.1).
+//
+// ⛔ THE POINT OF THESE FUNCTIONS IS THE PRECISION, not the identity. Anyone
+// could write `log(1+x)` and `exp(x)-1` and satisfy the first example of each
+// spec; both specs then assert that the answer for 1e-16 is close to 1e-16 and
+// **is not 0.0**, which the naive forms fail — the leading 1 swallows every
+// significant digit. The cases marked "the whole reason" below are those, and
+// the two after them show the naive forms really do lose it, so those
+// assertions are not holding either way.
+func TestMathLog1pAndExpm1(t *testing.T) {
+	cases := []struct{ src, want string }{
+		// log1p(x) == log(1 + x) in the ordinary range.
+		{`p Math.log1p(3) == Math.log(4)`, "true\n"},
+		{`p Math.expm1(3) == Math.exp(3) - 1`, "true\n"},
+
+		// ⛔ the whole reason these exist.
+		{`p Math.log1p(1e-16) == 0.0`, "false\n"},
+		{`p Math.expm1(1e-16) == 0.0`, "false\n"},
+		// and the naive forms answer 0.0, which is the control for the two above.
+		{`p Math.log(1 + 1e-16)`, "0.0\n"},
+		{`p Math.exp(1e-16) - 1`, "0.0\n"},
+
+		// log1p is real for x >= -1: -1 is -Infinity, which is a value, and
+		// below -1 is Math::DomainError with MRI's exact wording.
+		{`p Math.log1p(-1)`, "-Infinity\n"},
+		{mathRescue(`Math.log1p(-1 - 1e-15)`),
+			"Math::DomainError: Numerical argument is out of domain - log1p\n"},
+		{`p Math.expm1(-1000) == -1.0`, "true\n"},
+
+		// NaN in, NaN out.
+		{`p Math.log1p(Float::NAN).nan?`, "true\n"},
+		{`p Math.expm1(Float::NAN).nan?`, "true\n"},
+
+		// Float() coercion, with MRI's messages.
+		{mathRescue(`Math.log1p("10")`), "TypeError: can't convert String into Float\n"},
+		{mathRescue(`Math.expm1("test")`), "TypeError: can't convert String into Float\n"},
+		{mathRescue(`Math.log1p(nil)`), "TypeError: can't convert nil into Float\n"},
+		{mathRescue(`Math.expm1(nil)`), "TypeError: can't convert nil into Float\n"},
+
+		// ⛔ Arity, which core/math/log1p_spec.rb:27 asserts outright. These two
+		// are strict where the older unary functions are not: `Math.cbrt(9, 3)`
+		// still answers 2.080083823051904 instead of raising, a pre-existing
+		// divergence across the module that cannot be fixed by a blanket check
+		// (`Math.log` takes one OR two arguments) and is tracked on its own. A
+		// new method must not inherit it.
+		{mathRescue(`Math.log1p(9, 3)`),
+			"ArgumentError: wrong number of arguments (given 2, expected 1)\n"},
+		{mathRescue(`Math.expm1(9, 3)`),
+			"ArgumentError: wrong number of arguments (given 2, expected 1)\n"},
+		// ⛔ And with NO argument, where the older helpers do something worse than
+		// being lenient: they index args[0] out of range, and the VM's
+		// panic-to-exception net surfaces the Go runtime error as the Ruby
+		// message. Measured on main:
+		//
+		//	Math.cbrt   -> ArgumentError: cbrt: runtime error: index out of range [0] with length 0
+		//	Math.log1p  -> ArgumentError: wrong number of arguments (given 0, expected 1)
+		//
+		// The class is right by accident; the message is a Go internal leaked to
+		// Ruby code, and it would read differently on another Go version. These
+		// two cases pin MRI's wording so that cannot regress into it.
+		{mathRescue(`Math.log1p`),
+			"ArgumentError: wrong number of arguments (given 0, expected 1)\n"},
+		{mathRescue(`Math.expm1`),
+			"ArgumentError: wrong number of arguments (given 0, expected 1)\n"},
+
+		// module_function: reachable as a private instance method on an includer,
+		// which is the last example of each spec file.
+		{"class IML\n  include Math\nend\np IML.new.send(:log1p, 4.21).round(11)", "1.65057985577\n"},
+		{"class IME\n  include Math\nend\np IME.new.send(:expm1, 23.1415).round(4)", "11226018483.0012\n"},
+	}
+	for _, c := range cases {
+		if got := eval(t, c.src); got != c.want {
+			t.Errorf("src=%q got=%q want=%q", c.src, got, c.want)
+		}
+	}
+}