Fix Integrate returning unevaluated when integrand is constant w.r.t. variable

Add a general is_constant_wrt check at the top of integrate() so that
expressions like Integrate[x*x, y] correctly return x^2*y instead of
the unevaluated Integrate[x^2, y].

Author: ad-si

src/functions/calculus_ast.rs

@@ -1841,6 +1841,16 @@ fn gcd_i128_local(a: i128, b: i128) -> i128 {
 
 /// Integrate an expression with respect to a variable
 fn integrate(expr: &Expr, var: &str) -> Option<Expr> {
+  // General constant check: ∫ c dy = c*y for any expression c independent of y
+  // (handles compound expressions like x^2, Sin[x], etc. when integrating w.r.t. a different variable)
+  if is_constant_wrt(expr, var) {
+    return Some(Expr::BinaryOp {
+      op: crate::syntax::BinaryOperator::Times,
+      left: Box::new(expr.clone()),
+      right: Box::new(Expr::Identifier(var.to_string())),
+    });
+  }
+
   match expr {
     // Constant: ∫ c dx = c*x
     Expr::Integer(n) => Some(Expr::BinaryOp {

tests/interpreter_tests/calculus.rs

@@ -3,6 +3,14 @@ use super::*;
 mod integrate_with_sum {
   use super::*;
 
+  #[test]
+  fn integrate_constant_wrt_other_var() {
+    assert_eq!(interpret("Integrate[x*x, y]").unwrap(), "x^2*y");
+    assert_eq!(interpret("Integrate[Sin[x], y]").unwrap(), "y*Sin[x]");
+    assert_eq!(interpret("Integrate[Log[x], y]").unwrap(), "y*Log[x]");
+    assert_eq!(interpret("Integrate[x^2 + y, y]").unwrap(), "x^2*y + y^2/2");
+  }
+
   #[test]
   fn integrate_polynomial() {
     assert_eq!(interpret("Integrate[x^2, x]").unwrap(), "x^3/3");