Fix adding a BigNumber scalar to a number-datatype matrix

src/type/matrix/utils/matAlgo14xDs.js

@@ -33,12 +33,19 @@ export const createMatAlgo14xDs = /* #__PURE__ */ factory(name, dependencies, ({
 
     // process data types
     if (typeof adt === 'string') {
-      // datatype
-      dt = adt
-      // convert b to the same datatype
-      b = typed.convert(b, dt)
-      // callback
-      cf = typed.find(callback, [dt, dt])
+      // Take the datatype fast-path only when the scalar converts into the
+      // matrix's datatype. When it cannot (e.g. a BigNumber scalar and a
+      // `number` matrix), fall through to the generic callback so the values
+      // promote as usual instead of throwing on the conversion.
+      let converted
+      try {
+        converted = typed.convert(b, adt)
+      } catch {}
+      if (converted !== undefined) {
+        b = converted
+        dt = adt
+        cf = typed.find(callback, [dt, dt])
+      }
     }
 
     // populate cdata, iterate through dimensions

test/unit-tests/function/arithmetic/add.test.js

@@ -123,6 +123,13 @@ describe('add', function () {
       assert.deepStrictEqual(a4.size(), [2])
       assert.deepStrictEqual(a4.valueOf(), [math.bignumber(8), math.bignumber(10)])
     })
+
+    it('should promote when a scalar does not fit the matrix datatype (#3612)', function () {
+      const a = math.matrix([[1, 2]], 'dense', 'number')
+      const expected = math.matrix([[math.bignumber(4), math.bignumber(5)]])
+      assert.deepStrictEqual(add(a, math.bignumber(3)), expected)
+      assert.deepStrictEqual(add(math.bignumber(3), a), expected)
+    })
   })
 
   describe('SparseMatrix', function () {