fix(issue:3777) operation on invalid type resolved

* Fixes issue less#3777 where using CSS custom property in an operation
  resulted in a parsing error.

packages/less/src/less/parser/parser.js

@@ -796,6 +796,18 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
                 if (parserInput.currentChar() === '@' && (name = parserInput.$re(/^(@[\w-]+)\s*:/))) { return name[1]; }
             },
 
+            //
+            // The custom property part of a variable definition.
+            //
+            //     --fink:
+            //
+            customProperty: function () {
+                var name;
+                if (parserInput.currentChar() === '-' && (name = parserInput.$re(/^(--[\w-]+)\s*:/))) {
+                    return name[1];
+                }
+            },
+
             //
             // Call a variable value to retrieve a detached ruleset
             // or a value from a detached ruleset's rules.
@@ -1578,7 +1590,7 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
 
                 parserInput.save();
 
-                name = this.variable() || this.ruleProperty();
+                name = this.variable() || this.customProperty() || this.ruleProperty();
                 if (name) {
                     isVariable = typeof name === 'string';
 
@@ -1597,7 +1609,7 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
                         merge = !isVariable && name.length > 1 && name.pop().value;
 
                         // Custom property values get permissive parsing
-                        if (name[0].value && name[0].value.slice(0, 2) === '--') {
+                        if (isVariable && name.startsWith('--')) {
                             if (parserInput.$char(';')) {
                                 value = new Anonymous('');
                             } else {

packages/less/src/less/tree/operation.js

@@ -2,6 +2,8 @@ import Node from './node';
 import Color from './color';
 import Dimension from './dimension';
 import * as Constants from '../constants';
+import Variable from './variable';
+import Call from './call';
 const MATH = Constants.Math;
 
 
@@ -18,8 +20,51 @@ Operation.prototype = Object.assign(new Node(), {
         this.operands = visitor.visitArray(this.operands);
     },
 
+    find: function (obj, fun) {
+        for (var i_2 = 0, r = void 0; i_2 < obj.length; i_2++) {
+            r = fun.call(obj, obj[i_2]);
+            if (r) {
+                return r;
+            }
+        }
+        return null;
+    },
+
+    evalVariable: function (context, operand) {
+        if (operand.name === 'var' && operand.args.length === 1) {
+            var varName = operand.args[0].toCSS();
+            var variable = this.find(context.frames, function (frame) {
+                var v = frame.variable(varName);
+                if (v) {
+                    if (v.important) {
+                        var importantScope = context.importantScope[context.importantScope.length - 1];
+                        importantScope.important = v.important;
+                    }
+                    // If in calc, wrap vars in a function call to cascade evaluate args first
+                    if (context.inCalc) {
+                        return (new Call('_SELF', [v.value])).eval(context);
+                    }
+                    else {
+                        return v.value.eval(context);
+                    }
+                }
+            });
+            if (variable) {
+                return variable;
+            }
+        }
+    },
+
     eval(context) {
-        let a = this.operands[0].eval(context), b = this.operands[1].eval(context), op;
+        var a = this.evalVariable(context, this.operands[0])
+        if (!a) {
+            a = this.operands[0].eval(context)
+        }
+        var b = this.evalVariable(context, this.operands[1]);
+        if (!b) {
+            b = this.operands[1].eval(context);
+        }
+        var op;
 
         if (context.isMathOn(this.op)) {
             op = this.op === './' ? '/' : this.op;
@@ -29,6 +74,18 @@ Operation.prototype = Object.assign(new Node(), {
             if (b instanceof Dimension && a instanceof Color) {
                 b = b.toColor();
             }
+            if (a instanceof Dimension && b instanceof Call && b.name === 'var') {
+                if (b.args && b.args.length === 1) {
+                    b = new Variable(b.args[0].toCSS(), 0, {});
+                    return a.operate(context, op, b);
+                }
+            }
+            if (b instanceof Dimension && a instanceof Call && a.name === 'var') {
+                if (a.args && a.args.length === 1) {
+                    a = new Variable(a.args[0].toCSS(), 0, {});
+                    return b.operate(context, op, a);
+                }
+            }
             if (!a.operate || !b.operate) {
                 if (
                     (a instanceof Operation || b instanceof Operation)

packages/less/src/less/tree/ruleset.js

@@ -286,7 +286,7 @@ Ruleset.prototype = Object.assign(new Node(), {
     variables() {
         if (!this._variables) {
             this._variables = !this.rules ? {} : this.rules.reduce(function (hash, r) {
-                if (r instanceof Declaration && r.variable === true) {
+                if (r instanceof Declaration && (r.variable === true  || (typeof r.name ==='string' && r.name.startsWith('--')))) {
                     hash[r.name] = r;
                 }
                 // when evaluating variables in an import statement, imports have not been eval'd

packages/test-data/css/_main/custom-property.css

@@ -2,3 +2,7 @@
   --basic-deg: 20deg;
   --basic-deg-tan: tan(var(--basic-deg));
 }
+.test2 {
+  --some-var: 5px;
+  prop: min(100% - var(--some-var), 10px);
+}

packages/test-data/less/_main/custom-property.less

@@ -2,3 +2,8 @@
     --basic-deg: 20deg;
     --basic-deg-tan: tan(var(--basic-deg));
 }
+
+.test2 {
+    --some-var: 5px;
+    prop: min(100% - var(--some-var), 10px);
+}