Parser doesn't error on invalid variable interpolation syntax

To reproduce:

Playground link: https://lesscss.org/less-preview/#eyJjb2RlIjoiLmZvbyB7XG4gIEBmaW5rOiAjZmZmO1xuICBjb2xvcjogJ0B7ZGFya2VuKEBmaW5rLCA1MCUpfSc7XG59IiwiYWN0aXZlVmVyc2lvbiI6IjQuMy4wIiwibWF0aCI6InBhcmVucy1kaXZpc2lvbiIsInN0cmljdFVuaXRzIjpmYWxzZX0=

.foo {
  @fink: #fff;
  color: '@{darken(@fink, 50%)}';
}

Current behavior:

Compiles to the following with no errors:

.foo {
  color: '@{darken(@fink, 50%)}';
}

The value of .foo's color property is not interpolated with the value of @fink and is instead the exact same string as the input, down to keeping the @{} interpolation syntax.

Expected behavior:

A parse error on line 3, color: '@{darken(@fink, 50%)}';, given that invoking a function within the variable interpolation syntax is invalid.

[!NOTE]
To do what this attempts to do, you must invoke the function separately in a variable and then interpolate that variable:

.bar {
  @fink: #fff;
  @bink: darken(@fink, 50%);
  color: '@{bink}';
}

(Playground: https://lesscss.org/less-preview/#eyJjb2RlIjoiLmJhciB7XG4gIEBmaW5rOiAjZmZmO1xuICBAYmluazogZGFya2VuKEBmaW5rLCA1MCUpO1xuICBjb2xvcjogJ0B7Ymlua30nO1xufSIsImFjdGl2ZVZlcnNpb24iOiI0LjMuMCIsIm1hdGgiOiJwYXJlbnMtZGl2aXNpb24iLCJzdHJpY3RVbml0cyI6ZmFsc2V9)

Environment information:

• less version: v4.3.0
• nodejs version: n/a
• operating system: n/a

Reproduce the behavior using the linked Less playground and the provided interpolation example. Trace how the parser handles function calls inside variable interpolation; done means the invalid syntax raises a parse error on line 3 while the separately assigned variable example continues to compile correctly.