manual: Document the use of static as a proc call

doc/manual.rst

@@ -2975,6 +2975,20 @@ Even some code that has side effects is permitted in a static block:
   static:
     echo "echo at compile time"
 
+`static` can also be used like a routine.
+
+.. code-block:: nim
+
+  proc getNum(a: int): int = a
+
+  # Below calls "echo getNum(123)" at compile time.
+  static:
+    echo getNum(123)
+
+  # Below call evaluates the "getNum(123)" at compile time, but its
+  # result gets used at run time.
+  echo static(getNum(123))
+
 There are limitations on what Nim code can be executed at compile time;
 see `Restrictions on Compile-Time Execution
 <#restrictions-on-compileminustime-execution>`_ for details.

tests/system/tstatic_callable.nim

@@ -0,0 +1,12 @@
+# bug #16987
+
+proc getNum(a: int): int = a
+
+# Below calls "doAssert getNum(123) == 123" at compile time.
+static:
+  doAssert getNum(123) == 123
+
+# Below calls evaluate the "getNum(123)" at compile time, but the
+# results of those calls get used at run time.
+doAssert (static getNum(123)) == 123
+doAssert (static(getNum(123))) == 123

tests/system/tstatic_callable_error.nim

@@ -0,0 +1,14 @@
+# bug #16987
+
+discard """
+errormsg: "cannot evaluate at compile time: inp"
+nimout: '''
+tstatic_callable_error.nim(14, 21) Error: cannot evaluate at compile time: inp'''
+"""
+
+
+# line 10
+proc getNum(a: int): int = a
+
+let inp = 123
+echo (static getNum(inp))