Needs a "don't use truthy types in a conditional/boolean context" diagnostic #2609

How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

Which OS are you using?

Windows

What is the issue affecting?

Type Checking, Diagnostics/Syntax Checking

Expected Behaviour

When variable a has a truthy type (e.g. any type that does not contain boolean or nil), then the following lines should all give warnings, since (assuming the type annotations are correct and Weak Nil Check is off) a cannot be nil or false, and thus can never resolve to false in a boolean context, and so the operations are pointless:

if a then print("1") end  -- will always print 1
if not a then print("2") end  -- will never print 2
local b = a or "3"  -- equivalent to b = a
local c = a and "4" or "5"  -- equivalent to c = "4"

Actual Behaviour

None of those lines warn you about anything under the current diagnostics

Reproduction steps

1. Type the following into a lua file:

---@param a number (or string, table, etc.)
function A(a)
    if a then print("1") end
    if not a then print("2") end
    local b = a or "3"
    local c = a and "4" or "5"
end

2. Observe the lack of diagnostic warnings on each line inside the function

(I would also like a "no-unparameterized-tables" diagnostic where plain table without any <> does not count as a valid type)