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Fix try/catch/finally flow#1729

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Perryvw merged 3 commits into
TypeScriptToLua:masterfrom
SuperPaintman:1137-fix-try-catch-finally
Jun 30, 2026
Merged

Fix try/catch/finally flow#1729
Perryvw merged 3 commits into
TypeScriptToLua:masterfrom
SuperPaintman:1137-fix-try-catch-finally

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@SuperPaintman

@SuperPaintman SuperPaintman commented Jun 28, 2026 •
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TL;DR

This PR fixes the issue described in #1137.

try/catch/finally blocks didn't behave correctly when the catch block threw (or re-threw) an error. The finally block would never execute because the error from catch propagated immediately.

Example:

InputOutput (-before, +after)
 
declare function compute(this: void): void

function foo() {
    try {
        compute()
    } catch (err) {
        print("Reporting the error ...")
        throw err;
    } finally {
        print("Cleaning up resources...")
    }
}
 
 function foo(self)
     do
         local function ____catch(err)
             print("Reporting the error ...")
             error(err, 0)
         end
         local ____try, ____hasReturned = pcall(function()
             compute()
         end)
+        local ____rethrow
         if not ____try then
-            ____catch(____hasReturned)
+            ____try, ____rethrow = pcall(function()
+                ____catch(____hasReturned)
+            end)
         end
         do
             print("Cleaning up resources...")
         end
+        if not ____try then
+            error(____rethrow, 0)
+        end
     end
 end

Logs (-before, +after)

 Reporting the error ...
+Cleaning up resources...
 stdin:8: attempt to call global 'compute' (a nil value)
 stack traceback:
 	[C]: in function 'error'
-	stdin:5: in function '____catch'
-	stdin:11: in function 'foo'
+	stdin:20: in function 'foo'
 	stdin:1: in main chunk

How the fix works

Note: This change only affects try/catch/finally blocks. try/catch and try/finally are unchanged.

We capture any errors from catch block into a ____rethrow variable, then let the finally execute, and only then re-throw the error.

A few more examples

From unit tests (was broken) ("re-throw (%p)") 
--- ./testdata-old/testcase-0002.lua
+++ ./testdata/testcase-0002.lua
@@ -1,55 +1,67 @@
 local ____exports = {}
 function ____exports.__main(self)
     local innerTry = false
     local innerFinally = false
     local outerFinally = false
     local function foo(self)
         do
             local function ____catch(e)
                 error((e .. "->") .. "outer.catch", 0)
             end
             local ____try, ____hasReturned = pcall(function()
                 do
                     local function ____catch(e)
                         error((e .. "->") .. "inner.catch", 0)
                     end
                     local ____try, ____hasReturned = pcall(function()
                         if innerTry then
                             error("inner.try", 0)
                         end
                     end)
+                    local ____rethrow
                     if not ____try then
-                        ____catch(____hasReturned)
+                        ____try, ____rethrow = pcall(function()
+                            ____catch(____hasReturned)
+                        end)
                     end
                     do
                         if innerFinally then
                             error("inner.finally", 0)
                         end
                     end
+                    if not ____try then
+                        error(____rethrow, 0)
+                    end
                 end
             end)
+            local ____rethrow
             if not ____try then
-                ____catch(____hasReturned)
+                ____try, ____rethrow = pcall(function()
+                    ____catch(____hasReturned)
+                end)
             end
             do
                 if outerFinally then
                     error("outer.finally", 0)
                 end
             end
+            if not ____try then
+                error(____rethrow, 0)
+            end
         end
     end
     local result = "x"
     do
         local function ____catch(e)
             result = e
         end
         local ____try, ____hasReturned = pcall(function()
             foo(nil)
         end)
         if not ____try then
             ____catch(____hasReturned)
         end
     end
     return result
 end
 return ____exports
From unit tests ("return from catch->finally") 
--- ./testdata-old/testcase-0020.lua
+++ ./testdata/testcase-0020.lua
@@ -1,29 +1,35 @@
 local ____exports = {}
 function ____exports.__main(self)
     local x = "unevaluated"
     local function evaluate(self, arg)
         x = "evaluated"
         return arg
     end
     local function foobar(self)
         do
             local function ____catch(e)
                 return true, evaluate(nil, e)
             end
             local ____try, ____hasReturned, ____returnValue = pcall(function()
                 error("foobar", 0)
             end)
+            local ____rethrow
             if not ____try then
-                ____hasReturned, ____returnValue = ____catch(____hasReturned)
+                ____try, ____rethrow = pcall(function()
+                    ____hasReturned, ____returnValue = ____catch(____hasReturned)
+                end)
             end
             do
                 return "finally"
             end
+            if not ____try then
+                error(____rethrow, 0)
+            end
             if ____hasReturned then
                 return ____returnValue
             end
         end
     end
     return (tostring(foobar(nil)) .. " ") .. x
 end
 return ____exports

Resources

Closes: #1137

@SuperPaintman SuperPaintman marked this pull request as ready for review June 28, 2026 05:11
@SuperPaintman

SuperPaintman commented Jun 28, 2026

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CC: @Perryvw as the author of #1137

Perryvw
Perryvw approved these changes Jun 28, 2026
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@Perryvw Perryvw left a comment

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Looks fine, just needs a npm run fix:prettier

@SuperPaintman

SuperPaintman commented Jun 29, 2026

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@Perryvw should be formatted now

@Perryvw Perryvw merged commit b248e41 into TypeScriptToLua:master Jun 30, 2026
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@Perryvw Perryvw Perryvw approved these changes

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try/finally does not behave as expected when rethrowing

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@SuperPaintman @Perryvw