Report error for function declarations as statement children in strict mode

[thromel]

Fixes #62896

Problem

In strict mode, function declarations can only appear at the top level of a script, module, or function body, or inside a block. Code like if (true) function f() {} is a syntax error in strict mode, and JavaScript engines throw:

"In strict mode code, functions can only be declared at top level or inside a block."

However, TypeScript was not reporting this error, allowing invalid code to pass compilation.

"use strict";
if (true) function f() {}
f() // Runtime error: In strict mode code, functions can only be declared at top level or inside a block

Fix

Added a check in the binder to detect when a function declaration is a direct child of a statement node and report error TS1256 in strict mode.

The affected statement types are:

• IfStatement
• WhileStatement
• DoStatement
• ForStatement
• ForInStatement
• ForOfStatement
• WithStatement
• LabeledStatement

Examples

// Errors in strict mode (TS1256)
if (true) function f1() {}
while (true) function f2() {}
do function f3() {} while (false);
for (;;) function f4() {}
for (let x in {}) function f5() {}
for (let x of []) function f6() {}
label: function f7() {}

// Valid - function inside a block
if (true) { function g() {} }

// Valid - top level
function topLevel() {}

Test

Added tests/cases/compiler/functionDeclarationAsStatementInStrictMode.ts covering all error and valid cases.

[thromel]

@RyanCavanaugh In the issue you mentioned "now that we assume strict mode at all times, this can be a proper parse error."

I've implemented this to report unconditionally (regardless of the inStrictMode flag in the binder), since TypeScript assumes strict mode at all times. However, I noticed other similar checks like with statements still check inStrictMode:

function checkStrictModeWithStatement(node: WithStatement) {
    if (inStrictMode) {
        errorOnFirstToken(node, Diagnostics.with_statements_are_not_allowed_in_strict_mode);
    }
}

Could you elaborate on whether this new check should:

1. Report unconditionally (current implementation) - treating it as a parse error regardless of strict mode context
2. Only report when inStrictMode is true - consistent with other strict mode checks

Happy to adjust based on your guidance!

[jakebailey]

since TypeScript assumes strict mode at all times

This is not actually the case. I don't think we should do this until 7.0.

[jakebailey]

Hm, no: #54500

Probably should only report when isStrictMode is true, but we will just remove that option in a later PR (or, earlier?)

[thromel]

@jakebailey Thanks for the clarification! I've updated the PR to only report the error when inStrictMode is true, consistent with other strict mode checks.