[ASDAlexander77]

Problem

A value that can't be an interface, i.e. a number, string, boolean or array, compiled without any error when passed where an interface with members is expected:

interface IKind { kind: number; }
function f(i: IKind) { return i.kind; }
print(f([1, 2]));   // compiled; f read garbage
print(f(5));        // same

TypeScript rejects all of these. docs/bugs/array_of_ifaces_cast.ts exists for this case: it says that passing IStateToken[] for an interface parameter "should produce an error". tslang never reported it. That file fails earlier on unrelated errors, but a cleaned-up version compiled, with at most <unknown>:0: warning: invalid cast from '!llvm.ptr' to '!llvm.struct<(ptr, ptr, ptr)>'.

Cause

castToInterfaceSpecialCases handles classes, tuples and objects. For anything else it returned nothing, so the value fell through to a plain ts.Cast to the interface type. Lowering can't build an interface value from an array or a scalar, so it produced garbage.

Fix

For a numeric type (any width), number, string, boolean, char, bigint, an array or a const array cast to an interface that has members (getVTableSize() > 0), report:

error: type 's32[]' can't be cast to interface 'IKind': it has none of the interface's members

An interface without members still accepts any value, as in TypeScript.

Testing

• New error-text tests test-compile-interface-cast-{number,string,boolean,array} (each requires the message), plus test-compile-interface-cast-empty, which must compile cleanly. The current release compiles all five without an error.
• Full ctest: 3918/3918 pass.

🤖 Generated with Claude Code