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fix: report error when generic method attempts to implement non-generic interface method #3007

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fix: report error when generic method attempts to implement non-generic interface method #3007

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6fce518
fix: report error when generic method attempts to implement non-gener…
Changqing-JING Apr 3, 2026
c1f84f8
Fix
Changqing-JING Apr 3, 2026
fae279c
Add test
Changqing-JING Apr 3, 2026
dda9bbc
add testcase
Changqing-JING Apr 3, 2026
54dd10a
forbidden interface function and generic function to be generic
Changqing-JING Apr 7, 2026
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22 changes: 21 additions & 1 deletion src/resolver.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -3063,7 +3063,14 @@ export class Resolver extends DiagnosticEmitter {
let boundPrototype = classInstance.getMember(unboundOverridePrototype.name);
if (boundPrototype) { // might have errored earlier and wasn't added
assert(boundPrototype.kind == ElementKind.FunctionPrototype);
overrideInstance = this.resolveFunction(<FunctionPrototype>boundPrototype, instance.typeArguments);
let boundFuncPrototype = <FunctionPrototype>boundPrototype;
let overrideTypeParams = boundFuncPrototype.typeParameterNodes;
let numOverrideTypeParams = overrideTypeParams ? overrideTypeParams.length : 0;
let baseTypeArgs = instance.typeArguments;
let numBaseTypeArgs = baseTypeArgs ? baseTypeArgs.length : 0;
if (numOverrideTypeParams == numBaseTypeArgs) {
overrideInstance = this.resolveFunction(boundFuncPrototype, baseTypeArgs);
}
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@HerrCai0907 HerrCai0907 Apr 3, 2026

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should we allow this implement when the generic number are the same? I think it will crash also if the typing mismatched.
IMO, maybe we should forbidden all generic function in interface?

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@MaxGraey MaxGraey Apr 3, 2026

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Hmm, I see only one edge case in this scenario:

interface I {
  add<T, U>(x: T, y: U): U;
}

class C2 implements I {
  add<U, T>(x: T, y: U): U {
    return x;
  }
}

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@Changqing-JING Changqing-JING Apr 4, 2026 •
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Good point. If we follow C++ design:

  1. template function aka. generic function can't be member in interface
  2. template function also can't override virtual function in interface.

Shall we do the same?

I would prefer to follow C++ design because generic function in interface is a confused behaviour
But it may break some existing code on user side.
What do maintainers prefer?

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@Changqing-JING Changqing-JING Apr 7, 2026 •
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@HerrCai0907 @MaxGraey

Dear maintainers,

I have pushed a new commit to forbidden interface function and abstract function to be generic.
Could you check this design concept.

By the way, I just awared that if base class and child class one is generic, the other one is non-generic, then there is another crash. But I would prefer to fix it in another PR. I just left a fix me in current PR.

var called: string = "";

class A {
  foo<T>(x: T): void {
    called = "A";
  }
}

class B extends A {
  foo(x: i32): void { // non-generic overrides generic
    called = "B";
  }
}

// Static dispatch: B.foo called directly
var b = new B();
b.foo(1);
assert(called == "B");
called = "";

// Virtual dispatch through A reference: which gets called?
var a: A = new B();
a.foo<i32>(1);
assert(called == "B"); // if vtable works, "B"; if fallback to base, "A"

}
}
if (overrideInstance) overrides.add(overrideInstance);
Expand Down Expand Up @@ -3439,6 +3446,19 @@ export class Resolver extends DiagnosticEmitter {
default: assert(false);
}
if (!member.is(CommonFlags.Abstract)) {
// A generic method cannot implement a non-generic interface method
// because monomorphization requires a concrete type to generate code,
// but virtual dispatch through the interface has no type arguments.
if (unimplemented.has(memberName)
&& member.kind == ElementKind.FunctionPrototype
&& unimplemented.get(memberName)!.kind == ElementKind.FunctionPrototype
) {
let memberTypeParams = (<FunctionPrototype>member).typeParameterNodes;
let ifaceTypeParams = (<FunctionPrototype>unimplemented.get(memberName)).typeParameterNodes;
let numMemberTypeParams = memberTypeParams ? memberTypeParams.length : 0;
let numIfaceTypeParams = ifaceTypeParams ? ifaceTypeParams.length : 0;
if (numMemberTypeParams != numIfaceTypeParams) continue;
}
unimplemented.delete(memberName);
}
}
Expand Down
7 changes: 7 additions & 0 deletions tests/compiler/override-typeparam-mismatch.json
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@@ -0,0 +1,7 @@
{
"asc_flags": [],
"stderr": [
"TS2515: Non-abstract class 'override-typeparam-mismatch/CC' does not implement inherited abstract member 'foo' from 'override-typeparam-mismatch/I'.",
"EOF"
]
}
14 changes: 14 additions & 0 deletions tests/compiler/override-typeparam-mismatch.ts
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Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
interface I {
foo(x: i32): i32;
}

class CC implements I {
foo<T>(x: i32): i32 {
return x;
}
}

let c:I = new CC();
c.foo(1);

ERROR("EOF");
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