[stable] Don't merge function types with unknown return type

[kinke]

As that can cause a 'compatible' function type with resolved return type to wrongly get that deco. This is what happens with LDC for a particular test case as D v2.111 regression, causing an assertion in the frontend while generating a TypeInfo_Function for some TypeFunction, with TypeFunction.next being null (unknown return type).

The relevant genTypeInfo() calls before the patch, in the format genTypeInfo: <type> (<deco>), <type after merge2()> (<deco after merge2()>):

// const funcptr: TypeInfo_Const
genTypeInfo: const(Algebraic!() function(Function) nothrow @system) (xPFNbC4util8FunctionZSQq__T9AlgebraicZQl), const(Algebraic!() function(Function) nothrow @system) (xPFNbC4util8FunctionZSQq__T9AlgebraicZQl)
// mutable funcptr: TypeInfo_Pointer
genTypeInfo: Algebraic!() function(Function) nothrow @system (PFNbC4util8FunctionZSQq__T9AlgebraicZQl), Algebraic!() function(Function) nothrow @system (PFNbC4util8FunctionZSQq__T9AlgebraicZQl)
// function: TypeInfo_Function
// NOTE the missing Algebraic return type in the decos, this is the bug
genTypeInfo: nothrow @system Algebraic!()(Function) (FNbC4util8FunctionZ), nothrow @system (Function) (FNbC4util8FunctionZ)

With this patch:

genTypeInfo: const(Algebraic!() function(Function) nothrow @system) (xPFNbC4util8FunctionZSQq__T9AlgebraicZQl), const(Algebraic!() function(Function) nothrow @system) (xPFNbC4util8FunctionZSQq__T9AlgebraicZQl)
genTypeInfo: Algebraic!() function(Function) nothrow @system (PFNbC4util8FunctionZSQq__T9AlgebraicZQl), Algebraic!() function(Function) nothrow @system (PFNbC4util8FunctionZSQq__T9AlgebraicZQl)
genTypeInfo: nothrow @system Algebraic!()(Function) (FNbC4util8FunctionZSQq__T9AlgebraicZQl), nothrow @system Algebraic!()(Function) (FNbC4util8FunctionZSQq__T9AlgebraicZQl)

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + dmd#21350"

[kinke]

Unfortunately I wasn't able to quickly put together a testcase that would fail with DMD too.

@rainers: You're at least somewhat familiar with that type-merging stuff, right? Does the patch seem reasonable to you?

[rainers]

Yeah, I agree it's not reasonable to merge incomplete types. Not sure if the actual error is not somewhere earlier that allows such a function type with next being null to exist.