From adb1131d08f28f0bc5f729e88b73cf22846c86c5 Mon Sep 17 00:00:00 2001
From: Tim Foley <tfoleyNV@users.noreply.github.com>
Date: Fri, 5 Feb 2021 09:01:36 -0800
Subject: Initial implementation of interface conjunctions (#1691)

The basic feature here is the ability to use the `&` operator to produce the conjunction/intersection of two interfaces. That is, you can have interfaces:

    interface IFirst { int getFirst(); }
    interface ISecond { int getSecoond(); }

and if you need a generic function where the type parameter `T` must conform to *both* of these interfaces, you express that by constraining the parameter to the intersection of the interfaces:

    void someFunction<T : IFirst & ISecond>(T value) { ... }

Without this feature, the main alternative an application would have is to define an intermediate interface, like:

    interface IBoth : IFirst, ISecond {}

Forcing users to deal with an intermediate interface creates more work for type authors (they need to remember to inherit from the right combined interface(s)), or for `extension` authors (when you add `ISecond` to a type that used to just support `IFirst`, you had better also add `IBoth`). In the worst case, a family of N related "leaf" interfaces would give rise to an exponential number of intermediate interfaces to represnt the possible combinations.

A conjunction like `IFirst & ISecond` is officially its own type, and can be used to declare a type alias:

    typealias IBoth = IFirst & ISecond;

This change only includes the first pass of work on this feature, so there are several caveats to be aware of:

* Using a conjunction as part of an inheritance clause is not yet supported (e.g., `struct X : IFirst & ISecond`). This is true even if the conjunction was introduced by an intermediate `typealias`

* The `&` syntax introduced here is only parsed in places where only a type (not an expression) is possible. This means you cannot do things like cast to a conjunction with `(IFirst & ISecond)(someValue)`.

* This work *should* apply to conjunctions of more than two interfaces (like `IA & IB & IC`) but that has not yet been tested

* In the long run it may be sensible to allow conjunctions that use concrete types, but we really ought to have the semantic checking logic rule that out for now.

* During testing, I encountered compiler crashes when trying to use this feature together with `property` declarations. Further investigation and debugging is called for.

* The handling of conjunction types is currently incomplete, in that there are many equivalences the compiler does not yet understand. For example, it is clear that `IA & IB` is equivalent to `IB & IA`, but the compiler currently does not understand this and will treat them as different types. A deeper implementation approach is called for.

* Conjunctions are currently only supported for generic type parameter constraints, when performing full specialization. Use of conjunctions for existential-type value parameters or with dynamic dispatch is not yet supported.
---
 source/slang/slang-ir.cpp | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

(limited to 'source/slang/slang-ir.cpp')

diff --git a/source/slang/slang-ir.cpp b/source/slang/slang-ir.cpp
index 7e84ca66a..313e48502 100644
--- a/source/slang/slang-ir.cpp
+++ b/source/slang/slang-ir.cpp
@@ -2774,6 +2774,14 @@ namespace Slang
         return getType(kIROp_PseudoPtrType, SLANG_COUNT_OF(operands), operands);
     }
 
+    IRType* IRBuilder::getConjunctionType(
+        UInt            typeCount,
+        IRType* const*  types)
+    {
+        return getType(kIROp_ConjunctionType, typeCount, (IRInst* const*)types);
+    }
+
+
 
     void IRBuilder::setDataType(IRInst* inst, IRType* dataType)
     {
@@ -3043,8 +3051,30 @@ namespace Slang
         return emitIntrinsicInst(type, kIROp_MakeTuple, count, args);
     }
 
+    IRInst* IRBuilder::emitMakeTuple(UInt count, IRInst* const* args)
+    {
+        List<IRType*> types;
+        for(UInt i = 0; i < count; ++i)
+            types.add(args[i]->getFullType());
+
+        auto type = getTupleType(types);
+        return emitMakeTuple(type, count, args);
+    }
+
     IRInst* IRBuilder::emitGetTupleElement(IRType* type, IRInst* tuple, UInt element)
     {
+        // As a quick simplification/optimization, if the user requests
+        // `getTupleElement(makeTuple(a_0, a_1, ... a_N), i)` then we should
+        // just return `a_i`, provided that the index is properly in range.
+        //
+        if( auto makeTuple = as<IRMakeTuple>(tuple) )
+        {
+            if( element < makeTuple->getOperandCount() )
+            {
+                return makeTuple->getOperand(element);
+            }
+        }
+
         IRInst* args[] = { tuple, getIntValue(getIntType(), element) };
         return emitIntrinsicInst(type, kIROp_GetTupleElement, 2, args);
     }
-- 
cgit v1.2.3