Minimal optional constraints (#7422)

* Parse optional witness syntax

* Allow failing optional constraint

* Make `is` work with optional constraint

* Allow using optional constraint in checked if statements

* Fix tests

* Make it work with structs

* Fix MSVC build error

* Disallow using `as` with optional constraints

* Update test to match split is/as errors

* Add tests

* Fix uninitialized variables in tests

* Add tests of incorrect uses & fix related bugs

* Mention optional constraints in docs

* format code

* Fix type unification with NoneWitness

* Fix formatting

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>
Co-authored-by: Nathan V. Morrical <natemorrical@gmail.com>

6 files changed, 203 insertions, 2 deletions

diff --git a/tests/language-feature/generics/where-optional-1.slang b/tests/language-feature/generics/where-optional-1.slang
new file mode 100644
index 000000000..da4bdaacb
--- /dev/null
+++ b/tests/language-feature/generics/where-optional-1.slang
@@ -0,0 +1,11 @@
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK):
+interface IThing
+{
+    void thing();
+}
+
+void f<T>(T t) where optional T: IThing
+{
+    // Unchecked optional constraint is an error.
+    t.thing(); // CHECK: error 30403
+}
diff --git a/tests/language-feature/generics/where-optional-2.slang b/tests/language-feature/generics/where-optional-2.slang
new file mode 100644
index 000000000..67679bd8d
--- /dev/null
+++ b/tests/language-feature/generics/where-optional-2.slang
@@ -0,0 +1,63 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -cpu -shaderobj
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0 0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer;
+
+interface IThing
+{
+    [mutating]
+    int thing(int index);
+}
+
+struct MyThing: IThing
+{
+    int val;
+
+    [mutating]
+    int thing(int index)
+    {
+        val++;
+        outputBuffer[index] = val;
+        return val;
+    }
+}
+
+struct NotMyThing
+{
+    int val;
+}
+
+void f<T>(inout T t, int index) where optional T: IThing
+{
+    if (T is IThing)
+    {
+        outputBuffer[index+1] = 2 * t.thing(index);
+    }
+    else
+    {
+        outputBuffer[index] = 0;
+        outputBuffer[index+1] = 0;
+    }
+}
+
+[numthreads(1, 1, 1)]
+void computeMain(int3 dispatchThreadID: SV_DispatchThreadID)
+{
+    MyThing mt = MyThing(0);
+    NotMyThing nt = NotMyThing(1);
+
+    // CHECK: 1
+    // CHECK-NEXT: 2
+    f<MyThing>(mt, 0);
+    // CHECK-NEXT: 0
+    // CHECK-NEXT: 0
+    f<NotMyThing>(nt, 2);
+    // CHECK: 2
+    // CHECK-NEXT: 4
+    f(mt, 4);
+    // CHECK-NEXT: 0
+    // CHECK-NEXT: 0
+    f(nt, 6);
+}
diff --git a/tests/language-feature/generics/where-optional-3.slang b/tests/language-feature/generics/where-optional-3.slang
new file mode 100644
index 000000000..f8b3a4907
--- /dev/null
+++ b/tests/language-feature/generics/where-optional-3.slang
@@ -0,0 +1,95 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -cpu -shaderobj
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0 0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<int> outputBuffer;
+
+interface IReleaseable
+{
+    [mutating]
+    void release();
+}
+
+struct Container<K, V>
+    where optional K : IReleaseable
+    where optional V : IReleaseable
+{
+    K k[2];
+    V v[2];
+
+    [mutating]
+    void erase(int index)
+    {
+        if (K is IReleaseable)
+            k[index].release();
+        if (V is IReleaseable)
+            v[index].release();
+    }
+}
+
+struct HeavyEntry: IReleaseable
+{
+    int index;
+    int value;
+
+    [mutating]
+    void release()
+    {
+        outputBuffer[index] = value;
+    }
+};
+
+struct LightEntry
+{
+    int value;
+};
+
+[numthreads(1, 1, 1)]
+void computeMain(int3 dispatchThreadID: SV_DispatchThreadID)
+{
+    { // Neither is IReleaseable
+        var c = Container<LightEntry, LightEntry>();
+        c.k[0] = LightEntry(1);
+        c.k[1] = LightEntry(2);
+        c.v[0] = LightEntry(3);
+        c.v[1] = LightEntry(4);
+        c.erase(0);
+        c.erase(1);
+    }
+    { // K is IReleaseable
+        var c = Container<HeavyEntry, LightEntry>();
+        c.k[0] = HeavyEntry(0,1);
+        c.k[1] = HeavyEntry(1,2);
+        c.v[0] = LightEntry(3);
+        c.v[1] = LightEntry(4);
+        // CHECK: 1
+        c.erase(0);
+        // CHECK-NEXT: 2
+        c.erase(1);
+    }
+    { // V is IReleaseable
+        var c = Container<LightEntry, HeavyEntry>();
+        c.k[0] = LightEntry(1);
+        c.k[1] = LightEntry(2);
+        c.v[0] = HeavyEntry(2,3);
+        c.v[1] = HeavyEntry(3,4);
+        // CHECK-NEXT: 3
+        c.erase(0);
+        // CHECK-NEXT: 4
+        c.erase(1);
+    }
+    { // K and V are IReleaseable
+        var c = Container<HeavyEntry, HeavyEntry>();
+        c.k[0] = HeavyEntry(4,5);
+        c.k[1] = HeavyEntry(6,7);
+        c.v[0] = HeavyEntry(5,6);
+        c.v[1] = HeavyEntry(7,8);
+        // CHECK-NEXT: 5
+        // CHECK-NEXT: 6
+        c.erase(0);
+        // CHECK-NEXT: 7
+        // CHECK-NEXT: 8
+        c.erase(1);
+    }
+}
diff --git a/tests/language-feature/generics/where-optional-4.slang b/tests/language-feature/generics/where-optional-4.slang
new file mode 100644
index 000000000..6d72186d9
--- /dev/null
+++ b/tests/language-feature/generics/where-optional-4.slang
@@ -0,0 +1,15 @@
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK):
+interface IThing
+{
+    void thing();
+}
+
+void g<T>(T t) where T: IThing
+{
+}
+
+void f<T>(T t) where optional T: IThing
+{
+    // Error: cannot upgrade optional to non-optional witness in unchecked context.
+    g<T>(t); // CHECK: error 38029
+}
diff --git a/tests/language-feature/generics/where-optional-5.slang b/tests/language-feature/generics/where-optional-5.slang
new file mode 100644
index 000000000..3ce8041ad
--- /dev/null
+++ b/tests/language-feature/generics/where-optional-5.slang
@@ -0,0 +1,17 @@
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK):
+interface IThing
+{
+    void thing();
+}
+
+void f<T, U>(T t, U u)
+    where optional T: IThing
+    where optional U: IThing
+{
+    // Error: cannot upgrade optional to non-optional witness in unchecked context.
+    if (U is IThing)
+    {
+        // U being IThing doesn't justify using T as such!
+        t.thing(); // CHECK: error 30403
+    }
+}
diff --git a/tests/language-feature/interface-as-rhs-error.slang b/tests/language-feature/interface-as-rhs-error.slang
index 9ad71afde..7293b5134 100644
--- a/tests/language-feature/interface-as-rhs-error.slang
+++ b/tests/language-feature/interface-as-rhs-error.slang
@@ -21,13 +21,13 @@ struct AnotherType
 // These should produce errors - interface types as RHS
 bool testIsOperatorWithInterface<T>()
 {
-    //CHECK: ([[# @LINE+1]]): error 30301: 'is' and 'as' operators do not support interface types as the right-hand side
+    //CHECK: ([[# @LINE+1]]): error 30301: cannot use 'is' operator with an interface type as the right-hand side
     return (T is IMyInterface);
 }
 
 void testAsOperatorWithInterface<T>(T value)
 {
-    //CHECK: ([[# @LINE+1]]): error 30301: 'is' and 'as' operators do not support interface types as the right-hand side
+    //CHECK: ([[# @LINE+1]]): error 30302: cannot use 'as' operator with an interface type as the right-hand side
     let result = value as IMyInterface;
 }