Support `where` clause and type equality constraint. (#4986)

* Support `where` clause.

* Fix.

* Fix parser.

* Enhance test to cover traditional __generic syntax.

* Update user-guide.

* Support `where` clause on associatedtype.

* Fix.

* Put in more comments.

4 files changed, 141 insertions, 0 deletions

diff --git a/tests/language-feature/generics/where-1.slang b/tests/language-feature/generics/where-1.slang
new file mode 100644
index 000000000..904e03d52
--- /dev/null
+++ b/tests/language-feature/generics/where-1.slang
@@ -0,0 +1,20 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj -output-using-type
+
+// Test that we can use `where` clause to constrain generic type parameters.
+
+T process<T, int N>(vector<T, N> v) where T : IFloat
+{
+    return v[0];
+}
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[numthreads(1,1,1)]
+void computeMain()
+{
+    float a = 1.0;
+    outputBuffer[0] = process(a);
+    // CHECK: 1.0
+}
diff --git a/tests/language-feature/generics/where-2.slang b/tests/language-feature/generics/where-2.slang
new file mode 100644
index 000000000..b3e9bb86a
--- /dev/null
+++ b/tests/language-feature/generics/where-2.slang
@@ -0,0 +1,33 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj -output-using-type
+
+// Test that we can use `where` clause to constrain associatedtype of generic type parameters.
+
+interface IFoo
+{
+    associatedtype TA;
+}
+
+struct FooImpl : IFoo
+{
+    typealias TA = int;
+}
+
+__generic<typename T>
+T.TA process(T v)
+    where T : IFoo
+    where T.TA == int
+{
+    return 1;
+}
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[numthreads(1,1,1)]
+void computeMain()
+{
+    FooImpl fooImpl;
+    outputBuffer[0] = process(fooImpl);
+    // CHECK: 1.0
+}
diff --git a/tests/language-feature/generics/where-3.slang b/tests/language-feature/generics/where-3.slang
new file mode 100644
index 000000000..730373b76
--- /dev/null
+++ b/tests/language-feature/generics/where-3.slang
@@ -0,0 +1,36 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj -output-using-type
+
+// Test that we can use `where` clause to constrain the type of a type pack.
+
+interface IFoo
+{
+    associatedtype TA;
+}
+
+struct FooImpl : IFoo
+{
+    typealias TA = int;
+}
+
+void add(inout int a, int b)
+{
+    a += b;
+}
+
+int process<each T>(T v) where T == int
+{
+    int result = 0;
+    expand add(result, each v);
+    return result;
+}
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[numthreads(1,1,1)]
+void computeMain()
+{
+    outputBuffer[0] = process(1,2,3);
+    // CHECK: 6.0
+}
diff --git a/tests/language-feature/generics/where-4.slang b/tests/language-feature/generics/where-4.slang
new file mode 100644
index 000000000..9f86ca7dc
--- /dev/null
+++ b/tests/language-feature/generics/where-4.slang
@@ -0,0 +1,52 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -shaderobj -output-using-type
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK): -vk -shaderobj -output-using-type
+
+// Test that we can use `where` clause to constrain an associatedtype.
+
+interface IBar
+{
+    associatedtype TB;
+    TB get();
+}
+
+interface IFoo
+{
+    associatedtype TA : IBar where TA.TB == int;
+
+    TA getVal();
+}
+
+struct BarImpl : IBar
+{
+    typealias TB = int;
+    int x;
+    int get() { return x; }
+}
+
+struct FooImpl : IFoo
+{
+    typealias TA = BarImpl;
+    TA getVal() { TA a; a.x = 1; return a; }
+}
+
+int helper<T : IFoo>(T foo)
+{
+    // foo.getVal().get() has type `T.TA.TB`,
+    // because there is a type equality constraint defined on
+    // `IFoo.TA` such that `IFoo::TA.TB == int`, we should be able
+    // to conclude that `T.TA.TB` is `int` and the `return` here
+    // should type check.
+    return foo.getVal().get();
+}
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[numthreads(1,1,1)]
+void computeMain()
+{
+    FooImpl foo;
+
+    outputBuffer[0] = helper(foo);
+    // CHECK: 1.0
+}