Fix use of variadic generics with [Differentiable]. (#8736)

There was a bug that causes the compiler failing to treat a `no_diff
TypePack` as a type pack, and thus diagnose an error when resolving the
following call.

The fix is to unwrap any ModifiedType wrappers in `IsTypePack()` check.

5 files changed, 102 insertions, 3 deletions

diff --git a/docs/user-guide/06-interfaces-generics.md b/docs/user-guide/06-interfaces-generics.md
index 30efbada1..b7fca069e 100644
--- a/docs/user-guide/06-interfaces-generics.md
+++ b/docs/user-guide/06-interfaces-generics.md
@@ -1026,12 +1026,12 @@ void printNumbers<each T>(expand each T args) where T == int
 void compute<each T>(expand each T args) where T == int
 {
     // Maps every element in `args` to `elementValue + 1`, and forwards the
-    // new values as arguments to `printNumber`.
-    printNumber(expand (each args) + 1);
+    // new values as arguments to `printNumbers`.
+    printNumbers(expand (each args) + 1);
 
     // The above statement is equivalent to:
     // ```
-    // printNumber(args[0] + 1, args[1] + 1, ..., args[n-1] + 1);
+    // printNumbers(args[0] + 1, args[1] + 1, ..., args[n-1] + 1);
     // ```
 }
 void test()
diff --git a/source/slang/slang-ast-type.cpp b/source/slang/slang-ast-type.cpp
index e3f75cb2f..bfa347df3 100644
--- a/source/slang/slang-ast-type.cpp
+++ b/source/slang/slang-ast-type.cpp
@@ -4,6 +4,7 @@
 #include "slang-ast-builder.h"
 #include "slang-ast-dispatch.h"
 #include "slang-ast-modifier.h"
+#include "slang-check.h"
 #include "slang-syntax.h"
 
 #include <assert.h>
@@ -13,6 +14,7 @@ namespace Slang
 
 bool isAbstractTypePack(Type* type)
 {
+    type = unwrapModifiedType(type);
     if (as<ExpandType>(type))
         return true;
     if (isDeclRefTypeOf<GenericTypePackParamDecl>(type))
@@ -22,6 +24,7 @@ bool isAbstractTypePack(Type* type)
 
 bool isTypePack(Type* type)
 {
+    type = unwrapModifiedType(type);
     if (as<ConcreteTypePack>(type))
         return true;
     return isAbstractTypePack(type);
diff --git a/source/slang/slang-ast-val.h b/source/slang/slang-ast-val.h
index 7b9462f2f..d4edaae08 100644
--- a/source/slang/slang-ast-val.h
+++ b/source/slang/slang-ast-val.h
@@ -1014,6 +1014,10 @@ inline bool isTypeEqualityWitness(Val* witness)
         }
         return true;
     }
+    else if (auto expandWitness = as<ExpandSubtypeWitness>(witness))
+    {
+        return isTypeEqualityWitness(expandWitness->getPatternTypeWitness());
+    }
     return false;
 }
 
diff --git a/tests/language-feature/generics/variadic-generic-differentiable.slang b/tests/language-feature/generics/variadic-generic-differentiable.slang
new file mode 100644
index 000000000..55af3424c
--- /dev/null
+++ b/tests/language-feature/generics/variadic-generic-differentiable.slang
@@ -0,0 +1,59 @@
+//TEST:INTERPRET(filecheck=CHECK):
+
+// Test that we can call variadic generic [Differentiable] methods.
+
+interface IParameterExtractor
+{
+    [Differentiable]
+    void extract(no_diff uint x);
+}
+
+struct FloatParameterExtractor : IParameterExtractor
+{
+    [Differentiable]
+    void extract(no_diff uint x)
+    {
+        printf("fff\n");
+    }
+}
+
+[Differentiable]
+void extract_parameters_helper<T : IParameterExtractor>(
+    no_diff uint x,
+    T arg,
+    )
+{
+    arg.extract(x);
+}
+
+[Differentiable]
+void wrapper1<each T>(
+    uint x,
+    expand each T args, // compiler will add no_diff modifier here.
+    )
+    where T : IParameterExtractor
+{
+    expand extract_parameters_helper(x, each args);
+}
+
+[Differentiable]
+void wrapper2<each T>(
+    uint x,
+    expand each T args, // compiler will add no_diff modifier here.
+    )
+    where T : IParameterExtractor
+{
+    wrapper1(x, args);
+}
+
+
+void main()
+{
+    // There was a bug that causes the compiler failing to treat a `no_diff TypePack` as
+    // a type pack, and thus diagnose an error when resolving the following call.
+    //
+    wrapper2(1, FloatParameterExtractor(), FloatParameterExtractor());
+}
+
+// CHECK: fff
+// CHECK: fff
\ No newline at end of file
diff --git a/tests/language-feature/generics/variadic-user-guide.slang b/tests/language-feature/generics/variadic-user-guide.slang
new file mode 100644
index 000000000..1d1cfd790
--- /dev/null
+++ b/tests/language-feature/generics/variadic-user-guide.slang
@@ -0,0 +1,33 @@
+//TEST:INTERPRET(filecheck=CHECK):
+
+void printNumbers<each T>(expand each T args) where T == int
+{
+    // An single expression statement whose type will be `(void, void, ...)`.
+    // where each `void` is the result of evaluating expression `printf(...)` with
+    // each corresponding element in `args` passed as print operand.
+    //
+    expand printf("%d\n", each args);
+
+    // The above statement is equivalent to:
+    // ```
+    // (printf("%d\n", args[0]), printf("%d\n", args[1]), ..., printf("%d\n", args[n-1]));
+    // ```
+}
+void compute<each T>(expand each T args) where T == int
+{
+    // Maps every element in `args` to `elementValue + 1`, and forwards the
+    // new values as arguments to `printNumber`.
+    printNumbers(expand ((each args) + 1));
+
+    // The above statement is equivalent to:
+    // ```
+    // printNumber(args[0] + 1, args[1] + 1, ..., args[n-1] + 1);
+    // ```
+}
+void main()
+{
+    compute(1,2,3);
+    // CHECK: 2
+    // CHECK: 3
+    // CHECK: 4
+}
\ No newline at end of file