Fix NameExprType returning deleted canonical type when it's in a generic parent.

fixes #339

`NamedExpressionType::CreateCanonicalType()` may return a deleted pointer. The original implementation is as follows:
```
    Type* NamedExpressionType::CreateCanonicalType()
    {
        return GetType(declRef)->GetCanonicalType();
    }
```
If `GetType()` returns a newly constructed Type (this happens when the `typedef` is defined inside a generic parent, which triggers a non-trivial substitution), the temporary type will be deleted when the function returns. The fix is to store the temporary type as a field of NamedExpressionType (`innerType`).

A relevant fix (though not the true cause of issue #339) is to have `Type::GetCanonicalType()` also hold a `RefPtr` to the constructed canonical type, when the canonical type is not `this`. This prevents a returned canonical type being assigned to a RefPtr, which makes it possible for that RefPtr to be the sole owner of the canonical type and deleteing the canonical type when that RefPtr is destroyed.

2 files changed, 42 insertions, 0 deletions

diff --git a/tests/compute/typedef-member.slang b/tests/compute/typedef-member.slang
new file mode 100644
index 000000000..dbf4dcdc1
--- /dev/null
+++ b/tests/compute/typedef-member.slang
@@ -0,0 +1,38 @@
+//TEST(compute):COMPARE_COMPUTE:-xslang -use-ir
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):dxbinding(0),glbinding(0),out
+
+// Confirm that a struct type defined in a generic parent works
+
+RWStructuredBuffer<float> outputBuffer;
+
+struct SubType<T>
+{
+    T x;
+};
+
+struct GenStruct<T>
+{
+    typedef SubType<T> SubTypeT;
+    T getVal(SubTypeT v)
+    {
+        return v.x;
+    }
+};
+
+float test(float val)
+{
+    GenStruct<float>.SubTypeT sb;
+    sb.x = val;
+    GenStruct<float> obj;
+    return obj.getVal(sb);
+}
+
+
+[numthreads(4, 1, 1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    uint tid = dispatchThreadID.x;
+    float inVal = float(tid);
+    float outVal = test(inVal);
+    outputBuffer[tid] = outVal.x;
+}
\ No newline at end of file
diff --git a/tests/compute/typedef-member.slang.expected.txt b/tests/compute/typedef-member.slang.expected.txt
new file mode 100644
index 000000000..98798bd61
--- /dev/null
+++ b/tests/compute/typedef-member.slang.expected.txt
@@ -0,0 +1,4 @@
+0
+3F800000
+40000000
+40400000
\ No newline at end of file