Add error for forward references in generic constraints (#7615)

* Add error for forward references in generic constraints

Change addresses issue #6545.

The slang compiler's type checker is unable to support cases where
a generic type parameter is referenced as a constraint before it is
declared. For example code like:

````
interface IFoo<T : IFloat>
{
}

void bar<Foo : IFoo<T>, T : IFloat>()
{
}
````

Is not supported, but will currently report a generic error like,

"(0): error 99999: Slang compilation aborted due to an exception
of class Slang::InternalError: unexpected: generic type constraint
during lowering"

This change adds a new check for this kind of code and reports an
error like,

"error 30117: generic constraint for parameter 'Foo' references type
parameter 'T' before it is declared"

when detected. Basic testing of this error is also added in a new
diagnostic test.

* Add error for forward refs in generic constiants update 1

* Add error for forward refs in generic constraints update 2

Revised algo in checkForwardReferencesInGenericConstraint to run
in O(n) instead of the previous O(n^2) using HashSets.

* Add error for forward refs in generic constraints update 3

-Update logic adding referenced decl's in collectReferencedDecls.
-Simplified error string logic.

* Add error for forward refs in generic constraints update 4

-Declare collectReferencedDecls in slang-check.h such that it can
 be used as a general utility function.

* format code

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

2 files changed, 30 insertions, 0 deletions

diff --git a/tests/diagnostics/generic-constraint-forward-reference.slang b/tests/diagnostics/generic-constraint-forward-reference.slang
new file mode 100644
index 000000000..08a6c3478
--- /dev/null
+++ b/tests/diagnostics/generic-constraint-forward-reference.slang
@@ -0,0 +1,16 @@
+//DIAGNOSTIC_TEST:SIMPLE:

+// Test forward references in generic constraints

+

+interface IFloat {}

+interface IFoo<T : IFloat> {}

+interface IBar<T, U> {}

+

+// Test case 1: Simple forward reference in interface constraint

+void test1<Foo : IFoo<T>, T : IFloat>() {}

+

+// Test case 2: Multiple forward references in a single constraint

+void test2<A : IBar<B, C>, B : IFloat, C : IFloat>() {}

+

+// Valid case that should NOT produce our error:

+// Test case 3: Correct order - type parameter declared before use

+void test3<T : IFloat, Foo : IFoo<T>>() {}

diff --git a/tests/diagnostics/generic-constraint-forward-reference.slang.expected b/tests/diagnostics/generic-constraint-forward-reference.slang.expected
new file mode 100644
index 000000000..5c6b2707b
--- /dev/null
+++ b/tests/diagnostics/generic-constraint-forward-reference.slang.expected
@@ -0,0 +1,14 @@
+result code = -1
+standard error = {
+tests/diagnostics/generic-constraint-forward-reference.slang(9): error 30117: generic constraint for parameter 'Foo' references type parameter 'T' before it is declared
+void test1<Foo : IFoo<T>, T : IFloat>() {}
+                 ^~~~
+tests/diagnostics/generic-constraint-forward-reference.slang(12): error 30117: generic constraint for parameter 'A' references type parameter 'B' before it is declared
+void test2<A : IBar<B, C>, B : IFloat, C : IFloat>() {}
+               ^~~~
+tests/diagnostics/generic-constraint-forward-reference.slang(12): error 30117: generic constraint for parameter 'A' references type parameter 'C' before it is declared
+void test2<A : IBar<B, C>, B : IFloat, C : IFloat>() {}
+               ^~~~
+}
+standard output = {
+}