Add a better error message for common global generic failure (#634)

A common mistake that seems to come up when using global generic type parameters:

```hlsl
interface IHero { ... }
type_param H : IHero;
ParameterBlock<H> gHero;
```

is to accidentally try to specialize the type parameter `H` using `H` itself as the argument (instead of some concrete type like `Batman`). The current front-end checks naively let this pass, because `H` satisfies all the requirements (it sure does declare that it implements `IHero`, which is the only requirement we have). This currently leads to downstream failure when we generate code with generic type parameters still left in the IR.

This change implements a simple fix which is to:

- Check when we are trying to specialize a global generic parameter using another global generic parameter, since this is currently always a mistake

- Add a special-case diagnostic for the 99% case of this failure, which is specializing a type parameter to itself

This fix is primarily motivated by the way generics support will initially be implemented in Falcor.

2 files changed, 41 insertions, 0 deletions

diff --git a/source/slang/check.cpp b/source/slang/check.cpp
index 42bcfe4b5..7f91ac0c3 100644
--- a/source/slang/check.cpp
+++ b/source/slang/check.cpp
@@ -8635,6 +8635,42 @@ namespace Slang
                 SLANG_ASSERT(argIndex < globalGenericArgs.Count());
                 auto globalGenericArg = globalGenericArgs[argIndex];
 
+                // As a quick sanity check, see if the argument that is being supplied for a parameter
+                // is just the parameter itself, because this should always be an error:
+                //
+                if( auto argDeclRefType = globalGenericArg->As<DeclRefType>() )
+                {
+                    auto argDeclRef = argDeclRefType->declRef;
+                    if(auto argGenericParamDeclRef = argDeclRef.As<GlobalGenericParamDecl>())
+                    {
+                        if(argGenericParamDeclRef.getDecl() == globalGenericParam)
+                        {
+                            // We are trying to specialize a generic parameter using itself.
+                            sink->diagnose(globalGenericParam,
+                                Diagnostics::cannotSpecializeGlobalGenericToItself,
+                                globalGenericParam->getName());
+                            sink->diagnose(entryPointFuncDecl,
+                                Diagnostics::noteWhenCompilingEntryPoint,
+                                entryPointFuncDecl->getName());
+                            continue;
+                        }
+                        else
+                        {
+                            // We are trying to specialize a generic parameter using a *different*
+                            // global generic type parameter.
+                            sink->diagnose(globalGenericParam,
+                                Diagnostics::cannotSpecializeGlobalGenericToAnotherGenericParam,
+                                globalGenericParam->getName(),
+                                argGenericParamDeclRef.GetName());
+                            sink->diagnose(entryPointFuncDecl,
+                                Diagnostics::noteWhenCompilingEntryPoint,
+                                entryPointFuncDecl->getName());
+                            continue;
+                        }
+                    }
+                }
+
+
                 // Create a substitution for this parameter/argument.
                 RefPtr<GlobalGenericParamSubstitution> subst = new GlobalGenericParamSubstitution();
                 subst->paramDecl = globalGenericParam;
diff --git a/source/slang/diagnostic-defs.h b/source/slang/diagnostic-defs.h
index c0c74c78e..83e82fc81 100644
--- a/source/slang/diagnostic-defs.h
+++ b/source/slang/diagnostic-defs.h
@@ -307,9 +307,14 @@ DIAGNOSTIC(38102, Error, accessorMustBeInsideSubscriptOrProperty, "an accessor d
 DIAGNOSTIC(38020, Error, mismatchEntryPointTypeArgument, "expecting $0 entry-point type arguments, provided $1.")
 DIAGNOSTIC(38021, Error, typeArgumentDoesNotConformToInterface, "type argument `$1` for generic parameter `$0` does not conform to interface `$1`.")
 
+DIAGNOSTIC(38022, Error, cannotSpecializeGlobalGenericToItself, "the global type parameter '$0' cannot be specialized to itself")
+DIAGNOSTIC(38023, Error, cannotSpecializeGlobalGenericToAnotherGenericParam, "the global type parameter '$0' cannot be specialized using another global type parameter ('$1')")
+DIAGNOSTIC(-1, Note, noteWhenCompilingEntryPoint, "when compiling entry point '$0'")
+
 DIAGNOSTIC(38200, Error, recursiveModuleImport, "module `$0` recursively imports itself")
 DIAGNOSTIC(39999, Fatal, errorInImportedModule, "error in imported module, compilation ceased.")
 
+
 // 39xxx - Type layout and parameter binding.
 
 DIAGNOSTIC(39000, Error, conflictingExplicitBindingsForParameter, "conflicting explicit bindings for parameter '$0'")