Fix [mutating] generic methods (#1618)

Slang generates code that turns the implicit `this` parameter of a method into an explicit parameter. The logic that decides whether that parameter should be `inout` is a bit involved, and there was a bug where a generic method would lead to the use of an `in` modifier (the default) and override the `inout` modifier that was requested by the method itself.

This change fixes the logic to treat generic declarations in the parent chain of a leaf method as having no bearing on whether an implicit `this` parameter should be `inout` or not.

A test case is included that breaks with the old behavior, and demonstrates that a generic `[mutating]` method can now work correctly.

1 files changed, 8 insertions, 0 deletions

diff --git a/source/slang/slang-lower-to-ir.cpp b/source/slang/slang-lower-to-ir.cpp
index fefec805c..b2a7529c5 100644
--- a/source/slang/slang-lower-to-ir.cpp
+++ b/source/slang/slang-lower-to-ir.cpp
@@ -2231,6 +2231,14 @@ ParameterDirection getThisParamDirection(Decl* parentDecl, ParameterDirection de
         return defaultDirection;
     }
 
+    // A parent generic declaration should not change the
+    // mutating-ness of the inner declaration.
+    //
+    if( as<GenericDecl>(parentDecl) )
+    {
+        return defaultDirection;
+    }
+
     // For now we make any `this` parameter default to `in`.
     //
     return kParameterDirection_In;