Fix confusing error messages for interface return type mismatches (#7854)

* Initial plan

* Add improved diagnostic for interface return type mismatches

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Complete fix for interface return type mismatch error reporting

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Move diagnostic to synthesis phase for better interface return type mismatch errors

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Remove extraneous test file and update .gitignore

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Add diagnostic test for interface return type mismatch and apply formatting

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Address feedback: restore whitespace and use filecheck for diagnostic test

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Fix logic error in return type mismatch detection

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Remove unnecessary flag by using out parameter for diagnostic tracking

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Refactor witness synthesis failure reporting to use structured approach

Replace ad-hoc `outSpecificDiagnosticEmitted` parameter with `WitnessSynthesisFailureReason` enum and `MethodWitnessSynthesisFailureDetails` struct as requested in code review. This provides:

- Clear taxonomy of failure reasons (General, MethodResultTypeMismatch, MethodParameterMismatch)
- Centralized diagnostic emission in findWitnessForInterfaceRequirement
- Better extensibility for future failure types
- Improved maintainability by removing state tracking flags

The return type mismatch diagnostic continues to work correctly, showing error 38106 with precise location information.

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Remove unused MethodParameterMismatch enum and duplicate code

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Remove redundant requiredMethod field from MethodWitnessSynthesisFailureDetails

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Address feedback: add outFailureDetails guard and remove unnecessary hasReturnTypeError variable

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Fix regression: restore original diagnostic message for mutating method mismatch

Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>

* Fix.

* Fix.

* Remove `innerSink`.

* Print candidates considered  for interface match upon error.

* Fix tests.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: csyonghe <2652293+csyonghe@users.noreply.github.com>
Co-authored-by: Yong He <yonghe@outlook.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/tests/diagnostics/interfaces/return-type-mismatch.slang b/tests/diagnostics/interfaces/return-type-mismatch.slang
new file mode 100644
index 000000000..244f2f18e
--- /dev/null
+++ b/tests/diagnostics/interfaces/return-type-mismatch.slang
@@ -0,0 +1,20 @@
+// return-type-mismatch.slang
+
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK):
+
+enum LobeTypes : uint32_t { None = 0, Diffuse = 1 }
+
+interface IMaterialInstance {
+    LobeTypes get_lobe_types();
+}
+
+struct Broken : IMaterialInstance {
+    // CHECK: ([[# @LINE+1]]): error 38106:
+    uint get_lobe_types() { return LobeTypes::Diffuse; }
+}
+
+int test()
+{
+    Broken b;
+    return 0;
+}
\ No newline at end of file