Diagnose error when the function args can't satisfy constexpr parameter requirements (#7269)

## Summary
This PR enhances constexpr validation by adding proper error checking
when function arguments cannot satisfy constexpr parameter requirements,
addressing issue #6370.

## Problem
Previously, when a function declared constexpr parameters, the compiler
would attempt to propagate constexpr-ness to the call site arguments,
but there was insufficient validation and error reporting when this
propagation failed. This could lead silent failures where constexpr
requirements weren't properly enforced

## Solution
This PR adds checks that:

1. **Validates constexpr arguments**: When a function parameter is
marked as `constexpr`, the compiler now explicitly checks that the
corresponding argument can be marked as `constexpr`

2. **Issues clear compilation errors**: added
`Diagnostics::argIsNotConstexpr`)

3. **Handles both call scenarios**: The validation works for both:
   - Direct function calls with IR-level function definitions
   - Calls to function from external modules

Fixes #6370

---------

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

1 files changed, 56 insertions, 0 deletions

diff --git a/tests/diagnostics/constexpr-param.slang b/tests/diagnostics/constexpr-param.slang
new file mode 100644
index 000000000..d41410b16
--- /dev/null
+++ b/tests/diagnostics/constexpr-param.slang
@@ -0,0 +1,56 @@
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK):
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK1):
+//DIAGNOSTIC_TEST:SIMPLE(filecheck=CHECK2):
+
+// Failure to pass compile-time-constant argument
+// where it is expected.
+//
+// Test both callee and nested callee that require constexpr parameters.
+// Test both function defined in the same module and external function.
+
+Texture2D<int> input;
+RWTexture2D<int> output;
+
+int func_with_constexpr_params(constexpr int const_a)
+{
+    return const_a + const_a;
+}
+
+int func_internal(int a)
+{
+    // Error: arg passed through isn't compile-time constant
+    // CHECK: ([[# @LINE+1]]): error 40013:
+    return func_with_constexpr_params(a);
+}
+
+int func_external(int a)
+{
+    // CHECK1: ([[# @LINE+1]]): error 40012:
+    int2 offset = int2(a, a);
+    int3 location = int3(0, 0, 0);
+    // Load requires constexpr offset
+    return input.Load(location, offset);
+}
+
+[shader("compute")]
+void Test(uint3 thread_index : SV_DispatchThreadID)
+{
+    int result = 0;
+
+    // Okay, immediate constant
+    result += func_with_constexpr_params(1);
+
+
+    int ii = 0;
+    if (thread_index.x > 0)
+    {
+        ii = 1;
+    }
+    // Pass non-compile-time-constant argument to function that requires constexpr parameter
+    // CHECK2: ([[# @LINE+1]]): error 40013:
+    result += func_with_constexpr_params(ii);
+
+    // The nested function calls should issue compile errors
+    result += func_internal(ii);
+    result += func_external(ii);
+}