Warning on lossy implicit casts. (#2367)

* Warning on bool to float conversion.

* Fix test cases.

* Improve.

* LanguageServer: don't show constant value for non constant variables.

* Fix tests.

* Fix warnings in tests.

Co-authored-by: Yong He <yhe@nvidia.com>

1 files changed, 28 insertions, 4 deletions

diff --git a/source/slang/slang-check-conversion.cpp b/source/slang/slang-check-conversion.cpp
index 4bcf07b2b..a3273f942 100644
--- a/source/slang/slang-check-conversion.cpp
+++ b/source/slang/slang-check-conversion.cpp
@@ -955,15 +955,39 @@ namespace Slang
             // but then emit a diagnostic when actually reifying
             // the result expression.
             //
-            if( cost >= kConversionCost_Explicit )
+            if (outToExpr)
             {
-                if( outToExpr )
+                if (cost >= kConversionCost_Explicit)
                 {
                     getSink()->diagnose(fromExpr, Diagnostics::typeMismatch, toType, fromType);
-                    getSink()->diagnose(fromExpr, Diagnostics::noteExplicitConversionPossible, fromType, toType);
+                    getSink()->diagnose(
+                        fromExpr, Diagnostics::noteExplicitConversionPossible, fromType, toType);
+                }
+                else if (cost >= kConversionCost_Default)
+                {
+                    // For general types of implicit conversions, we issue a warning, unless `fromExpr` is a known constant
+                    // and we know it won't cause a problem.
+                    bool shouldEmitGeneralWarning = true;
+                    if (isScalarIntegerType(toType))
+                    {
+                        if (auto intVal = tryFoldIntegerConstantExpression(fromExpr, nullptr))
+                        {
+                            if (auto val = as<ConstantIntVal>(intVal))
+                            {
+                                if (isIntValueInRangeOfType(val->value, toType))
+                                {
+                                    // OK.
+                                    shouldEmitGeneralWarning = false;
+                                }
+                            }
+                        }
+                    }
+                    if (shouldEmitGeneralWarning)
+                    {
+                        getSink()->diagnose(fromExpr, Diagnostics::unrecommendedImplicitConversion, fromType, toType);
+                    }
                 }
             }
-
             if(outCost)
                 *outCost = cost;