From 409235aee65ce5fd807a50b32a2a4ba664aaab70 Mon Sep 17 00:00:00 2001
From: bbbradsmith <bbbradsmith@users.noreply.github.com>
Date: Tue, 2 May 2023 22:27:28 -0400
Subject: [PATCH] Optional warning for implicit constant conversion overflow

---
 doc/cc65.sgml       | 2 ++
 src/cc65/error.c    | 2 ++
 src/cc65/error.h    | 1 +
 src/cc65/typeconv.c | 5 +++++
 4 files changed, 10 insertions(+)

diff --git a/doc/cc65.sgml b/doc/cc65.sgml
index 683249bda..5a094571a 100644
--- a/doc/cc65.sgml
+++ b/doc/cc65.sgml
@@ -754,6 +754,8 @@ Here is a description of all the command line options:
         Warn about unused function parameters.
   <tag><tt/unused-var/</tag>
         Warn about unused variables.
+  <tag><tt/const-overflow/</tag>
+        Warn if numerical constant conversion implies overflow. (Disabled by default.)
   </descrip>
 
   The full list of available warning names can be retrieved by using the
diff --git a/src/cc65/error.c b/src/cc65/error.c
index 3f36d9e97..6ac3e594b 100644
--- a/src/cc65/error.c
+++ b/src/cc65/error.c
@@ -79,6 +79,7 @@ IntStack WarnUnusedLabel    = INTSTACK(1);  /* - unused labels */
 IntStack WarnUnusedParam    = INTSTACK(1);  /* - unused parameters */
 IntStack WarnUnusedVar      = INTSTACK(1);  /* - unused variables */
 IntStack WarnUnusedFunc     = INTSTACK(1);  /* - unused functions */
+IntStack WarnConstOverflow  = INTSTACK(0);  /* - overflow conversion of numerical constants */
 
 /* Map the name of a warning to the intstack that holds its state */
 typedef struct WarnMapEntry WarnMapEntry;
@@ -102,6 +103,7 @@ static WarnMapEntry WarnMap[] = {
     { &WarnUnusedLabel,         "unused-label"          },
     { &WarnUnusedParam,         "unused-param"          },
     { &WarnUnusedVar,           "unused-var"            },
+	{ &WarnConstOverflow,       "const-overflow"        },
 };
 
 Collection DiagnosticStrBufs;
diff --git a/src/cc65/error.h b/src/cc65/error.h
index 7fcb03467..83be8c782 100644
--- a/src/cc65/error.h
+++ b/src/cc65/error.h
@@ -76,6 +76,7 @@ extern IntStack WarnUnusedLabel;        /* - unused labels */
 extern IntStack WarnUnusedParam;        /* - unused parameters */
 extern IntStack WarnUnusedVar;          /* - unused variables */
 extern IntStack WarnUnusedFunc;         /* - unused functions */
+extern IntStack WarnConstOverflow;      /* - overflow conversion of numerical constants */
 
 /* Forward */
 struct StrBuf;
diff --git a/src/cc65/typeconv.c b/src/cc65/typeconv.c
index f77ec3951..49dfcc597 100644
--- a/src/cc65/typeconv.c
+++ b/src/cc65/typeconv.c
@@ -128,6 +128,7 @@ static void DoConversion (ExprDesc* Expr, const Type* NewType)
         ** internally already represented by a long.
         */
         if (NewBits <= OldBits) {
+            unsigned long OldVal = Expr->IVal;
 
             /* Cut the value to the new size */
             Expr->IVal &= (0xFFFFFFFFUL >> (32 - NewBits));
@@ -139,6 +140,10 @@ static void DoConversion (ExprDesc* Expr, const Type* NewType)
                     Expr->IVal |= shl_l (~0UL, NewBits);
                 }
             }
+
+            if ((OldVal != Expr->IVal) && IS_Get (&WarnConstOverflow)) {
+                Warning ("Implicit conversion of constant overflows %d-bit destination", NewBits);
+            }
         }
 
         /* Do the integer constant <-> absolute address conversion if necessary */