fix for #1941, taken from 22d435b689

src/cc65/shiftexpr.c

@@ -169,27 +169,21 @@ void ShiftExpr (struct ExprDesc* Expr)
                 }
             }
 
-            /* If the shift count is zero, nothing happens. If the left hand
-            ** side is a constant, the result is constant.
-            */
-            if (Expr2.IVal == 0 || lconst) {
-
-                /* Set the type */
+            /* If the left hand side is a constant, the result is constant */
+            if (lconst) {
+                /* Set the result type */
                 Expr->Type = ResultType;
 
-                if (lconst) {
-
-                    /* Evaluate the result */
-                    switch (Tok) {
-                        case TOK_SHL: Expr->IVal <<= Expr2.IVal; break;
-                        case TOK_SHR: Expr->IVal >>= Expr2.IVal; break;
-                        default: /* Shutup gcc */                break;
-                    }
-
-                    /* Limit the calculated value to the range of its type */
-                    LimitExprValue (Expr, 1);
+                /* Evaluate the result */
+                switch (Tok) {
+                    case TOK_SHL: Expr->IVal <<= Expr2.IVal; break;
+                    case TOK_SHR: Expr->IVal >>= Expr2.IVal; break;
+                    default: /* Shutup gcc */                break;
                 }
 
+                /* Limit the calculated value to the range of its type */
+                LimitExprValue (Expr, 1);
+
                 /* Result is already got, remove the generated code */
                 RemoveCode (&Mark1);
 
@@ -197,6 +191,15 @@ void ShiftExpr (struct ExprDesc* Expr)
                 continue;
             }
 
+            /* If the shift count is zero, nothing happens */
+            if (Expr2.IVal == 0) {
+                /* Result is already got, remove the pushing code */
+                RemoveCode (&Mark2);
+
+                /* Be sure to mark the value as in the primary */
+                goto MakeRVal;
+            }
+
             /* If we're shifting an integer or unsigned to the right, the lhs
             ** has a quasi-const address, and the shift count is larger than 8,
             ** we can load just the high byte as a char with the correct

test/val/bug1941-shift-by-zero.c

@@ -0,0 +1,28 @@
+/* Bug 1941 - Bitwise shift char types by 0 count results in out-of-range access */
+
+#include <stdio.h>
+#include <stdint.h>
+
+uint8_t foo = 42U;      /* "Low byte" */
+uint8_t goo = 1U;       /* "High byte" - you need it to reproduce the issue */
+int16_t bar = 256;      /* ...or just do it with this */
+
+_Static_assert (sizeof (foo >> 0) == sizeof (int), "Shift result should be int-promoted");
+_Static_assert (sizeof ((int8_t)bar << 0) == sizeof (int), "Shift result should be int-promoted");
+
+unsigned failures;
+
+int main(void)
+{
+    if (foo >> 0 != foo) {
+        ++failures;
+        printf("foo failed\n");
+    }
+
+    if ((int8_t)bar << 0 != (int8_t)bar) {
+        ++failures;
+        printf("bar failed\n");
+    }
+
+    return failures;
+}