diff --git a/tcg/tcg.c b/tcg/tcg.c
index 7ba6c45824..095008ac25 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -871,6 +871,7 @@ static int tcg_out_pool_finalize(TCGContext *s)
 #define C_N1_O1_I4(O1, O2, I1, I2, I3, I4) C_PFX6(c_n1_o1_i4_, O1, O2, I1, I2, I3, I4),
 
 typedef enum {
+    C_NotImplemented = -1,
 #include "tcg-target-con-set.h"
 } TCGConstraintSetIndex;
 
@@ -3326,7 +3327,7 @@ static void process_op_defs(TCGContext *s)
     for (TCGOpcode op = 0; op < NB_OPS; op++) {
         const TCGOpDef *def = &tcg_op_defs[op];
         const TCGConstraintSet *tdefs;
-        unsigned con_set;
+        TCGConstraintSetIndex con_set;
         TCGType type;
 
         if (def->flags & TCG_OPF_NOT_PRESENT) {
@@ -3342,11 +3343,10 @@ static void process_op_defs(TCGContext *s)
 
         /*
          * Macro magic should make it impossible, but double-check that
-         * the array index is in range.  Since the signness of an enum
-         * is implementation defined, force the result to unsigned.
+         * the array index is in range.
          */
         con_set = tcg_target_op_def(op);
-        tcg_debug_assert(con_set < ARRAY_SIZE(constraint_sets));
+        tcg_debug_assert(con_set >= 0 && con_set < ARRAY_SIZE(constraint_sets));
 
         /* The constraint arguments must match TCGOpcode arguments. */
         tdefs = &constraint_sets[con_set];
@@ -3359,7 +3359,7 @@ static const TCGArgConstraint *opcode_args_ct(const TCGOp *op)
 {
     TCGOpcode opc = op->opc;
     const TCGOpDef *def = &tcg_op_defs[opc];
-    unsigned con_set;
+    TCGConstraintSetIndex con_set;
 
     if (def->flags & TCG_OPF_NOT_PRESENT) {
         return empty_cts;
@@ -3368,7 +3368,7 @@ static const TCGArgConstraint *opcode_args_ct(const TCGOp *op)
     tcg_debug_assert(tcg_op_supported(opc, op->type));
 
     con_set = tcg_target_op_def(opc);
-    tcg_debug_assert(con_set < ARRAY_SIZE(constraint_sets));
+    tcg_debug_assert(con_set >= 0 && con_set < ARRAY_SIZE(constraint_sets));
     return all_args_cts[con_set];
 }
 
diff --git a/tcg/aarch64/tcg-target.c.inc b/tcg/aarch64/tcg-target.c.inc
index c0fe10e657..72e64c7dba 100644
--- a/tcg/aarch64/tcg-target.c.inc
+++ b/tcg/aarch64/tcg-target.c.inc
@@ -3149,7 +3149,7 @@ static TCGConstraintSetIndex tcg_target_op_def(TCGOpcode op)
         return C_O1_I2(w, 0, w);
 
     default:
-        g_assert_not_reached();
+        return C_NotImplemented;
     }
 }
 
diff --git a/tcg/arm/tcg-target.c.inc b/tcg/arm/tcg-target.c.inc
index cd770d2e20..d0c39f7122 100644
--- a/tcg/arm/tcg-target.c.inc
+++ b/tcg/arm/tcg-target.c.inc
@@ -2295,7 +2295,7 @@ static TCGConstraintSetIndex tcg_target_op_def(TCGOpcode op)
     case INDEX_op_bitsel_vec:
         return C_O1_I3(w, w, w, w);
     default:
-        g_assert_not_reached();
+        return C_NotImplemented;
     }
 }
 
diff --git a/tcg/i386/tcg-target.c.inc b/tcg/i386/tcg-target.c.inc
index c73f1039e1..b6b15723b4 100644
--- a/tcg/i386/tcg-target.c.inc
+++ b/tcg/i386/tcg-target.c.inc
@@ -3908,7 +3908,7 @@ static TCGConstraintSetIndex tcg_target_op_def(TCGOpcode op)
         return C_O1_I4(x, x, x, xO, x);
 
     default:
-        g_assert_not_reached();
+        return C_NotImplemented;
     }
 }
 
diff --git a/tcg/loongarch64/tcg-target.c.inc b/tcg/loongarch64/tcg-target.c.inc
index 395579774d..900558d301 100644
--- a/tcg/loongarch64/tcg-target.c.inc
+++ b/tcg/loongarch64/tcg-target.c.inc
@@ -2401,7 +2401,7 @@ static TCGConstraintSetIndex tcg_target_op_def(TCGOpcode op)
         return C_O1_I3(w, w, w, w);
 
     default:
-        g_assert_not_reached();
+        return C_NotImplemented;
     }
 }
 
diff --git a/tcg/mips/tcg-target.c.inc b/tcg/mips/tcg-target.c.inc
index a8987528e4..d2ace88aae 100644
--- a/tcg/mips/tcg-target.c.inc
+++ b/tcg/mips/tcg-target.c.inc
@@ -2305,7 +2305,7 @@ static TCGConstraintSetIndex tcg_target_op_def(TCGOpcode op)
                 : C_O0_I4(rZ, rZ, r, r));
 
     default:
-        g_assert_not_reached();
+        return C_NotImplemented;
     }
 }
 
diff --git a/tcg/ppc/tcg-target.c.inc b/tcg/ppc/tcg-target.c.inc
index 2b3d46af46..62110619f0 100644
--- a/tcg/ppc/tcg-target.c.inc
+++ b/tcg/ppc/tcg-target.c.inc
@@ -4374,7 +4374,7 @@ static TCGConstraintSetIndex tcg_target_op_def(TCGOpcode op)
         return C_O1_I4(v, v, v, vZM, v);
 
     default:
-        g_assert_not_reached();
+        return C_NotImplemented;
     }
 }
 
diff --git a/tcg/riscv/tcg-target.c.inc b/tcg/riscv/tcg-target.c.inc
index 9e981c5111..8e0a2d967e 100644
--- a/tcg/riscv/tcg-target.c.inc
+++ b/tcg/riscv/tcg-target.c.inc
@@ -2789,7 +2789,7 @@ static TCGConstraintSetIndex tcg_target_op_def(TCGOpcode op)
     case INDEX_op_cmpsel_vec:
         return C_O1_I4(v, v, vL, vK, vK);
     default:
-        g_assert_not_reached();
+        return C_NotImplemented;
     }
 }
 
diff --git a/tcg/s390x/tcg-target.c.inc b/tcg/s390x/tcg-target.c.inc
index 9696ff6515..cb0b7ff25e 100644
--- a/tcg/s390x/tcg-target.c.inc
+++ b/tcg/s390x/tcg-target.c.inc
@@ -3449,7 +3449,7 @@ static TCGConstraintSetIndex tcg_target_op_def(TCGOpcode op)
                 : C_O1_I4(v, v, v, vZ, v));
 
     default:
-        g_assert_not_reached();
+        return C_NotImplemented;
     }
 }
 
diff --git a/tcg/sparc64/tcg-target.c.inc b/tcg/sparc64/tcg-target.c.inc
index caa462249f..c104636cf7 100644
--- a/tcg/sparc64/tcg-target.c.inc
+++ b/tcg/sparc64/tcg-target.c.inc
@@ -1632,7 +1632,7 @@ static TCGConstraintSetIndex tcg_target_op_def(TCGOpcode op)
         return C_O1_I2(r, r, r);
 
     default:
-        g_assert_not_reached();
+        return C_NotImplemented;
     }
 }
 
diff --git a/tcg/tci/tcg-target.c.inc b/tcg/tci/tcg-target.c.inc
index 4ab19e93cc..57293e1458 100644
--- a/tcg/tci/tcg-target.c.inc
+++ b/tcg/tci/tcg-target.c.inc
@@ -178,7 +178,7 @@ static TCGConstraintSetIndex tcg_target_op_def(TCGOpcode op)
         return TCG_TARGET_REG_BITS == 64 ? C_O0_I2(r, r) : C_O0_I4(r, r, r, r);
 
     default:
-        g_assert_not_reached();
+        return C_NotImplemented;
     }
 }
 
