math: Give a proper error for division by zero

This errored out *later* because the result was infinite or NaN, but
it didn't actually stop evaluation.

I'm not sure if there is a way to get floating point math to turn an
infinity back into something that doesn't depend on a literal
infinity, but division by zero conceptually isn't a thing we can
support.

There's entire branches of maths dedicated to figuring out what
dividing by "basically zero" means and we don't have to get into it.

src/builtins/math.cpp

@@ -177,6 +177,8 @@ static const wchar_t *math_describe_error(const te_error_t &error) {
             return _(L"Unexpected token");
         case TE_ERROR_LOGICAL_OPERATOR:
             return _(L"Logical operations are not supported, use `test` instead");
+        case TE_ERROR_DIV_BY_ZERO:
+            return _(L"Division by zero");
         case TE_ERROR_UNKNOWN:
             return _(L"Expression is bogus");
         default:

src/tinyexpr.cpp

@@ -114,7 +114,10 @@ struct state {
 
     [[nodiscard]] te_error_t error() const {
         if (type_ == TOK_END) return {TE_ERROR_NONE, 0};
-        te_error_t err{error_, static_cast<int>(next_ - start_) + 1};
+        // If we have an error position set, use that,
+        // otherwise the current position.
+        const wchar_t *tok = errpos_ ? errpos_ : next_;
+        te_error_t err{error_, static_cast<int>(tok - start_) + 1};
         if (error_ == TE_ERROR_NONE) {
             // If we're not at the end but there's no error, then that means we have a
             // superfluous token that we have no idea what to do with.
@@ -129,6 +132,7 @@ struct state {
 
     const wchar_t *start_;
     const wchar_t *next_;
+    const wchar_t *errpos_{nullptr};
 
     te_fun_t current_{NAN};
     void next_token();
@@ -520,8 +524,17 @@ double state::term() {
     auto ret = factor();
     while (type_ == TOK_INFIX && (current_ == mul || current_ == divide || current_ == fmod)) {
         auto fn = current_;
+        auto tok = next_;
         next_token();
-        ret = fn(ret, factor());
+        auto ret2 = factor();
+        if (ret2 == 0 && (fn == divide || fn == fmod)) {
+            // Division by zero (also for modulo)
+            type_ = TOK_ERROR;
+            error_ = TE_ERROR_DIV_BY_ZERO;
+            // Error position is the "/" or "%" sign for now
+            errpos_ = tok;
+        }
+        ret = fn(ret, ret2);
     }
     return ret;
 }

src/tinyexpr.h

@@ -37,7 +37,8 @@ typedef enum {
     TE_ERROR_MISSING_OPERATOR = 6,
     TE_ERROR_UNEXPECTED_TOKEN = 7,
     TE_ERROR_LOGICAL_OPERATOR = 8,
-    TE_ERROR_UNKNOWN = 9
+    TE_ERROR_DIV_BY_ZERO = 9,
+    TE_ERROR_UNKNOWN = 10
 } te_error_type_t;
 
 typedef struct te_error_t {

tests/checks/math.fish

@@ -151,9 +151,14 @@ not math -s 12
 not math 2^999999
 # CHECKERR: math: Error: Result is infinite
 # CHECKERR: '2^999999'
-not math 1 / 0
-# CHECKERR: math: Error: Result is infinite
-# CHECKERR: '1 / 0'
+printf '<%s>\n' (not math 1 / 0 2>&1)
+# CHECK: <math: Error: Division by zero>
+# CHECK: <'1 / 0'>
+# CHECK: <   ^>
+printf '<%s>\n' (math 1 % 0 - 5 2>&1)
+# CHECK: <math: Error: Division by zero>
+# CHECK: <'1 % 0 - 5'>
+# CHECK: <   ^>
 
 # Validate "x" as multiplier
 math 0x2 # Hex
@@ -259,8 +264,9 @@ math pow 2 x cos'(-pi)', 2
 # This used to take ages, see #8170.
 # If this test hangs, that's reintroduced!
 math 'ncr(0/0, 1)'
-# CHECKERR: math: Error: Result is infinite
+# CHECKERR: math: Error: Division by zero
 # CHECKERR: 'ncr(0/0, 1)'
+# CHECKERR:       ^
 
 # Variadic functions require at least one argument
 math min