Report a domain error when const folding ** yields a complex number (#228)

src/fpy/semantics.py

@@ -2706,6 +2706,12 @@ def visit_AstBinaryOp(self, node: AstBinaryOp, state: CompileState):
                 folded_value = lhs_value / rhs_value
             elif node.op == BinaryStackOp.EXPONENT:
                 folded_value = lhs_value**rhs_value
+                if isinstance(folded_value, complex):
+                    # float ** float returns a complex number for a negative
+                    # base and a fractional exponent, where the Decimal path
+                    # raises decimal.InvalidOperation
+                    state.err("Domain error", node)
+                    return
             elif node.op == BinaryStackOp.FLOOR_DIVIDE:
                 # Floor toward -inf (Python `//`), matching the runtime backends.
                 if isinstance(lhs_value, int) and isinstance(rhs_value, int):

test/fpy/test_arithmetic.py

@@ -56,6 +56,24 @@ def test_const_complex_pow(self, fprime_test_api):
 
         assert_compile_failure(fprime_test_api, seq)
 
+    def test_const_complex_pow_float_operands(self, fprime_test_api):
+        """A negative base with a fractional exponent is a domain error even
+        when an operand has already been folded to a Python float, where
+        ``float ** float`` returns a complex number instead of raising."""
+        seq = """
+x: F64 = F64(-8.0) ** 0.5
+"""
+
+        assert_compile_failure(fprime_test_api, seq, match="Domain error")
+
+    def test_const_complex_pow_float_exponent(self, fprime_test_api):
+        """Same when it is the exponent that carries the float type."""
+        seq = """
+y: F64 = (-2.0) ** F64(0.5)
+"""
+
+        assert_compile_failure(fprime_test_api, seq, match="Domain error")
+
     def test_very_large_const_pow(self, fprime_test_api):
         seq = """
 10.0 ** 1000