Freeze casts of bare constants to typed PsConstantExprs

The type of the argument to a cast is hard to infer because there is no context from the enclosing scope. Unless a typed symbol appears in the argument, the typifier has no way to determine the type of the argument. For more discussion see #97.

This MR fixes the issue for casts of bare constants, e.g. CastFunc(0, float64) by freezing them to constants of the target type directly. Additionally, the typifier now raises an error if the type of an argument to a cast can not be determined.

src/pystencils/backend/kernelcreation/freeze.py

@@ -58,7 +58,7 @@ from ..ast.expressions import (
 )
 
 from ..constants import PsConstant
-from ...types import PsStructType, PsType
+from ...types import PsNumericType, PsStructType, PsType
 from ..exceptions import PsInputError
 from ..functions import PsMathFunction, MathFunctions
 
@@ -462,7 +462,7 @@ class FreezeExpressions:
             ]
         return cast(PsCall, args[0])
 
-    def map_CastFunc(self, cast_expr: CastFunc) -> PsCast:
+    def map_CastFunc(self, cast_expr: CastFunc) -> PsCast | PsConstantExpr:
         dtype: PsType
         match cast_expr.dtype:
             case DynamicType.NUMERIC_TYPE:
@@ -472,7 +472,19 @@ class FreezeExpressions:
             case other if isinstance(other, PsType):
                 dtype = other
 
-        return PsCast(dtype, self.visit_expr(cast_expr.expr))
+        arg = self.visit_expr(cast_expr.expr)
+        if (
+            isinstance(arg, PsConstantExpr)
+            and arg.constant.dtype is None
+            and isinstance(dtype, PsNumericType)
+        ):
+            # As of now, the typifier can not infer the type of a bare constant.
+            # However, untyped constants may not appear in ASTs from which
+            # kernel functions are generated. Therefore, we annotate constants
+            # instead of casting them.
+            return PsConstantExpr(arg.constant.interpret_as(dtype))
+        else:
+            return PsCast(dtype, arg)
 
     def map_Relational(self, rel: sympy.core.relational.Relational) -> PsRel:
         arg1, arg2 = [self.visit_expr(arg) for arg in rel.args]