Add return-type-widening guard to Refaster template engine

[knutwannheden]

When a Refaster @AfterTemplate has a wider return type than the @BeforeTemplate, and the matched expression is used in a context that depends on the narrower type (e.g., as a receiver for a chained method call), the replacement can break compilation.

Example

Given a Refaster rule where before returns Integer and after returns Number:

class ToNumber {
    @BeforeTemplate
    Integer before(int value) {
        return Integer.valueOf(value);
    }
    @AfterTemplate
    Number after(int value) {
        return value;
    }
}

This input would be broken by the replacement:

// Before: compiles fine — compareTo() exists on Integer
Integer.valueOf(42).compareTo(10);

// After: compile error — Number doesn't have compareTo()
((Number) 42).compareTo(10);

Error Prone Refaster has the same behavior, but it only presents suggestions for developer review. OpenRewrite applies fixes automatically, so it needs to be more conservative.

Changes

• Added isAssignableToTargetType(String) helper on AbstractRefasterJavaVisitor that checks whether the after template's return type is compatible with the target type expected by the surrounding context (receiver of chained call, method argument, assignment target)
• RecipeWriter.generateVisitMethod() now detects return type widening at annotation-processor time using javac's Types.isSubtype() on erased types, and emits a guard that skips the match when the wider type would be incompatible with the context
• Updated PreconditionsVerifierRecipes.java expected test output to include the guard for templates with String → Object widening

[timtebeek]

Nice! Good to avoid changes where unsafe to do so.