enhance: Add tests for premature caching of contextual parameter types

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Comment

Add tests for premature caching of contextual parameter types

The checker currently handles the following code correctly.

But the existing cases in contextuallyTypedParametersWithInitializers2.ts do not appear to cover the following case: the combination of premature type caching and parameter whose type is widened during contextual checking.

Example:

It is ok:

const fn: (reader: () => number, value: 1) => void =
  (get = () => x, x = 0) => {
    x.toFixed();
  };

Why this needs regression coverage:

when caching the symbol type, we need to check if it is context sensitive, as the author intentionly said:

func (c *Checker) getTypeOfVariableOrParameterOrProperty(symbol *ast.Symbol) *Type {
	links := c.valueSymbolLinks.Get(symbol)
	if links.resolvedType == nil {
		t := c.getTypeOfVariableOrParameterOrPropertyWorker(symbol)
		if t == nil {
			panic("Unexpected nil type")
		}
		// For a contextually typed parameter it is possible that a type has already
		// been assigned (in assignTypeToParameterAndFixTypeParameters), and we want
		// to preserve this type. In fact, we need to _prefer_ that type, but it won't
		// be assigned until contextual typing is complete, so we need to defer in
		// cases where contextual typing may take place.
		if links.resolvedType == nil && !c.isParameterOfContextSensitiveSignature(symbol) {
			links.resolvedType = t
		}
		return t
	}
	return links.resolvedType
}

Checking get = () => x queries x before its contextual parameter checking is complete:

1. The initial query get the contextual type 1.
2. Later, checking the default value 0 and widens the parameter's type to number.
3. If the initial result 1 is cached, assignParameterType returns early and cannot assign the final type.

This would incorrectly produce the error:

Type '0' is not assignable to type '1'.

The existing guard in getTypeOfVariableOrParameterOrProperty prevents this premature caching:

!c.isParameterOfContextSensitiveSignature(symbol)

The example passes with the current implementation. Removing the guard will produce the incorrect TS2322 diagnostic.

Proposed change:

Just add this case to contextuallyTypedParametersWithInitializers2.ts

const test4: (reader: () => number, value: 1) => void =
  (get = () => x, x = 0) => {
    x.toFixed();
  };

Add this case to the contextual parameter initializer tests, together with type and symbol baselines, to protect the existing behavior against regressions.

Existing tests cover the caching guard’s effect on implicit any diagnostics. This test adds coverage of type widening for parameters with default initializers, ensuring that prematurely cached literal types do not cause spurious errors.