NaN and ±Infinity double literals are emitted as undefined identifiers (NaNd, Infinity, -Infinity) in every language except JavaScript

[matt-edmondson]

What's wrong

LanguageGeneratorBase.FormatDouble (Coder/Languages/LanguageGeneratorBase.cs) formats with ToString("R") and, as its remarks say, leaves a non-finite value "as it is". CSharpGenerator writes {value.ToString(InvariantCulture)}d (Coder/Languages/CSharpGenerator.cs:105-106). Neither maps NaN or infinity to a spelling the target language understands.

Repro

gen.Generate(Literal.DecimalValue(double.NaN)), and the same for PositiveInfinity and NegativeInfinity:

Target	Emitted	Result
C#	NaNd, Infinityd, -Infinityd	not a valid token
C / C++	NaN, Infinity	gcc: 'NaN' undeclared
Go	NaN	undefined: NaN
Rust	NaN	E0425 cannot find value NaN
Python	NaN	NameError: name 'NaN' is not defined
JavaScript	NaN, Infinity	correct

These results come from compiling the generated text with each real toolchain.

Why it matters

NaN and infinity are common sentinel and default values, for example const double Unset = NaN or an initial min = Infinity. Any AST that contains one produces code that fails to compile in 6 of the 7 targets, and nothing warns about it at generation time.

Suggested fix

Add a per-language hook for non-finite values, such as a virtual FormatDouble or a FormatNonFinite(double):

Language	NaN	+Inf / −Inf
C#	double.NaN	double.PositiveInfinity / double.NegativeInfinity
C++	std::numeric_limits<double>::quiet_NaN()	std::numeric_limits<double>::infinity() (negated)
C	NAN	INFINITY / -INFINITY (needs <math.h>)
Go	math.NaN()	math.Inf(1) / math.Inf(-1) (needs import "math")
Rust	f64::NAN	f64::INFINITY / f64::NEG_INFINITY
Python	float("nan")	float("inf") / float("-inf")
JavaScript	NaN	Infinity / -Infinity

The C and Go spellings also need an include or import. If the generators can't add one yet, they should throw NotSupportedException rather than emit an undefined name.

Acceptance: a data-driven test covers NaN, +∞ and −∞ for each generator.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. NaN and infinity values produce output that fails to compile in 6 of the 7 targets.
• Area / suggested assignee: LanguageGeneratorBase.FormatDouble and CSharpGenerator. Owner: @matt-edmondson, who is already assigned to Whole-number double literals are emitted as integers: 1.0/2.0 becomes integer division (0) in C/C++/Go and fails to compile in Rust #78.
• Possible duplicate: this overlaps Whole-number double literals are emitted as integers: 1.0/2.0 becomes integer division (0) in C/C++/Go and fails to compile in Rust #78, which is open and assigned. Whole-number double literals are emitted as integers: 1.0/2.0 becomes integer division (0) in C/C++/Go and fails to compile in Rust #78's "Non-finite values" section and its suggested fix already call for per-language spellings of infinity and NaN. Open PR Read .inf, -.inf and .nan back as double literals #95 ("Part of Whole-number double literals are emitted as integers: 1.0/2.0 becomes integer division (0) in C/C++/Go and fails to compile in Rust #78") covers only the YAML .inf/.nan read-back and says the per-language spellings are still to do. This issue gives a more complete spec for that remaining part, with the full per-language table and the include/import caveat for C and Go. Either close it as a duplicate of Whole-number double literals are emitted as integers: 1.0/2.0 becomes integer division (0) in C/C++/Go and fails to compile in Rust #78, or narrow Whole-number double literals are emitted as integers: 1.0/2.0 becomes integer division (0) in C/C++/Go and fails to compile in Rust #78 to whole-number formatting and track the non-finite spellings here.
• Next step: decide which issue owns the non-finite spellings, then add the per-language hook and the data-driven NaN/±∞ test.

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[matt-edmondson]

Decision (maintainer, 2026-09-28)

• Generators add the required imports and includes for NaN/±∞. This is option 1 on Whole-number double literals are emitted as integers: 1.0/2.0 becomes integer division (0) in C/C++/Go and fails to compile in Rust #78. The C spelling (NAN/INFINITY plus <math.h>), the C++ spelling (std::numeric_limits plus <limits>) and the Go spelling (math.NaN()/math.Inf(±1) plus import "math") are emitted together with their import. The generator does not throw NotSupportedException for them.
• The spelling table in this issue is the spec. Use it for every target, including the C# fix: double.NaN / double.PositiveInfinity rather than NaNd.
• This covers the same remaining work as Whole-number double literals are emitted as integers: 1.0/2.0 becomes integer division (0) in C/C++/Go and fails to compile in Rust #78. Implement it once for both issues.

Next reader: add the per-language non-finite hook and the "required imports" mechanism in SourceFile generation, then add the data-driven NaN/+∞/−∞ test for each generator. Close this issue and #78 together.

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