go/compile: compiler cannot infer type of type parameter from function return value

[ray-harris]

What version of Go are you using (go version)?

$ go version
go version go1.18 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/ray/Library/Caches/go-build"
GOENV="/Users/ray/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/ray/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/ray/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/ray/.gimme/versions/go1.18.darwin.amd64"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/ray/.gimme/versions/go1.18.darwin.amd64/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/ray/go/src/github.com/go-math/float/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/cd/c35rv7yn5s3104f8dwqk3tqh0000gn/T/go-build3869542070=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

main.go

package main

import (
	"fmt"
	"math"
)

type Float interface {
	~float32 | ~float64
}

func NaN[T Float]() T {
	return T(math.NaN())
}

func main() {
	var n float32
	n = NaN()
	fmt.Println(n)
}

Run

go run main.go

What did you expect to see?

NaN

What did you see instead?

./main.go:18:9: cannot infer T

In order for the code to work, you have to provide the type of the variable n:

    n = NaN[float32]()

Since the function NaN returns a value of type T and the the type of variable n is known to be float32, the compiler should that T should be float32, but it does not.

[ianlancetaylor]

Thanks for the report. That is correct. We only infer type arguments from function arguments or from other type arguments, not from the context in which a function occurs. We aren't going to be adding new kinds of type inference until we have more experience with the forms of inference we already have, so closing. We can reexamine this idea later.

[seankhliao]

Duplicate of #50285