Explicitly define FunctionExpression

This sets the stage for modifying it to fix #3245

spec/syntax.md

@@ -71,6 +71,30 @@ No whitespace is allowed between components of an `InterpolatedUnquotedUrlConten
 [identifier code point]: https://drafts.csswg.org/css-syntax-3/#identifier-code-point
 [escape]: https://drafts.csswg.org/css-syntax-3/#escape-diagram
 
+### `FunctionExpression`
+
+<x><pre>
+**FunctionExpression**¹ ::= [CssMinMax]
+&#32;                     | SpecialFunctionExpression
+&#32;                     | [CalculationExpression]
+&#32;                     | FunctionCall
+**FunctionCall**        ::= FunctionName FunctionArguments ')'
+**FunctionName**²       ::= [NamespacedIdentifier] '('
+</pre></x>
+
+[CssMinMax]: types/calculation.md#cssminmax
+[CalculationExpression]: types/calculation.md#calculationexpression
+[NamespacedIdentifier]: modules.md#syntax
+
+1: If both `CssMinMax` and `FunctionCall` could be consumed, `CssMinMax`
+   takes precedence.
+
+2: `FunctionName` may not contain any whitespace, may not match
+   [`SpecialFunctionName`], and may not be case-insensitively equal to `'calc('`
+   or `'clamp('`.
+
+[`SpecialFunctionName`]: #specialfunctionexpression
+
 ### `SpecialFunctionExpression`
 
 > These functions are "special" in the sense that their arguments don't use the

spec/types/calculation.md

@@ -73,21 +73,13 @@ case-insensitively.
 > expression regardless of syntax, for full compatibility it's necessary to
 > parse it very expansively.
 
-### `MinMaxExpression`
-
-This production is parsed in a SassScript context when an expression is expected
-and the input stream starts with an identifier with value `min` or `max`
-(ignoring case) followed immediately by `(`.
+### `CssMinMax`
 
 <x><pre>
-**MinMaxExpression**¹ ::= CssMinMax | FunctionExpression
-**CssMinMax**         ::= ('min(' | 'max(')² CalcArgument (',' CalcArgument)* ')'
+**CssMinMax**         ::= ('min(' | 'max(')¹ CalcArgument (',' CalcArgument)* ')'
 </pre></x>
 
-1: If both `CssMinMax` and `FunctionExpression` could be consumed, `CssMinMax`
-   takes precedence.
-
-2: The strings `min(` and `max(` are matched case-insensitively.
+1: The strings `min(` and `max(` are matched case-insensitively.
 
 ## Types