llvm/flang/documentation/extensions.md

As a general principle, this compiler will accept by default and without complaint many legacy features, extensions to the standard language, and features that have been deleted from the standard, so long as the recognition of those features would not cause a standard-conforming program to be rejected or misinterpreted.

Other non-standard features, which do conflict with the current standard specification of the Fortran programming language, are accepted if enabled by command-line options.

Extensions, deletions, and legacy features supported by default

• Tabs in source
• <> as synonym for .NE. and /=
• $ and @ as legal characters in names
• Initialization in type declaration statements using /values/
• Kind specification with *, e.g. REAL*4
• DOUBLE COMPLEX
• Signed complex literal constants
• DEC STRUCTURE, RECORD, UNION, and MAP
• Structure field access with .field
• NCHARACTER type and NC Kanji character literals
• BYTE as synonym for INTEGER(KIND=1)
• Quad precision REAL literals with Q
• X prefix/suffix as synonym for Z on hexadecimal literals
• B, O, Z, and X accepted as suffixes as well as prefixes
• Triplets allowed in array constructors
• Old-style PARAMETER pi=3.14 statement without parentheses
• %LOC, %VAL, and %REF
• Leading comma allowed before I/O item list
• Empty parentheses allowed in PROGRAM P()
• Missing parentheses allowed in FUNCTION F
• Cray based POINTER(p,x)
• Arithmetic IF. (Which branch should NaN take? Fall through?)
• ASSIGN statement, assigned GO TO, and assigned format
• PAUSE statement
• Hollerith literals and edit descriptors
• NAMELIST allowed in the execution part
• Omitted colons on type declaration statements with attributes
• COMPLEX constructor expression, e.g. (x+y,z)
• + and - before all primary expressions, e.g. x*-y
• .NOT. .NOT. accepted
• NAME= as synonym for FILE=
• DISPOSE=
• Data edit descriptors without width or other details
• D lines in fixed form as comments or debug code
• CONVERT= on the OPEN statement
• Leading semicolons are ignored before any statement that could have a label
• The character & in column 1 in fixed form source is a variant form of continuation line.

Extensions supported when enabled by options

• C-style backslash escape sequences in quoted CHARACTER literals (but not Hollerith) [-fbackslash]
• Logical abbreviations .T., .F., .N., .A., .O., and .X. [-flogical-abbreviations]
• .XOR. as a synonym for .NEQV. [-fxor-operator]

Extensions and legacy features deliberately not supported

• .LG. as synonym for .NE.
• REDIMENSION
• Allocatable COMMON
• Expressions in formats
• ACCEPT as synonym for READ *
• TYPE as synonym for PRINT
• ARRAY as synonym for DIMENSION
• VIRTUAL as synonym for DIMENSION
• ENCODE and DECODE as synonyms for internal I/O
• IMPLICIT AUTOMATIC, IMPLICIT STATIC
• Default exponent of zero, e.g. 3.14159E
• Characters in defined operators that are neither letters nor digits
• B suffix on unquoted octal constants
• Z prefix on unquoted hexadecimal constants (dangerous)
• T and F as abbreviations for .TRUE. and .FALSE. (dangerous)