From 9b7155865768f7987be2bbae93978bfbc689555a Mon Sep 17 00:00:00 2001
From: Steve Bronder <sbronder@flatironinstitute.org>
Date: Thu, 16 May 2024 12:50:19 -0400
Subject: [PATCH] update to use logic to check compiler version for c++17
 compatability

---
 .../contributor_help_pages/developer_doc.md   |  2 +-
 make/compiler_flags                           | 26 ++++++++++++++++---
 make/libraries                                |  4 +--
 3 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/doxygen/contributor_help_pages/developer_doc.md b/doxygen/contributor_help_pages/developer_doc.md
index 0961a5ba9c3..687eb1b157d 100644
--- a/doxygen/contributor_help_pages/developer_doc.md
+++ b/doxygen/contributor_help_pages/developer_doc.md
@@ -184,7 +184,7 @@ These are the more common make flags that could be set:
 These are the rest of the variables that can be set:
 
 - C++ compiler flags
-    - `CXXFLAGS_LANG`: sets the language. Currently defaults to `-std=c++1y`
+    - `CXXFLAGS_LANG`: sets the language. Currently defaults to `-std=c++17`
     - `CXXFLAGS_WARNINGS`: compiler options to squash compiler warnings
     - `CXXFLAGS_BOOST`: Boost-specific compiler flags
     - `CXXFLAGS_EIGEN`: Eigen-specific compiler flags
diff --git a/make/compiler_flags b/make/compiler_flags
index 407d5932082..73268a95204 100644
--- a/make/compiler_flags
+++ b/make/compiler_flags
@@ -70,7 +70,6 @@ CXX_VERSION :=  $(shell $(CXX) -dumpfullversion -dumpversion 2>&1)
 CXX_MAJOR := $(word 1,$(subst ., ,$(CXX_VERSION)))
 CXX_MINOR := $(word 2,$(subst ., ,$(CXX_VERSION)))
 
-
 ################################################################################
 # Set optional compiler flags for performance
 #
@@ -122,13 +121,34 @@ CPPFLAGS_BOOST ?= -DBOOST_DISABLE_ASSERTS
 CPPFLAGS_SUNDIALS ?= -DNO_FPRINTF_OUTPUT $(CPPFLAGS_OPTIM_SUNDIALS) $(CXXFLAGS_FLTO_SUNDIALS)
 #CPPFLAGS_GTEST ?=
 
-
-ifeq ($(STAN_USE_CPP14),)
+ifeq ($(CXX_TYPE), gcc)
+  GCC_GE_73 := $(shell [ $(CXX_MAJOR) -gt 7 -o \( $(CXX_MAJOR) -eq 7 -a $(CXX_MINOR) -ge 1 \) ] && echo true)
+  ifeq ($(GCC_GE_73),true)
+    CXXFLAGS_LANG ?= -std=c++17
+    CXXFLAGS_STANDARD ?= c++17
+  endif
+else ifeq ($(CXX_TYPE), clang)
+  CLANG_GE_5 := $(shell [ $(CXX_MAJOR) -gt 5 -o \( $(CXX_MAJOR) -eq 5 -a $(CXX_MINOR) -ge 0 \) ] && echo true)
+  ifeq ($(CLANG_GE_5),true)
+    CXXFLAGS_LANG ?= -std=c++17
+    CXXFLAGS_STANDARD ?= c++17
+  endif
+else ifeq ($(CXX_TYPE), mingw32-gcc)
+  MINGW_GE_50 := $(shell [ $(CXX_MAJOR) -gt 5 -o \( $(CXX_MAJOR) -eq 5 -a $(CXX_MINOR) -ge 0 \) ] && echo true)
+  ifeq ($(MINGW_GE_50),true)
+    CXXFLAGS_LANG ?= -std=c++17
+    CXXFLAGS_STANDARD ?= c++17
+  endif
+else ifeq ($(STAN_USE_CPP14),)
   CXXFLAGS_LANG ?= -std=c++17
+  CXXFLAGS_STANDARD ?= c++17
 else
   $(warning "Because STAN_USE_CPP14 is set C++14 standard is requested. This will not be possible in the next release!")
   CXXFLAGS_LANG ?= -std=c++1y
+  CXXFLAGS_STANDARD ?= c++1y
 endif
+
+
 #CXXFLAGS_BOOST ?=
 CXXFLAGS_SUNDIALS ?= -pipe $(CXXFLAGS_OPTIM_SUNDIALS) $(CPPFLAGS_FLTO_SUNDIALS)
 #CXXFLAGS_GTEST
diff --git a/make/libraries b/make/libraries
index f6beafcda46..e2b347d8388 100644
--- a/make/libraries
+++ b/make/libraries
@@ -176,11 +176,11 @@ endif
 $(TBB_BIN)/tbb.def: $(TBB_BIN)/tbb-make-check
 	@mkdir -p $(TBB_BIN)
 	touch $(TBB_BIN)/version_$(notdir $(TBB))
-	tbb_root="$(TBB_RELATIVE_PATH)" WINARM64="$(WINARM64)" CXX="$(CXX)" CC="$(TBB_CC)" LDFLAGS='$(LDFLAGS_TBB)' '$(MAKE)' -C "$(TBB_BIN)" -r -f "$(TBB_ABSOLUTE_PATH)/build/Makefile.tbb" compiler=$(TBB_CXX_TYPE) cfg=release stdver=c++1y  CXXFLAGS="$(TBB_CXXFLAGS)"
+	tbb_root="$(TBB_RELATIVE_PATH)" WINARM64="$(WINARM64)" CXX="$(CXX)" CC="$(TBB_CC)" LDFLAGS='$(LDFLAGS_TBB)' '$(MAKE)' -C "$(TBB_BIN)" -r -f "$(TBB_ABSOLUTE_PATH)/build/Makefile.tbb" compiler=$(TBB_CXX_TYPE) cfg=release stdver=$(CXXFLAGS_STANDARD)  CXXFLAGS="$(TBB_CXXFLAGS)"
 
 $(TBB_BIN)/tbbmalloc.def: $(TBB_BIN)/tbb-make-check
 	@mkdir -p $(TBB_BIN)
-	tbb_root="$(TBB_RELATIVE_PATH)" WINARM64="$(WINARM64)" CXX="$(CXX)" CC="$(TBB_CC)" LDFLAGS='$(LDFLAGS_TBB)' '$(MAKE)' -C "$(TBB_BIN)" -r -f "$(TBB_ABSOLUTE_PATH)/build/Makefile.tbbmalloc" compiler=$(TBB_CXX_TYPE) cfg=release stdver=c++1y malloc CXXFLAGS="$(TBB_CXXFLAGS)"
+	tbb_root="$(TBB_RELATIVE_PATH)" WINARM64="$(WINARM64)" CXX="$(CXX)" CC="$(TBB_CC)" LDFLAGS='$(LDFLAGS_TBB)' '$(MAKE)' -C "$(TBB_BIN)" -r -f "$(TBB_ABSOLUTE_PATH)/build/Makefile.tbbmalloc" compiler=$(TBB_CXX_TYPE) cfg=release stdver=$(CXXFLAGS_STANDARD) malloc CXXFLAGS="$(TBB_CXXFLAGS)"
 
 $(TBB_BIN)/libtbb.dylib: $(TBB_BIN)/tbb.def
 $(TBB_BIN)/libtbbmalloc.dylib: $(TBB_BIN)/tbbmalloc.def